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The en route phase of flight has seen some of the mostdramatic improvements in the way pilots navigate fromdeparture to destination. Developments in technologyhave played a significant role in most of theseimprovements. Computerized avionics and advancednavigation systems are common place in both generaland commercial aviation.

The procedures employed in the en route phase offlight are governed by a set of specific flight standardsestablished by Title 14 of the Code of FederalRegulations (14 CFR), Federal Aviation Administration(FAA) Order 8260.3, United States Standard forTerminal Instrument Procedures (TERPS), and relatedpublications. These standards establish courses to beflown, obstacle clearance criteria, minimum altitudes,navigation performance, and communications require-ments. For the purposes of this discussion, the en routephase of flight is defined as that segment of flight fromthe termination point of a departure procedure to theorigination point of an arrival procedure.

EN ROUTE NAVIGATIONPart 91.181 is the basis for the course to be flown. Tooperate an aircraft within controlled airspace underinstrument flight rules (IFR), pilots must fly the centerof a Federal airway or the direct course between navi-gational aids or fixes defining a route. The regulationallows maneuvering to pass well clear of other air traf-fic or, if in visual flight rules (VFR) conditions, to clearthe flight path both before and during climb or descent.

En route IFR navigation is evolving from the navi-gational aid (NAVAID)-based airway system to asophisticated, computer-based system that can generateinfinite courses to suit the operational requirements ofany flight. Although the promise of the new navigationsystems is limitless, the present system of navigationserves a valuable function and is expected to remainfor a number of years.

The procedures pilots employ in the en route phase offlight take place in the structure of the NationalAirspace System (NAS) consisting of three strata. Thefirst, or lower stratum, is an airway structure thatextends from the base of controlled airspace up to butnot including 18,000 feet mean sea level (MSL). The

second stratum is an area containing identifiable jetroutes as opposed to designated airways, and extendsfrom 18,000 feet MSL to Flight Level (FL) 450. Thethird stratum, FL 450 and above is intended for ran-dom, point-to-point navigation.

AIR ROUTE TRAFFIC CONTROL CENTERSThe Air Route Traffic Control Center (ARTCC)encompasses the en route air traffic control systemair/ground radio communications, that provides safeand expeditious movement of aircraft operating oninstrument flight rules (IFR) within the controlled air-space of the Center. ARTCCs provide the centralauthority for issuing IFR clearances and nationwidemonitoring of each IFR flight. This applies primarily tothe en route phase of flight, and includes weather infor-mation and other inflight services. There are 20ARTCCs in the conterminous United States (U.S.), andeach Center contains between 20 to 80 sectors, withtheir size, shape, and altitudes determined by trafficflow, airway structure, and workload. Appropriateradar and communication sites are connected to theCenters by microwave links and telephone lines.[Figure 3-1]

The CFRs require the pilot in command under IFR incontrolled airspace to continuously monitor an appro-priate Center or control frequency. When climbing aftertakeoff, an IFR flight is either in contact with a radar-equipped local departure control or, in some areas, anARTCC facility. As a flight transitions to the en routephase, pilots typically expect a handoff from departurecontrol to a Center frequency if not already in contactwith the Center. The FAA National AeronauticalCharting Office (NACO) publishes en route chartsdepicting Centers and sector frequencies, as shown infigure 3-2. During handoff from one Center to another,the previous controller assigns a new frequency. Incases where flights may be still out of range, the Centerfrequencies on the face of the chart are very helpful. Infigure 3-2, notice the boundary between Memphis andAtlanta Centers, and the remoted sites with discretevery high frequency (VHF) and ultra high frequency(UHF) for communicating with the appropriateARTCC. These Center frequency boxes can be used forfinding the nearest frequency within the aircraft range.They also can be used for making initial contact with

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the Center for airfiled “pop up” clearances. The exactlocation for the Center transmitter is not shown;although the frequency boxes are placed as close aspossible to the known location.

During the en route phase, as a flight transitions fromone Center facility to the next, a handoff or transfer ofcontrol is required as previously described. The hand-off procedure is similar to the handoff between other

ClevelandCenter

AlbuquerqueCenter

SeattleCenter

Atlanta Center

ChicagoCenter

Boston Center

Washington Center (DC)

Denver Center

Fort Worth Center

Houston Center

IndianapolisCenter

Jacksonville Center

Kansas City Center

Los Angeles Center

Salt Lake City Center

Miami Center

MemphisCenter

Minneapolis Center

New York Center

ZID

ZMP

ZOB

ZBW

ZNYZNY

ZDC

ZAU

ZKC

ZMEZTL

ZJX

ZMA

ZHU

ZFW

ZAB

ZDV

ZLA

ZOA

ZLC

ZSE

Oakland Center

HonoluluCenter

ZHN

AnchorageCenter

ZAN

Figure 3-1. Air Route Traffic Control Centers.

Figure 3-2. ARTCC Centers and Sector Frequencies.

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radar facilities, such as departure or approach control.During the handoff, the controller whose airspace isbeing vacated issues instructions that include the nameof the facility to contact, appropriate frequency, andother pertinent remarks.

While the Center is monitoring a flight by radar, it ispossible that air traffic control (ATC) computers maygo down, power fails, or radio communications fail,leaving a flight without ATC guidance. In other cases,there may be gaps in radar coverage along the route offlight. Accepting radar vectors from controllers doesnot relieve pilots of their responsibility for safety offlight. Pilots must maintain a safe altitude and keeptrack of their position, and it is their obligation to ques-tion controllers, request an amended clearance, or, inan emergency, deviate from their instructions if theybelieve that the safety of flight is in doubt. Keepingtrack of altitude and position when climbing, andduring all other phases of flight, are basic elementsof situational awareness. Aircraft equipped with aground proximity warning system (GPWS) and traf-fic alert and collision avoidance system (TCAS) helppilots detect and correct unsafe altitudes and trafficconflicts. Regardless of equipment, pilots must alwaysmaintain situational awareness regarding their locationand the location of traffic in their vicinity.

PREFERRED IFR ROUTESA system of preferred IFR routes helps pilots, flightcrews, and dispatchers plan a route of flight to mini-mize route changes, and to aid in the efficient, orderlymanagement of air traffic using Federal airways.Preferred IFR routes are designed to serve the needs ofairspace users and to provide for a systematic flow ofair traffic in the major terminal and en route flight envi-ronments. Cooperation by all pilots in filing preferred

routes results in fewer air traffic delays and better effi-ciency for departure, en route, and arrival air trafficservice. [Figure 3-3]

Preferred IFR routes are published in theAirport/Facility Directory for the low and high altitudestratum. If they begin or end with an airway number, itindicates that the airway essentially overlies the airportand flights normally are cleared directly on the airway.Preferred IFR routes beginning or ending with a fixindicate that pilots may be routed to or from these fixesvia a standard instrument departure (SID) route, radarvectors, or a standard terminal arrival route (STAR).Routes for major terminals are listed alphabeticallyunder the name of the departure airport. Where severalairports are in proximity they are listed under the prin-cipal airport and categorized as a metropolitan area;e.g., New York Metro Area. One way preferred IFRroutes are listed numerically showing the segment fixesand the direction and times effective. Where more thanone route is listed, the routes have equal priority foruse. Official location identifiers are used in the routedescription for very high frequency omnidirectionalranges (VORs) and very high frequency omnidirec-tional ranges/tactical air navigation (VORTACs), andintersection names are spelled out. The route is directwhere two NAVAIDs, an intersection and a NAVAID, aNAVAID and a NAVAID radial and distance point, orany navigable combination of these route descriptionsfollow in succession.

SUBSTITUTE EN ROUTE FLIGHT PROCEDURESAir route traffic control centers are responsible for speci-fying essential substitute airway and route segments andfixes for use during VOR/VORTAC shutdowns.Scheduled shutdowns of navigational facilities requireplanning and coordination to ensure an uninterrupted

flow of air traffic. A schedule ofproposed facility shutdownswithin the region is maintainedand forwarded as far in advanceas possible to enable the substi-tute routes to be published.Substitute routes are normallybased on VOR/VORTAC facili-ties established and publishedfor use in the appropriate altitudestrata. In the case of substituteroutes in the upper airspace stra-tum, it may be necessary toestablish routes by reference toVOR/VORTAC facilities usedin the low altitude system.Nondirectional radio beacon(NDB) facilities may only beused where VOR/VORTACcoverage is inadequate and ATCrequirements necessitate use ofFigure 3-3. Preferred IFR Routes.

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such NAVAIDs. Where operational necessity dictates,navigational aids may be used beyond their standardservice volume (SSV) limits, provided that the routescan be given adequate frequency protection.

The centerline of substitute routes must be containedwithin controlled airspace, although substitute routesfor off-airway routes may not be in controlled air-space. Substitute routes are flight inspected to verifyclearance of controlling obstacles and to check forsatisfactory facility performance. To provide pilotswith necessary lead time, the substitute routes aresubmitted in advance of the en route chart effectivedate. If the lead time cannot be provided, the shut-down may be delayed or a special graphic NOTAMmay be considered. Normally, shutdown of facilitiesscheduled for 28 days (half the life of the en routechart) or less will not be charted. The format fordescribing substitute routes is from navigational fixto navigational fix. A minimum en route altitude(MEA) and a maximum authorized altitude (MAA) isprovided for each route segment. Temporary reportingpoints may be substituted for the out-of-service facilityand only those other reporting points that are essentialfor air traffic control. Normally, temporary reportingpoints over intersections are not necessary whereCenter radar coverage exists. A minimum receptionaltitude (MRA) is established for each temporaryreporting point.

TOWER EN ROUTE CONTROLWithin the NAS it is possible to fly an IFR flight with-out leaving approach control airspace, using tower enroute control (TEC) service. This helps expedite airtraffic and reduces air traffic control and pilot commu-nication requirements. TEC is referred to as “tower enroute,” or “tower-to-tower,” and allows flight beneaththe en route structure. Tower en route control reallo-cates airspace both vertically and geographically toallow flight planning between city pairs while remain-ing with approach control airspace. All users areencouraged to use the TEC route descriptions in theAirport/Facility Directory when filing flight plans. Allpublished TEC routes are designed to avoid en routeairspace, and the majority are within radar coverage.[Figure 3-4]

The graphic depiction of TEC routes is not to be usedfor navigation or for detailed flight planning. Not allcity pairs are depicted. It is intended to show geo-graphic areas connected by tower en route control.Pilots should refer to route descriptions for specificflight planning. The word “DIRECT” appears as theroute when radar vectors are used or no airway exists.Also, this indicates that a SID or STAR may beassigned by ATC. When a NAVAID or intersectionidentifier appears with no airway immediately preced-ing or following the identifier, the routing is understood

to be direct to or from that point unless otherwisecleared by ATC. Routes beginning and ending with anairway indicate that the airway essentially overfliesthe airport, or radar vectors will be issued. Wheremore than one route is listed to the same destination,ensure that the correct route for the type of aircraftclassification has been filed. These are denoted afterthe route in the altitude column using J (jet powered),M (turbo props/special, cruise speed 190 knots orgreater), P (non-jet, cruise speed 190 knots orgreater), or Q (non-jet, cruise speed 189 knots or less).Although all airports are not listed under the destina-tion column, IFR flights may be planned to satelliteairports in the proximity of major airports via thesame routing. When filing flight plans, the codedroute identifier, i.e., BURL1, VTUL4, or POML3,may be used in lieu of the route of flight.

AIRWAY AND ROUTE SYSTEMThe present en route system is based on the VHF air-way/route navigation system. Low frequency (LF) andintegrated LF/VHF airways and routes have graduallybeen phased out in the conterminous U.S., with someremaining in Alaska.

MONITORING OF NAVIGATION FACILITIESVOR, VORTAC, and instrument landing system (ILS)facilities, as well as most nondirectional radio beacons(NDBs) and marker beacons installed by the FAA, areprovided with an internal monitoring feature. Internalmonitoring is provided at the facility through the use ofequipment that causes a facility shutdown if perform-ance deteriorates below established tolerances. Aremote status indicator also may be provided throughthe use of a signal sampling receiver, microwave link,or telephone circuit. Older FAA NDBs and some non-Federal NDBs do not have the internal feature andmonitoring is accomplished by manually checking theoperation at least once each hour. FAA facilities suchas automated flight service stations (AFSSs) andARTCCs/sectors are usually the control point forNAVAID facility status. Pilots can query the appropri-ate FAA facility if they have questions in flight regard-ing NAVAID status, in addition to checking notices toairmen (NOTAMs) prior to flight, since NAVAIDs andassociated monitoring equipment are continuouslychanging.

LF AIRWAYS/ROUTESNumerous low frequency airways still exist in Alaska,as depicted in this NACO en route low altitude chartexcerpt near Nome, Alaska. [Figure 3-5] Colored LFeast and west airways G7, G212 (green), and R35 (red),are shown, along with north and south airways B2, B27(blue), and A1 (amber), all based upon the Fort DavisNDB en route NAVAID. The nearby Nome VORTACVHF en route NAVAID is used with victor airwaysV452, V333, V507, V506, and V440.

3-5

Figure 3-4.Tower En Route Control.

Figure 3-5. LF and VHF Airways — Alaska.

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VHF AIRWAYS/ROUTESFigure 3-6 depicts numerous arrowed, single directionjet routes on this excerpt from a NACO en route highaltitude chart, effective at and above 18,000 feet MSLup to and including FL 450. Notice the MAAs of 41,000and 29,000 associated with J24 and J193, respectively.Additionally, note the BAATT, NAGGI, FUMES, andMEYRA area navigation (RNAV) waypoints.Waypoints are discussed in detail later in this chapter.

VHF EN ROUTE OBSTACLE CLEARANCE AREASAll published routes in the NAS are based on specificobstacle clearance criteria. An understanding of enroute obstacle clearance areas helps with situationalawareness and may help avoid controlled flight intoterrain (CFIT). Obstacle clearance areas for the enroute phase of flight are identified as primary, second-ary, and turning areas.

The primary and secondary area obstacle clearance cri-teria, airway and route widths, and the ATC separationprocedures for en route segments are a function of safetyand practicality in flight procedures. These flight proce-dures are dependent upon the pilot, the aircraft, and thenavigation system being used, resulting in a total VORsystem accuracy factor, along with an associated proba-bility factor. The pilot/aircraft information componentof this criteria includes pilot ability to track the radialand the flight track resulting from turns at variousspeeds and altitudes under different wind conditions.

The navigation system information includes navigationfacility radial alignment displacement, transmittermonitor tolerance, and receiver accuracy. All of thesefactors were considered during development of enroute criteria. From this analysis, the computationsresulted in a total system accuracy of ±4.5° 95 percentof the time and ±6.7° 99 percent of the time. The 4.5°figure became the basis for primary area obstacleclearance criteria, airway and route widths, and theATC separation procedures. The 6.7° value providessecondary obstacle clearance area dimensions. Figure3-7 depicts the primary and secondary obstacle clear-ance areas.

PRIMARY AREAThe primary obstacle clearance area has a protectedwidth of 8 nautical miles (NM); 4 NM on each side ofthe centerline. The primary area has widths of routeprotection based upon system accuracy of a ±4.5° anglefrom the NAVAID. These 4.5° lines extend out fromthe NAVAID and intersect the boundaries of the pri-mary area at a point approximately 51 NM from theNAVAID. Ideally, the 51 NM point is where pilotswould change over from navigating away from thefacility, to navigating toward the next facility, althoughthis ideal is rarely achieved.

If the distance from the NAVAID to the changeoverpoint (COP) is more than 51 NM, the outer boundaryof the primary area extends beyond the 4 NM widthalong the 4.5° line when the COP is at midpoint. This

Figure 3-6. VHF Jet Routes.

3-7

means the primary area, along with its obstacle clear-ance criteria, is extended out into what would havebeen the secondary area. Additional differences in theobstacle clearance area result in the case of the effect ofan offset COP or dogleg segment. For protected enroute areas the minimum obstacle clearance in the pri-mary area, not designated as mountainous under Part95 — IFR altitude is 1,000 feet over the highest obsta-cle. [Figure 3-8]

Mountainous areas for the Eastern and Western U.S.are designated in Part 95, as shown in figure 3-9.Additional mountainous areas are designated forAlaska, Hawaii, and Puerto Rico. With some excep-tions, the protected en route area minimum obstacleclearance over terrain and manmade obstacles inmountainous areas is 2,000 feet. Obstacle clearance issometimes reduced to not less than 1,500 feet aboveterrain in the designated mountainous areas of theEastern U.S., Puerto Rico, and Hawaii, and may bereduced to not less than 1,700 feet in mountainousareas of the Western U.S. and Alaska. Consideration isgiven to the following points before any altitudes pro-viding less than 2,000 feet of terrain clearance areauthorized:

• Areas characterized by precipitous terrain.

• Weather phenomena peculiar to the area.

• Phenomena conducive to marked pressure differ-entials.

• Type of and distance between navigational facilities.

• Availability of weather services throughout thearea.

• Availability and reliability of altimeter resettingpoints along airways and routes in the area.

Altitudes providing at least 1,000 feet of obstacle clear-ance over towers and/or other manmade obstacles maybe authorized within designated mountainous areas ifthe obstacles are not located on precipitous terrain whereBernoulli Effect is known or suspected to exist.

4.5°

4.5°51

51 4.5°

4.5°

4 NM

4 NM

4 NM

4 NM

2 NM

2 NM

6.7°

6.7°

6.7°

6.7° 51

51

Primary Obstacle Clearance Area

Secondary Obstacle Clearance Area

Figure 3-7. VHF En Route Obstacle Clearance Areas.

1,000 Feet AboveHighest Obstacle

Primary En RouteObstacle Clearance Area

Nonmountainous Area

Figure 3-8. Obstacle Clearance - Primary Area.

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Bernoulli Effect, atmospheric eddies, vortices, waves,and other phenomena that occur in conjunction with dis-turbed airflow associated with the passage of strongwinds over mountains can result in pressure deficienciesmanifested as very steep horizontal pressure gradients.Since downdrafts and turbulence are prevalent underthese conditions, potential hazards may be multiplied.

SECONDARY AREAThe secondary obstacle clearance area extends along aline 2 NM on each side of the primary area. Navigationsystem accuracy in the secondary area has widths ofroute protection of a ±6.7° angle from the NAVAID.These 6.7° lines intersect the outer boundaries of the sec-ondary areas at the same point as primary lines, 51 NMfrom the NAVAID. If the distance from the NAVAID tothe COP is more than 51 NM, the secondary areaextends along the 6.7° line when the COP is at mid-point. In all areas, mountainous and nonmountainous,obstacles that are located in secondary areas are con-sidered as obstacles to air navigation if they extendabove the secondary obstacle clearance plane. Thisplane begins at a point 500 feet above the obstaclesupon which the primary obstacle clearance area isbased, and slants upward at an angle that causes it tointersect the outer edge of the secondary area at a point500 feet higher. [Figure 3-10]

The obstacle clearance areas for LF airways and routesare different than VHF, with the primary and secondaryarea route widths both being 4.34 NM. The accuracylines are 5.0° in the primary obstacle clearance area and7.5° in the secondary area. Obstacle clearance in theprimary area of LF airways and routes is the same asthat required for VHF, although the secondary areaobstacle clearance requirements are based upon dis-tance from the facility and location of the obstaclerelative to the inside boundary of the secondary area.

Figure 3-9. Designated Mountainous Areas.

Puerto RicoMountainous Area

67° 66°30' 66°

20°

25°

67° 66°30' 66°

30°

35°

40°

45°

75°85°95°105°115°

125°

45°

40°

35°

30°

25°

20°

125°

115°

105°

95°85°

75°

LEGEND

WASHINGTON

OREGON

MONTANA

IDAHO

NEVADA

WYOMING

UTAH COLORADO

CALIFORNIA

ARIZONA NEW MEXICO

NORTHDAKOTA

SOUTHDAKOTA

NEBRASKA

KANSAS

OKLAHOMA

TEXASLA

MS

FL

GEORGIA

SC

MD

DE

RI

ME

VTNHMA

CTNEW YORK

PA

WV

VIRGINA

MI

IN OHIO

WISCONSIN

MINNESOTA

IOWA

ILLINOIS

MISSOURI

ARKANSAS

KENTUCKY

NJ

NO CAROLINATENNESSEE

AL

(NOT DRAWN TO SCALE)

MountainousAreas

NonmountainousArea

MountainousArea

Primary Area

Secondary Area

500 Feet

Total Widthof SecondaryArea

Distance fromObstacle toOuter Edge ofSecondary Area

Figure 3-10. Obstacle Clearance - Secondary Area.

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When a VHF airway or route terminates at aNAVAID or fix, the primary area extends beyond thattermination point. Figure 3-11 and its inset show theconstruction of the primary and secondary areas atthe termination point. When a change of course onVHF airways and routes is necessary, the en routeobstacle clearance turning area extends the primaryand secondary obstacle clearance areas to accommo-date the turn radius of the aircraft. Since turns at orafter fix passage may exceed airway and routeboundaries, pilots are expected to adhere to airwayand route protected airspace by leading turns earlybefore a fix. The turn area provides obstacle clear-ance for both turn anticipation (turning prior to thefix) and flyover protection (turning after crossing thefix). This does not violate the requirement to fly thecenterline of the airway. Many factors enter into theconstruction and application of the turning area toprovide pilots with adequate obstacle clearance pro-tection. These may include aircraft speed, the amountof turn versus NAVAID distance, flight track, curveradii, MEAs, and minimum turning altitude (MTA). Atypical protected airspace is shown in figure 3-11.Turning area system accuracy factors must be appliedto the most adverse displacement of the NAVAID orfix and the airway or route boundaries at which theturn is made.

If applying nonmountainous en route turning area cri-teria graphically, depicting the vertical obstruction

clearance in a typical application, the template mightappear as in figure 3-12.

Turns that begin at or after fix passage may exceed theprotected en route turning area obstruction clearance.Figure 3-13 contains an example of a flight trackdepicting a turn at or after fix passage, together with anexample of an early turn. Without leading a turn, an air-craft operating in excess of 290 knots true airspeed(TAS) can exceed the normal airway or route bound-aries depending on the amount of course changerequired, wind direction and velocity, the character ofthe turn fix (DME, overhead navigation aid, or inter-section), and pilot technique in making a coursechange. For example, a flight operating at 17,000 feetMSL with a TAS of 400 knots, a 25° bank, and a coursechange of more than 40° would exceed the width of theairway or route; i.e., 4 NM each side of centerline. Dueto the high airspeeds used at 18,000 feet MSL andabove, additional IFR separation protection for coursechanges is provided.

NAVAID SERVICE VOLUMEEach class of VHF NAVAID has an established opera-tional service volume to ensure adequate signal coverageand frequency protection from other NAVAIDs on thesame frequency. The maximum distances vary withaltitude. When using VORs for direct route navigation,the maximum distances between NAVAIDs specifiedwith the appropriate altitudes are as follows:

Figure 3-11.Turning Area, Intersection Fix, NAVAID Distance less than 51 NM.

RadiiCenter

SecondaryArcs

PrimaryArcs

PrimaryIndexes"Outside"

"Inside"

TerminationAreas

CENTERLINE

CENTERLI

NE

En RouteFacility Facility

Providing IntersectionRadial

FixDisplacement

Area

4.5° 3.6°

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• 12,000 feet and below (T facilities) 50 NM

• Below 18,000 feet 80 NM

• 14,500 feet to 17,999 feet 200 NM

• 18,000 feet to FL 450 260 NM

• Above FL 450 200 NM

There may be a time when ATC clears pilots for a directroute that exceeds the stated distances. When that hap-pens, ATC provides radar monitoring and navigationalassistance as necessary. For more information on direct

route navigation, using RNAV procedures and tech-niques refer to the En Route RNAV Procedures sectionlater in this chapter.

NAVIGATIONAL GAPSWhere a navigational course guidance gap exists,referred to as an MEA gap, the airway or route segmentmay still be approved for navigation. The navigationalgap may not exceed a specific distance that variesdirectly with altitude, from zero NM at sea level to 65NM at 45,000 feet MSL and not more than one gap mayexist in the airspace structure for the airway or routesegment. Additionally, a gap usually does not occur atany airway or route turning point. To help ensure themaximum amount of continuous positive course guid-ance available when flying, there are established enroute criteria for both straight and turning segments.Where large gaps exist that require altitude changes,MEA “steps” may be established at increments of notless than 2,000 feet below 18,000 feet MSL, or not lessthan 4,000 feet at 18,000 MSL and above, provided thata total gap does not exist for the entire segment withinthe airspace structure. MEA steps are limited to onestep between any two facilities to eliminate continu-ous or repeated changes of altitude in problem areas.The allowable navigational gaps pilots can expect tosee are determined, in part, by reference to the graphdepicted in figure 3-14. Notice the en route chartexcerpt depicting that the MEA is established with agap in navigation signal coverage northwest of theCarbon VOR/DME on V134. At the MEA of 13,000,the allowable navigation course guidance gap isapproximately 18.5 NM, as depicted by Sample 2. Thenavigation gap area is not identified on the chart bydistances from the navigation facilities.

Secondary AreaPrimary Area

500 Feet

500 Feet

Figure 3-12.Turning Area Obstruction Clearance.

Airway Route Boundary

Airway Route Boundary

TurningFix

Early Turn

Turn at or afterFix Passage

Figure 3-13. Adhering to Airway/Route Turning Area.

3-11

CHANGEOVER POINTSWhen flying airways, pilots normally change frequen-cies midway between navigation aids, although thereare times when this is not practical. If the navigation

signals cannot be received from the second VOR at themidpoint of the route, a changeover point (COP) isdepicted and shows the distance in NM to each NAVAID,as depicted in figure 3-15. COPs indicate the point

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Sample 2: Enter with MEA of 13,000 Feet.

Read Allowable Gap 18.5 NM

ALLOWABLE NAVIGATION COURSE GUIDANCE GAP (NM)

Figure 3-14. Navigational Course Guidance Gaps.

Figure 3-15. Changeover Points.

22

45

13,000 ft

13,000V 344

3-12

where a frequency change is necessary to receivecourse guidance from the facility ahead of the aircraftinstead of the one behind. These changeover pointsdivide an airway or route segment and ensure continu-ous reception of navigation signals at the prescribedminimum en route IFR altitude. They also ensure thatother aircraft operating within the same portion of anairway or route segment receive consistent azimuth sig-nals from the same navigation facilities regardless ofthe direction of flight.

Where signal coverage from two VORs overlaps at theMEA, the changeover point normally is designated atthe midpoint. Where radio frequency interference orother navigation signal problems exist, the COP isplaced at the optimum location, taking into considera-tion the signal strength, alignment error, or any otherknown condition that affects reception. Thechangeover point has an effect on the primary and sec-ondary obstacle clearance areas. On long airway orroute segments, if the distance between two facilities isover 102 NM and the changeover point is placed atthe midpoint, the system accuracy lines extendbeyond the minimum widths of 8 and 12 NM, and aflare or spreading outward results at the COP, asshown in figure 3-16. Offset changeover points anddogleg segments on airways or routes can also resultin a flare at the COP.

IFR EN ROUTE ALTITUDESMinimum en route altitudes, minimum reception alti-tudes, maximum authorized altitudes, minimumobstruction clearance altitudes, minimum crossingaltitudes, and changeover points are established bythe FAA for instrument flight along Federal airways,as well as some off-airway routes. The altitudes areestablished after it has been determined that the nav-igation aids to be used are adequate and so orientedon the airways or routes that signal coverage isacceptable, and that flight can be maintained withinprescribed route widths.

For IFR operations, regulations require that pilots operatethe aircraft at minimum altitudes. Except when necessaryfor takeoff or landing, pilots may not operate an aircraftunder IFR below applicable minimum altitudes, or if noapplicable minimum altitude is prescribed, in the case ofoperations over an area designated as mountainous, analtitude of 2,000 feet above the highest obstacle withina horizontal distance of 4 NM from the course to beflown. In any other case, an altitude of 1,000 feet abovethe highest obstacle within a horizontal distance of 4NM from the course to be flown must be maintained asa minimum altitude. If both a MEA and a minimumobstruction clearance altitude (MOCA) are prescribedfor a particular route or route segment, pilots may oper-ate an aircraft below the MEA down to, but not below,the MOCA, only when within 22 NM of the VOR.When climbing to a higher minimum IFR altitude(MIA), pilots must begin climbing immediately afterpassing the point beyond which that minimum altitudeapplies, except when ground obstructions intervene,the point beyond which that higher minimum altitudeapplies must be crossed at or above the applicable min-imum crossing altitude (MCA) for the VOR.

If on an IFR flight plan, but cleared by ATC to maintainVFR conditions on top, pilots may not fly below mini-mum en route IFR altitudes. Minimum altitude rulesare designed to ensure safe vertical separation betweenthe aircraft and the terrain. These minimum altituderules apply to all IFR flights, whether in IFR or VFRweather conditions, and whether assigned a specificaltitude or VFR conditions on top.

MINIMUM EN ROUTE ALTITUDEThe minimum enroute altitude (MEA) is the lowestpublished altitude between radio fixes that assuresacceptable navigational signal coverage and meetsobstacle clearance requirements between those fixes.The MEA prescribed for a Federal airway or segment,RNAV low or high route, or other direct route applies

Figure 3-16. Changeover Point Effect on Long Airway or Route Segment.

4 NM

4 NM

2 NM

2 NM

6.7°

4.5°

4.5°

6.7° 70

70

Secondary Areas

Primary Area Flare

Flare

3-13

to the entire width of the airway, segment, or routebetween the radio fixes defining the airway, segment,or route. MEAs for routes wholly contained withincontrolled airspace normally provide a buffer above thefloor of controlled airspace consisting of at least 300feet within transition areas and 500 feet within controlareas. MEAs are established based upon obstacle clear-ance over terrain and manmade objects, adequacy ofnavigation facility performance, and communicationsrequirements, although adequate communication at theMEA is not guaranteed.

MINIMUM OBSTRUCTION CLEARANCE ALTITUDEThe minimum obstruction clearance altitude (MOCA)is the lowest published altitude in effect between radiofixes on VOR airways, off-airway routes, or route seg-ments that meets obstacle clearance requirements forthe entire route segment. This altitude also assuresacceptable navigational signal coverage only within 22NM of a VOR. The MOCA seen on the NACO en routechart, may have been computed by adding the requiredobstacle clearance (ROC) to the controlling obstacle inthe primary area or computed by using a TERPS chartif the controlling obstacle is located in the secondaryarea. This figure is then rounded to the nearest 100 -foot increment, i.e., 2,049 feet becomes 2,000, and2,050 feet becomes 2,100 feet. An extra 1,000 feet isadded in mountainous areas, in most cases. The MOCAis based upon obstacle clearance over the terrain orover manmade objects, adequacy of navigation facilityperformance, and communications requirements.

ATC controllers have an important role in helping pilotsremain clear of obstructions. Controllers are instructedto issue a safety alert if the aircraft is in a position that,in their judgement, places the pilot in unsafe proximityto terrain, obstructions, or other aircraft. Once pilotsinform ATC of action being taken to resolve the situa-tion, the controller may discontinue the issuance offurther alerts. A typical terrain/obstruction alert maysound like this: “Low altitude alert. Check your alti-tude immediately. The MOCA in your area is12,000.”

MINIMUM VECTORING ALTITUDESMinimum vectoring altitudes (MVAs) are establishedfor use by ATC when radar ATC is exercised. The mini-mum vectoring altitude provides 1,000 feet of clearanceabove the highest obstacle in nonmountainous areas and2,000 feet above the highest obstacle in designatedmountainous areas. Because of the ability to isolate spe-cific obstacles, some MVAs may be lower than MEAs,MOCAs, or other minimum altitudes depicted on chartsfor a given location. While being radar vectored, IFRaltitude assignments by ATC are normally at or abovethe MVA.

Controllers use MVAs only when they are assured anadequate radar return is being received from the air-craft. Charts depicting minimum vectoring altitudesare normally available to controllers but not availableto pilots. Situational awareness is always important,especially when being radar vectored during a climbinto an area with progressively higher MVA sectors,similar to the concept of minimum crossing altitude.Except where diverse vector areas have been estab-lished, when climbing, pilots should not be vectoredinto a sector with a higher MVA unless at or above thenext sector’s MVA. Where lower MVAs are requiredin designated mountainous areas to achieve compati-bility with terminal routes or to permit vectoring to aninstrument approach procedure, 1,000 feet of obstacleclearance may be authorized with the use of AirportSurveillance Radar (ASR). The MVA will provide atleast 300 feet above the floor of controlled airspace.Minimum vectoring altitude charts are developed tothe maximum radar range. Sectors provide separationfrom terrain and obstructions. Each MVA chart hassectors large enough to accommodate vectoring ofaircraft within the sector at the MVA. [Figure 3-17]

MINIMUM RECEPTION ALTITUDEMinimum reception altitudes (MRAs) are deter-mined by FAA flight inspection traversing an entireroute of flight to establish the minimum altitude thenavigation signal can be received for the route andfor off-course NAVAID facilities that determine a fix.When the MRA at the fix is higher than the MEA, anMRA is established for the fix, and is the lowest alti-tude at which an intersection can be determined.

MINIMUM CROSSING ALTITUDEA minimum crossing altitude (MCA) is the lowest alti-tude at certain fixes at which the aircraft must cross whenproceeding in the direction of a higher minimum en routeIFR altitude, as depicted in figure 3-18. MCAs are estab-lished in all cases where obstacles intervene to preventpilots from maintaining obstacle clearance during a nor-mal climb to a higher MEA after passing a point beyondwhich the higher MEA applies. The same protected enroute area vertical obstacle clearance requirements forthe primary and secondary areas are considered in thedetermination of the MCA. The standard for determiningthe MCA is based upon the following climb gradients,and is computed from the flight altitude:

• Sea level through 5,000 feet MSL — 150 feet perNM

• 5000 feet through 10,000 feet MSL — 120 feetper NM

• 10,000 feet MSL and over — 100 feet per NM

To determine the MCA seen on a NACO en route chart,the distance from the obstacle to the fix is computed

3-14

Figure 3-17. Example of an Air Route Traffic Control Center MVA Chart.

10,000

RIW15,800

14,50012,000

13,70010,700

11,000

12,40012,000

15,500

14,200

11,00013,800

13,300

12,300

CKW

VEL

FBR

RKS

RK

S 325

RK

S 0

03

070

065

243 142RKS 269

211

RKS 080

270

00

1

065

110

18

0R

KS

16

0

20

40

60

80

100

120

14,500

X

V 3615900

V 3615200

V 361 5900 ESQWID

Figure 3-18. Minimum Crossing Altitude.

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from the point where the centerline of the en routecourse in the direction of flight intersects the farthestdisplacement from the fix, as shown in figure 3-19.When a change of altitude is involved with a coursechange, course guidance must be provided if thechange of altitude is more than 1,500 feet and/or if thecourse change is more than 45°, although there is anexception to this rule. In some cases, course changes ofup to 90° may be approved without course guidance pro-vided that no obstacles penetrate the established MEArequirement of the previous airway or route segment.

Outside U. S. airspace, pilots may encounter differentflight procedures regarding minimum crossing altitudeand transitioning from one MEA to a higher MEA. Inthis case, pilots are expected to be at the higher MEAcrossing the fix, similar to an MCA. Pilots must thor-oughly review flight procedure differences when flyingoutside U.S. airspace. On NACO en route charts, routesand associated data outside the conterminous U.S. areshown for transitional purposes only and are not part ofthe high altitude jet route and RNAV route systems.[Figure 3-20]

Figure 3-19. MCA Determination Point.

3200'

700'

2000'

2000'

6 NM

4620' MSL120' per NM Required

Multiply by 6 NM -720 Ft.

Maximum Displacement

MSL

Obstacle Line

Obstruction Height 4620' Required Clearance +2000' MOCA At Obstruction = 6620' Climb Value* - 720'MCA Required = 5900' * Based upon 6 NM @ 120 Feet Per NM

X

MEA 5200'

MCA 5900' E

Figure 3-20. Crossing a Fix to a Higher MEA in Canada.

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MAXIMUM AUTHORIZED ALTITUDEA maximum authorized altitude (MAA) is a pub-lished altitude representing the maximum usable alti-tude or flight level for an airspace structure or routesegment. It is the highest altitude on a Federal airway,jet route, RNAV low or high route, or other direct routefor which an MEA is designated at which adequatereception of navigation signals is assured. MAAs rep-resent procedural limits determined by technical limi-tations or other factors such as limited airspace orfrequency interference of ground based facilities.

IFR CRUISING ALTITUDE OR FLIGHT LEVELIn controlled airspace, pilots must maintain the altitudeor flight level assigned by ATC, although if the ATCclearance assigns “VFR conditions on-top,” an altitudeor flight level as prescribed by Part 91.159 must bemaintained. In uncontrolled airspace (except while in aholding pattern of 2 minutes or less or while turning) ifoperating an aircraft under IFR in level cruising flight,an appropriate altitude as depicted in the legend ofNACO IFR en route high and low altitude charts mustbe maintained. [Figure 3-21]

When operating on an IFR flight plan below 18,000feet MSL in accordance with a VFR-on-top clearance,any VFR cruising altitude appropriate to the directionof flight between the MEA and 18,000 feet MSL maybe selected that allows the flight toremain in VFR conditions. Any changein altitude must be reported to ATC andpilots must comply with all other IFRreporting procedures. VFR-on-top isnot authorized in Class A airspace.When cruising below 18,000 feet MSL,the altimeter must be adjusted to thecurrent setting, as reported by a stationwithin 100 NM of your position. Inareas where weather reporting stationsare more than 100 NM from the route,the altimeter setting of a station that isclosest may be used. During IFRflight, ATC advises flights periodi-cally of the current altimeter setting,but it remains the responsibility of thepilot or flight crew to update altimetersettings in a timely manner. Altimetersettings and weather information areavailable from weather reportingfacilities operated or approved by theU.S. National Weather Service, or asource approved by the FAA. Somecommercial operators have the author-ity to act as a government approvedsource of weather information, includingaltimeter settings, through certificationunder the FAA’s Enhanced WeatherInformation System.

Flight level operations at or above 18,000 feet MSLrequire the altimeter to be set to 29.92. A flight level (FL)is defined as a level of constant atmospheric pressurerelated to a reference datum of 29.92 in. Hg. Each flightlevel is stated in three digits that represents hundreds offeet. For example, FL 250 represents an altimeter indica-tion of 25,000 feet. Conflicts with traffic operating below18,000 feet MSL may arise when actual altimeter set-tings along the route of flight are lower than 29.92.Therefore, Part 91.121 specifies the lowest usable flightlevels for a given altimeter setting range.

LOWEST USABLE FLIGHT LEVELWhen the barometric pressure is 31.00 inches of mer-cury or less and pilots are flying below 18,000 feetMSL, use the current reported altimeter setting. This isimportant because the true altitude of an aircraft islower than indicated when sea level pressure is lowerthan standard. When an aircraft is en route on an instru-ment flight plan, air traffic controllers furnish thisinformation at least once while the aircraft is in the con-troller’s area of jurisdiction. According to Part 91.144,when the barometric pressure exceeds 31.00 inchesHg., the following procedures are placed in effect byNOTAM defining the geographic area affected: Set31.00 inches for en route operations below 18,000 feetMSL and maintain this setting until beyond the affectedarea. Air traffic control issues actual altimeter settings

Figure 3-21. IFR Cruising Altitude or Flight Level.

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and advises pilots to set 31.00 inches in their altimeter,for en route operations below 18,000 feet MSL inaffected areas. If an aircraft has the capability of settingthe current altimeter setting and operating into airportswith the capability of measuring the current altimetersetting, no additional restrictions apply. At or above18,000 feet MSL, altimeters should be set to 29.92inches of mercury (standard setting). Additional proce-dures exist beyond the en route phase of flight.

The lowest usable flight level is determined by theatmospheric pressure in the area of operation. As localaltimeter settings fall below 29.92, pilots operating inClass A airspace must cruise at progressively higherindicated altitudes to ensure separation from aircraftoperating in the low altitude structure as follows:

Current Altimeter Setting Lowest Usable Flight Level

• 29.92 or higher 180

• 29.91 to 29.42 185

• 29.41 to 28.92 190

• 28.91 to 28.42 195

• 28.41 to 27.92 200

When the minimum altitude, as prescribed in Parts91.159 and 91.177, is above 18,000 feet MSL, the low-est usable flight level is the flight level equivalent ofthe minimum altitude plus the number of feet specifiedaccording to the lowest flight level correction factor asfollows:

Altimeter Setting Correction Factor

• 29.92 or higher none

• 29.91 to 29.42 500 Feet

• 29.41 to 28.92 1000 Feet

• 28.91 to 28.42 1500 Feet

• 28.41 to 27.92 2000 Feet

• 27.91 to 27.42 2500 Feet

OPERATIONS IN OTHER COUNTRIESWhen flight crews transition from the U.S. NAS toanother country’s airspace, they should be aware ofdifferences not only in procedures but also airspace.For example, when flying into Canada regardingaltimeter setting changes, as depicted in figure 3-22,notice the change from QNE to QNH when flyingnorthbound into the Moncton flight information

region (FIR), an airspace of defined dimensions whereflight information service and alerting service are pro-vided. Transition altitude (QNH) is the altitude in thevicinity of an airport at or below which the verticalposition of the aircraft is controlled by reference to alti-tudes (MSL). The transition level (QNE) is the lowestflight level available for use above the transition alti-tude. Transition height (QFE) is the height in thevicinity of an airport at or below which the verticalposition of the aircraft is expressed in height above theairport reference datum. The transition layer is the air-space between the transition altitude and the transitionlevel. If descending through the transition layer, set thealtimeter to local station pressure. When departing andclimbing through the transition layer, use the standardaltimeter setting (QNE) of 29.92 inches of Mercury,1013.2 millibars, or 1013.2 hectopascals. Rememberthat most pressure altimeter errors are subject tomechanical, elastic, temperature, and installationerrors. Extreme cold temperature differences also mayrequire a correction factor.

REPORTING PROCEDURESIn addition to acknowledging a handoff to anotherCenter en route controller, there are reports that shouldbe made without a specific request from ATC. Certainreports should be made at all times regardless ofwhether a flight is in radar contact with ATC, whileothers are necessary only if radar contact has been lostor terminated. Refer to figure 3-23 for a review ofthese reports.

NONRADAR POSITION REPORTSIf radar contact has been lost or radar service ter-minated, the CFRs require pilots to provide ATCwith position reports over designated VORs andintersections along their route of flight. Thesecompulsory reporting points are depicted onNACO IFR en route charts by solid triangles.Position reports over fixes indicated by open trian-gles are noncompulsory reporting points, and areonly necessary when requested by ATC. If on adirect course that is not on an established airway,report over the fixes used in the flight plan thatdefine the route, since they automatically becomecompulsory reporting points. Compulsory report-ing points also apply when conducting an IFRflight in accordance with a VFR-on-top clearance.Whether a route is on airways or direct, positionreports are mandatory in a nonradar environment,and they must include specific information. A typicalposition report includes information pertaining to air-craft position, expected route, and estimated timeof arrival (ETA). Time may be stated in minutesonly when no misunderstanding is likely to occur.[Figure 3-24]

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COMMUNICATION FAILURETwo-way radio communication failure procedures forIFR operations are outlined in Part 91.185. Unless other-wise authorized by ATC, pilots operating under IFR areexpected to comply with this regulation. Expanded pro-cedures for communication failures are found in theAIM. Pilots can use the transponder to alert ATC to aradio communication failure by squawking code 7600.[Figure 3-25] If only the transmitter is inoperative, listenfor ATC instructions on any operational receiver, includ-ing the navigation receivers. It is possible ATC may tryto make contact with pilots over a VOR, VORTAC,NDB, or localizer frequency. In addition to moni-toring NAVAID receivers, attempt to reestablishcommunications by contacting ATC on a previouslyassigned frequency, calling a FSS or AeronauticalRadio Incorporated (ARINC).

The primary objective of the regulations governingcommunication failures is to preclude extended IFRoperations within the ATC system since these operations

may adversely affect other users of the airspace. If theradio fails while operating on an IFR clearance, butin VFR conditions, or if encountering VFR condi-tions at any time after the failure, continue the flightunder VFR conditions, if possible, and land as soonas practicable. The requirement to land as soon aspracticable should not be construed to mean as soonas possible. Pilots retain the prerogative of exercis-ing their best judgment and are not required to land atan unauthorized airport, at an airport unsuitable for thetype of aircraft flown, or to land only minutes short oftheir intended destination. However, if IFR conditionsprevail, pilots must comply with procedures designatedin the CFRs to ensure aircraft separation.

If pilots must continue their flight under IFR afterexperiencing two-way radio communication failure,they should fly one of the following routes:

• The route assigned by ATC in the last clearancereceived.

Figure 3-22. Altimeter Setting Changes.

3-19

• If being radar vectored, the direct route from thepoint of radio failure to the fix, route, or airwayspecified in the radar vector clearance.

• In the absence of an assigned route, the route ATChas advised to expect in a further clearance.

• In the absence of an assigned or expected route,the route filed in the flight plan.

It is also important to fly a specific altitude shouldtwo-way radio communications be lost. The altitudeto fly after a communication failure can be found inPart 91.185 and must be the highest of the followingaltitudes for each route segment flown.

• The altitude or flight level assigned in the lastATC clearance.

• The minimum altitude or flight level for IFRoperations.

• The altitude or flight level ATC has advised toexpect in a further clearance.

In some cases, the assigned or expected altitude maynot be as high as the MEA on the next route segment.In this situation, pilots normally begin a climb to thehigher MEA when they reach the fix where the MEArises. If the fix also has a published minimum cross-ing altitude, they start the climb so they will be at orabove the MCA when reaching the fix. If the nextsucceeding route segment has a lower MEA, descendto the applicable altitude either the last assignedaltitude or the altitude expected in a further clearance when reaching the fix where the MEA decreases.

Figure 3-23. ATC Reporting Procedure Examples.

Leaving one assigned flight altitude or flight level for another

VFR-on-top change in altitude

Leaving any assigned holding fix or point

Missed approach

Unable to climb or descend at least 500 feet per minute

TAS variation from filed speed of 5% or 10 knots, whicheveris greater

Time and altitude or flight level upon reaching a holding fixor clearance limit

Loss of nav/comm capability (required by Part 91.187)

Unforecast weather conditions or other information relatingto the safety of flight (required by Part 91.183)

"Marathon 564, leaving 8,000, climb to 10,000."

"Marathon 564, VFR-on-top, climbing to 10,500."

"Marathon 564, leaving FARGO Intersection."

"Marathon 564, missed approach, request clearance to Chicago."

"Marathon 564, maximum climb rate 400 feet per minute."

"Marathon 564, advises TAS decrease to140 knots."

"Marathon 564, FARGO Intersection at 05, 10,000, holding east."

"Marathon 564, ILS receiver inoperative."

"Marathon 564, experiencing moderate turbulence at 10,000."

Leaving FAF or OM inbound on final approach

Revised ETA of more than three minutes

Position reporting at compulsory reporting points (requiredby Part 91.183)

"Marathon 564, outer marker inbound, leaving 2,000."

"Marathon 564, revising SCURRY estimate to 55."

See figure 3-24 for position report items.

RADAR/NONRADAR REPORTS

These reports should be made at all times without a specific ATC request.

NONRADAR REPORTS

When you are not in radar contact, these reports should be made without a specific request from ATC.

REPORTS EXAMPLE:

REPORTS EXAMPLE:

3-20

CLIMBING AND DESCENDING EN ROUTEBefore the days of nationwide radar coverage, en routeaircraft were separated from each other primarily byspecific altitude assignments and position reportingprocedures. Much of the pilot’s time was devoted toinflight calculations, revising ETAs, and relaying

position reports to ATC. Today, pilots and air trafficcontrollers have far more information and better toolsto make inflight computations and, with the expan-sion of radar, including the use of an en route flightprogress strip shown in figure 3-26, position reportsmay only be necessary as a backup in case of radarfailure or for RNAV random route navigation. Figure3-26 also depicts the numerous en route data entriesused on a flight progress strip, generated by theARTCC computer. Climbing, level flight, anddescending during the en route phase of IFR flightinvolves staying in communication with ATC, mak-ing necessary reports, responding to clearances,

monitoring position, and staying abreast of anychanges to the airplane’s equipment status or weather.

PILOT/CONTROLLER EXPECTATIONSWhen ATC issues a clearance or instruction, pilots areexpected to execute its provisions upon receipt. Insome cases, ATC includes words that modify theirexpectation. For example, the word “immediately” in aclearance or instruction is used to impress urgency toavoid an imminent situation, and expeditious compli-ance is expected and necessary for safety. The additionof a climb point or time restriction, for example, doesnot authorize pilots to deviate from the route of flightor any other provision of the ATC clearance. If you

Identification

Position

Time

Altitude/Flight Level

IFR or VFR (in a report to an FSS only)

ETA over the next reporting fix

Following reporting point

Pertinent remarks

"Marathon 564,

Sidney

15, (minutes after the hour)

9,000,

IFR,

Akron 35, (minutes after the hour)

Thurman next."

(If necessary)

Figure 3-24. Nonradar Position Reports.

Figure 3-25.Two-Way Radio Communication FailureTransponder Code.

When an aircraft squawks code 7600 during a two-way radio communication failure, the information block on the radar screen flashes RDOF (radio failure) to alert the controller.

3-21

receive a term “climb at pilot’s discretion” in the alti-tude information of an ATC clearance, it means thatyou have the option to start a climb when you wish,that you are authorized to climb at any rate, and to tem-porarily level off at any intermediate altitude asdesired, although once you vacate an altitude, you may

not return to that altitude. When ATC has not used theterm “at pilot’s discretion” nor imposed any climbrestrictions, you should climb promptly on acknowl-edgment of the clearance. Climb at an optimum rateconsistent with the operating characteristics of youraircraft to 1,000 feet below the assigned altitude, and

Figure 3-26. En Route Flight Progress Strip and Data Entries.

3-22

then attempt to climb at a rate of between 500 and1,500 feet per minute until you reach your assignedaltitude. If at anytime you are unable to climb at a rateof at least 500 feet a minute, advise ATC. If it is neces-sary to level off at an intermediate altitude duringclimb, advise ATC.

“Expedite climb” normally indicates you should usethe approximate best rate of climb without an excep-tional change in aircraft handling characteristics.Normally controllers will inform you of the reason foran instruction to expedite. If you fly a turbojet airplaneequipped with afterburner engines, such as a militaryaircraft, you should advise ATC prior to takeoff if youintend to use afterburning during your climb to the enroute altitude. Often, the controller may be able to plantraffic to accommodate a high performance climb andallow you to climb to the planned altitude withoutrestriction. If you receive an “expedite” clearance fromATC, and your altitude to maintain is subsequentlychanged or restated without an expedite instruction, theexpedite instruction is canceled.

During en route climb, as in any other phase offlight, it is essential that you clearly communicatewith ATC regarding clearances. In the followingexample, a flight crew experienced an apparentclearance readback/hearback error, that resulted inconfusion about the clearance, and ultimately, toinadequate separation from another aircraft.“Departing IFR, clearance was to maintain 5,000feet, expect 12,000 in ten minutes. After handoff toCenter, we understood and read back, ‘Leaving5,000 turn left heading 240° for vector on course.’The First Officer turned to the assigned headingclimbing through 5,000 feet. At 5,300 feet Centeradvised assigned altitude was 5,000 feet. We imme-diately descended to 5,000. Center then informed uswe had traffic at 12 o’clock and a mile at 6,000.After passing traffic, a higher altitude was assignedand climb resumed. We now believe the clearancewas probably ‘reaching’ 5,000, etc. Even our read-back to the controller with ‘leaving’ didn’t catch thedifferent wording.” “Reaching” and “leaving” arecommonly used ATC terms having different usages.They may be used in clearances involving climbs,descents, turns, or speed changes. In the cockpit, thewords “reaching” and “leaving” sound much alike.

For altitude awareness during climb, professional pilotsoften call out altitudes on the flight deck. The pilot notflying may call 2,000 and 1,000 feet prior to reaching anassigned altitude. The callout may be, “two to go” and

“one to go.” Climbing through the transition altitude(QNH), both pilots set their altimeters to 29.92 inchesof mercury and announce “2992 inches” (or ‘standard,’on some airplanes) and the flight level passing. Forexample, “2992 inches” (‘standard’), flight level oneeight zero.” The second officer on three pilot crewsmay ensure that both pilots have inserted the properaltimeter setting. On international flights, pilots mustbe prepared to differentiate, if necessary, between baro-metric pressure equivalents with inches of mercury,and millibars or hectopascals, to eliminate any poten-tial for error, for example, 996 millibars erroneouslybeing set as 2996.

For a typical IFR flight, the majority of inflight timeoften is flown in level flight at cruising altitude, fromtop of climb to top of descent (TOD). Generally, TODis used in airplanes with a flight management system(FMS), and represents the point at which descent is firstinitiated from cruise altitude. FMSs also assist in levelflight by cruising at the most fuel saving speed, provid-ing continuing guidance along the flight plan route,including great circle direct routes, and continuousevaluation and prediction of fuel consumption alongwith changing clearance data.

ATC will issue a descent clearance so that an aircraftwill arrive in approach control airspace at an appropri-ate minimum altitude. There are two basic descentclearances that ATC issues. ATC may ask pilots todescend to and maintain a specific altitude. Generally,this clearance is for en route traffic separation pur-poses, and pilots need to respond to it promptly.Descend at the optimum rate for the specific aircraft,until 1,000 feet above the assigned altitude. The last1,000 feet of descent should be made at a rate of 500 to1,500 feet per minute. The second type of descentclearance allows pilots to descend “…at pilot’s discre-tion.” When ATC issues a clearance at pilot’s discre-tion, pilots may begin the descent whenever theychoose. They also are authorized to level off, temporar-ily, at any intermediate altitude during the descent,although once they leave an altitude, they may notreturn to it.

A descent clearance also may include a segment wherethe descent is at your discretion, such as “ …cross theJoliet VOR at or above 12,000, descend and maintain5,000.” This clearance authorizes descent from theassigned altitude whenever the pilot and flight crewchooses, so long as they cross the Joliet VOR at orabove 12,000 feet MSL. After that, they shoulddescend at a normal rate until they reach the assigned

3-23

altitude of 5,000 feet MSL. A descent clearance whichspecifies a crossing altitude allows descent at your dis-cretion, but only for that flight segment to which thealtitude restriction applies. [Figure 3-27]

Clearances to descend at pilot’s discre-tion are not just an option for ATC. Pilotsalso may request this type of clearanceso that they can operate more efficiently.If you are enroute above anovercast layer,you may ask for adescent at yourdiscretion, whichallows you toremain above the clouds for as long aspossible. Pilots may find this particularlyimportant, for example, if the outside airtemperature is conducive to icing condi-tions. The request permits you to stay atyour cruising altitude longer, in order toconserve fuel and avoid prolonged peri-ods of IFR flight in icing conditions. Thistype of descent also minimizes yourexposure to turbulence, by allowing youto level off at an altitude where the air issmoother.

AIRCRAFT SPEED AND ALTITUDEDuring the en route descent phase offlight, an additional benefit of flightmanagement systems is that the FMSprovides fuel saving idle thrust descentto your destination airport. This allowsan uninterrupted profile descent fromlevel cruising altitude to an appro-priate minimum IFR altitude (MIA),except where level flight is requiredfor speed adjustment. Controllers antic-ipate and plan that you may level off at10,000 feet MSL on descent to complywith the Part 91 indicated airspeed limitof 250 knots. Leveling off at any othertime on descent may seriously affect airtraffic handling by ATC. It is imperative that you makeevery effort to fulfill ATC expected actions on descentto aid in safely handling and expediting air traffic.

ATC issues speed adjustments if you are being radarcontrolled to achieve or maintain required or desiredspacing. They express speed adjustments in terms ofknots based on indicated airspeed in 10 knot incrementsexcept that at or above FL 240 speeds may be expressedin terms of Mach numbers in 0.01 increments. The useof Mach numbers is restricted to turbojet airplanes withMach meters. If complying with speed adjustments,

pilots are expected to maintain that speed within plus orminus 10 knots or 0.02 Mach.

Speed and altitude restrictions in clearances are sub-ject to misinterpretation, as evidenced in this case

where instructions in apublished procedure weretreated as a clearance by acorporate flight crew.“…We were at FL 310 andhad already programmedthe ‘expect-crossing alti-tude’ of 17,000 feet at theVOR. When the altitudealerter sounded, I advisedCenter that we were leav-ing FL 310. ATC acknowl-edged with a ‘Roger.’ AtFL 270, Center quizzed usabout our descent. I toldthe controller we weredescending so as to crossthe VOR at 17,000 feet.ATC advised us that wedid not have clearance to

descend. What we thoughtwas a clearance was in factan ‘expect’ clearance. Weare both experiencedpilots…which just meansthat experience is no substi-tute for a direct question toCenter when you are indoubt about a clearance.Also, the term ‘Roger’ onlymeans that ATC receivedthe transmission, not that

they understood the transmission. The AIM indicatesthat ‘expect’ altitudes are published for planning pur-poses. ‘Expect’ altitudes are not considered crossingrestrictions until verbally issued by ATC.”

HOLDING PROCEDURESThe criteria for holding pattern airspace is developedboth to provide separation of aircraft, as well as obstacleclearance The alignment of holding patterns typicallycoincides with the flight course you fly after leaving theholding fix. For level holding, a minimum of 1,000 feetobstacle clearance is provided throughout the primary

You are southbound on V333 at 13,000 feet MSL over theFIBKE Intersection, and ATC issues the following clearance:

"Piper 1030P descend now to 11,000, cross JELLO at orabove 7,000 feet, descend and maintain 5,000."

Descend promptly to 11,000 feet MSL uponreceiving the clearance. Then, at your discretion,descend to cross JELLO Intersection at or above7,000 feet MSL. Once you reach JELLO, you must descend to 5,000 feet MSL.

Figure 3-27. Descending from the En RouteSegment.

3-24

area. In the secondary area 500 feet of obstacle clearanceis provided at the inner edge, tapering to zero feet at theouter edge. Allowance for precipitous terrain is consid-ered, and the altitudes selected for obstacle clearancemay be rounded to the nearest 100 feet. When criteria fora climb in hold is applied, no obstacle penetrates theholding surface. [Figure 3-28]

There are many factors that affect aircraft during hold-ing maneuvers, including navigational aid ground andairborne tolerance, effect of wind, flight procedures,application of air traffic control, outbound leg length,maximum holding airspeeds, fix to NAVAID distance,DME slant range effect, holding airspace size, andaltitude holding levels. In order to allow for these fac-tors when establishing holding patterns, procedurespecialists must apply complex criteria contained inOrder 7130.3, Holding Pattern Criteria.

ATC HOLDING INSTRUCTIONSWhen controllers anticipate a delay at a clearance limitor fix, pilots will usually be issued a holding clearanceat least five minutes before the ETA at the clearancelimit or fix. If the holding pattern assigned by ATC isdepicted on the appropriate aeronautical chart, pilotsare expected to hold as published. In this situation, thecontroller will issue a holding clearance which includes

the name of the fix, directs you to hold as published,and includes an expect further clearance (EFC) time.An example of such a clearance is: “Marathon fivesixty four, hold east of MIKEY Intersection as pub-lished, expect further clearance at 1521.” When ATCissues a clearance requiring you to hold at a fix where aholding pattern is not charted, you will be issued com-plete holding instructions. This information includesthe direction from the fix, name of the fix, course, leglength, if appropriate, direction of turns (if left turnsare required), and the EFC time. You are required tomaintain your last assigned altitude unless a new alti-tude is specifically included in the holding clearance,and you should fly right turns unless left turns areassigned. Note that all holding instructions shouldinclude an EFC time. If you lose two-way radio com-munication, the EFC allows you to depart the holdingfix at a definite time. Plan the last lap of your holdingpattern to leave the fix as close as possible to the exacttime. [Figure 3-29]

If you are approaching your clearance limit and havenot received holding instructions from ATC, you areexpected to follow certain procedures. First, call ATCand request further clearance before you reach the fix.If you cannot obtain further clearance, you are expectedto hold at the fix in compliance with the published

Figure 3-28.Typical Holding Pattern Design Criteria Template.

Fix Displacement Area

Facility

Facility

Secondary Area

Primary Area Holding Pattern Airspace Area

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holding pattern. If a holding pattern is not charted atthe fix, you are expected to hold on the inbound courseusing right turns. This procedure ensures that ATC willprovide adequate separation. [Figure 3-30] Assume youare eastbound on V214 and the Cherrelyn VORTAC isyour clearance limit. If you have not been able to obtainfurther clearance and have not received holdinginstructions, you should plan to hold southwest on the221° radial using left-hand turns, as depicted. If thisholding pattern was not charted, you would hold westof the VOR on V214 using right-hand turns.

Where required for aircraft separation, ATC mayrequest that you hold at any designated reporting pointin a standard holding pattern at the MEA or the MRA,whichever altitude is the higher at locations where aminimum holding altitude has not been established.Unplanned holding at en route fixes may be expectedon airway or route radials, bearings, or courses. If thefix is a facility, unplanned holding could be on anyradial or bearing. There may be holding limitationsrequired if standard holding cannot be accomplished atthe MEA or MRA.

MAXIMUM HOLDING SPEEDAs you have seen, the size of the holdingpattern is directly proportional to thespeed of the airplane. In order to limit theamount of airspace that must be protectedby ATC, maximum holding speeds inknots indicated airspeed (KIAS) havebeen designated for specific altituderanges [Figure 3-31]. Even so, some hold-ing patterns may have additional speedrestrictions to keep faster airplanes fromflying out of the protected area. If a hold-ing pattern has a nonstandard speedrestriction, it will be depicted by an iconwith the limiting airspeed. If the holding

Figure 3-29. ATC Holding Instructions.

A clearance for an uncharted holding pattern contains additional information:

There are at least three items in aclearance for a charted holding pattern:

• Direction to hold from the holding fix

• Holding fix

• Expect further clearance time

"...Hold southeast

of PINNE Intersection as published.

Expect further clearance at 1645."

• Direction to hold from holding fix

• Holding fix

• The holding course (a specified radial, magnetic bearing, airway or route number)

• The outbound leg length in minutes or nautical miles when DME is used

• Nonstandard pattern, if used

• Expect further clearance time

"...Hold west

of Horst Intersection

on Victor 8

5 mile legs

left turns

expect further clearance at 1430."

CHERRELYND

( H )117.2 CHL

V 2 1 4

331°

269°

221°

126°

Figure 3-30. Clearance Limit Holding.

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speed limit is less than you feel is necessary, youshould advise ATC of your revised holding speed. Also,if your indicated airspeed exceeds the applicable maxi-mum holding speed, ATC expects you to slow to thespeed limit within three minutes of your ETA at theholding fix. Often pilots can avoid flying a holding pat-tern, or reduce the length of time spent in the holdingpattern, by slowing down on the way to the holding fix.

HIGH PERFORMANCE HOLDINGCertain limitations come into play when you operateat higher speeds; for instance, aircraft do not makestandard rate turns in holding patterns if the bank anglewill exceed 30°. If your aircraft is using a flight direc-tor system, the bank angle is limited to 25°. Since anyaircraft must be traveling at over 210 knots TAS for thebank angle in a standard rate turn to exceed 30°, thislimit applies to relatively fast airplanes. An aircraftusing a flight director would have to be holding at morethan 170 knots TAS to come up against the 25° limit.These true airspeeds correspond to indicated airspeedsof about 183 and 156 knots, respectively, at 6,000 feetin a standard atmosphere. Since some military air-planes need to hold at higher speeds than the civilianlimits, the maximum at military airfields is higher. Forexample, the maximum holding airspeed at USAF air-fields is 310 KIAS. [Figure 3-32]

FUEL STATE AWARENESSIn order to increase fuel state awareness, commercialoperators and other professional flight crews are

required to record the timeand fuel remaining duringIFR flight. For example, on aflight scheduled for one houror less, the flight crew mayrecord the time and fuelremaining at the top of climb(TOC) and at one additionalwaypoint listed in the flightplan. Generally, TOC is usedin airplanes with a flightmanagement system, andrepresents the point at whichcruise altitude is first reached.TOC is calculated based oncurrent airplane altitude,climb speed, and cruise alti-tude. The captain may elect todelete the additional waypointrecording requirement if theflight is so short that therecord will not assist in themanagement of the flight. Forflights scheduled for morethan one hour, the flight crewmay record the time and fuelremaining shortly after the

top of climb and at selected waypoints listed in theflight plan, conveniently spaced approximately onehour apart. The flight crew compares actual fuel burnto planned fuel burn. Each fuel tank must be moni-tored to verify proper burnoff and appropriate fuelremaining. On two pilot airplanes, the pilot not flying(PNF) keeps the flight plan record. On three pilot air-planes, the second officer and PNF coordinate record-ing and keeping the flight plan record. In all cases, thepilot making the recording communicates the infor-mation to the pilot flying.

DIVERSION PROCEDURESOperations Specifications (OpsSpecs) for commercialoperators include provisions for en route emergency

Figure 3-31. Maximum Holding Speed Examples.

Maximum Holding Airspeed: 200 KIAS

14,000'MSL

6,000'MSL

Maximum Holding Airspeed: 265 KIAS

Maximum Holding Airspeed: 230 KIAS

MinimumHoldingAltitude(MHA)

6,001'MSL

14,001'MSL

Figure 3-32. High Performance Holding.

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diversion airport requirements. Operators are expectedto develop a sufficient set of emergency diversionairports, such that one or more can be reasonablyexpected to be available in varying weather condi-tions. The flight must be able to make a safe landing,and the airplane maneuvered off of the runway at theselected diversion airport. In the event of a disabledairplane following landing, the capability to move thedisabled airplane must exist so as not to block theoperation of any recovery airplane. In addition, thoseairports designated for use must be capable of protect-ing the safety of all personnel by being able to:

• Offload the passengers and flight crew in a safemanner during possible adverse weather condi-tions.

• Provide for the physiological needs of the pas-sengers and flight crew for the duration until safeevacuation.

• Be able to safely extract passengers and flightcrew as soon as possible. Execution and comple-tion of the recovery is expected within 12 to 48hours following diversion.

Part 91 operators also need to be prepared for a diver-sion. Designation of an alternate in the IFR flightplan is a good first step; although, changing weatherconditions or equipment issues may require pilots toconsider other options.

EN ROUTE RNAV PROCEDURESRNAV is a method of navigation that permits aircraftoperations on any desired course within the coverageof station-referenced signals, or within the limits ofself-contained system capability. The continued growthin aviation creates increasing demands on airspacecapacity and emphasizes the need for optimum utiliza-tion of available airspace. These factors, allied with therequirement for NAS operationalefficiency, along with the enhancedaccuracy of current navigation sys-tems, resulted in the required navi-gation performance (RNP) concept.RNAV is the primary means ofmeeting RNP requirements.

OFF AIRWAY ROUTESPart 95 prescribes altitudes govern-ing the operation of your aircraftunder IFR on Federal airways, jetroutes, RNAV low or high altituderoutes, and other direct routes forwhich an MEA is designated in this

regulation. In addition, it designates mountainousareas and changeover points. Off-airway routes areestablished in the same manner, and in accordancewith the same criteria as airways and jet routes. If youfly for a scheduled air carrier or operator for compen-sation or hire, any requests for the establishment ofoff-airway routes are initiated by your companythrough your principal operations inspector (POI)who works directly with your company and coordinatesFAA approval. Air carrier authorized routes are con-tained in the company’s OpsSpecs under the auspicesof the air carrier operating certificate. [Figure 3-33]

Off-airway routes predicated on public navigation facil-ities and wholly contained within controlled airspaceare published as direct Part 95 routes. Off-airway routespredicated on privately owned navigation facilities ornot contained wholly within controlled airspace arepublished as off-airway non-Part 95 routes. In evaluat-ing the adequacy of off-airway routes, the followingitems are considered; the type of aircraft and naviga-tion systems used; proximity to military bases, trainingareas, low level military routes; and the adequacy ofcommunications along the route. If you are a commer-cial operator, and you plan to fly off-airway routes,your OpsSpecs will likely address en route limitationsand provisions regarding en route authorizations to usethe global positioning system (GPS) or other RNAVsystems in the NAS. Your POI must ensure that yourlong range navigation program incorporates therequired practices and procedures. These proceduresmust be in your manuals and in checklists, as appro-priate. Training on the use of long range navigationequipment and procedures must be included in your train-ing curriculums, and your minimum equipment lists(MELs) and maintenance programs must address thelong range navigation equipment. Examples of otherselected areas requiring specialized en route authoriza-tion include the following:

Note 3 - Only B-747 and DC-10 operations authorized in these areas.

AUTHORIZED AREAS OF EN ROUTE OPERATION

LIMITATIONS, PROVISIONS, AND REFERENCE PARAGRAPHS

The 48 contiguous United States and the District of Columbia

Note 1

Canada, excluding Canadian MNPS airspace and the areas of magnetic unreliability as established in the Canadian AIP

Note 3

SPECIAL REQUIREMENTS:

Note 1 - B-737 Class II navigation operations with a single long-range system is authorized only within this area of en route operation.

Figure 3-33. Excerpt of Authorized Areas of En Route Operation.

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• Class I navigation in the U.S. Class A airspaceusing area or long range navigation systems.

• Class II navigation using multiple long rangenavigation systems.

• Operations in central East Pacific airspace.

• North Pacific operations.

• Operations within North Atlantic (NAT) mini-mum navigation performance specifications(MNPS) airspace.

• Operations in areas of magnetic unreliability.

• North Atlantic operation (NAT/OPS) with twoengine airplanes under Part 121.

• Extended range operations (ER-OPS) with twoengine airplanes under Part 121.

• Special fuel reserves in international operations.

• Planned inflight redispatch or rerelease en route.

• Extended over water operations using a singlelong-range communication system.

• Operations in reduced vertical separation mini-mum (RVSM) airspace.

DIRECT FLIGHTSThere are a number of ways to create shorter routes andfly off the airways. You can use NACO low and highaltitude en route charts to create routes for directflights, although many of the charts do not share thesame scale as the adjacent chart, so a straight line isvirtually impossible to use as a direct route for longdistances. Generally speaking, NACO charts are plot-ted accurately enough to draw a direct route that canbe flown. A straight line drawn on a NACO en routechart can be used to determine if a direct route willavoid airspace such as Class B airspace, restrictedareas, prohibited areas, etc. Because NACO en routecharts use the Lambert Conformal Conic projection, astraight line is as close as possible to a geodesic line(better than a great circle route). The closer that yourroute is to the two standard parallels of 33° and 45°on the chart, the better your straight line. There arecautions, however. Placing our round earth on a flatpiece of paper causes distortions, particularly on longeast-west routes. If your route is 180° or 360°, there isvirtually no distortion in the course line.

About the only way you can confidently avoid pro-tected airspace is by the use of some type of airborne

database, including a graphic display of the airspace onthe long-range navigation system moving map, forexample. When not using an airborne database, leavinga few miles as a buffer helps ensure that you stay awayfrom protected airspace.

In figure 3-34, a straight line on a magnetic course fromSCRAN intersection of 270° direct to the Fort SmithRegional Airport in Arkansas will pass just north ofrestricted area R-2401A and B, and R-2402. Since theairport and the restricted areas are precisely plotted,there is an assurance that you will stay north of therestricted areas. From a practical standpoint, it mightbe better to fly direct to the Wizer NDB. This routegoes even further north of the restricted areas andplaces you over the final approach fix to Runway 25 atFort Smith.

One of the most common means for you to fly directroutes is to use conventional navigation such as VORs.When flying direct off-airway routes, remember toapply the FAA distance limitations, based uponNAVAID service volume.

RANDOM RNAV ROUTESRandom RNAV routes may be an integral solution inmeeting the worldwide demand for increased air traf-fic system capacity and safety. Random RNAV routesare direct routes, based on RNAV capability. They aretypically flown between waypoints defined in terms oflatitude and longitude coordinates, degree and distancefixes, or offsets from established routes and airways ata specified distance and direction. Radar monitoring byATC is required on all random RNAV routes.

With IFR certified RNAV units (GPS or FMS), thereare several questions to be answered, including“Should I fly airways or should I fly RNAV direct?”One of the considerations is the determination of theMIA. In most places in the world at FL 180 and above,the MIA is not significant since you are well above anyterrain or obstacles. On the other hand, a direct route at8,000 feet from Salt Lake City, Utah to Denver,Colorado, means terrain and obstacles are very impor-tant. This RNAV direct route across the RockyMountains reduces your distance by about 17 NM, butradar coverage over the Rockies at lower altitudes ispretty spotty. This raises numerous questions. Whatwill air traffic control allow on direct flights? What willthey do if radar coverage is lost? What altitudes willthey allow when they can’t see you on radar? Do theyhave altitudes for direct routes? The easy answer is tofile the airways, then all the airway MIAs becomeusable. But with RNAV equipment, a direct route ismore efficient. Even though on some routes the mileagedifference may be negligible, there are many other caseswhere the difference in distance is significant.

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All air route traffic control centers have MIAs for theirareas of coverage. Although these altitudes are not pub-lished anywhere, they are available when airborne fromATC.

OFF ROUTE OBSTACLE CLEARANCE ALTITUDEAn off-route obstacle clearance altitude (OROCA)is an off-route altitude that provides obstruction clear-ance with a 1,000-foot buffer in nonmountainous terrainareas and a 2,000-foot buffer in designated mountain-ous areas within the U.S. This altitude may not providesignal coverage from ground-based navigational aids,air traffic control radar, or communications coverage.OROCAs are intended primarily as a pilot tool foremergencies and situational awareness. OROCAsdepicted on NACO en route charts do not provide youwith an acceptable altitude for terrain and obstructionclearance for the purposes of off-route, random RNAVdirect flights in either controlled or uncontrolled air-space. If you depart an airport VFR intending to orneeding to obtain an airfiled (popup) IFR clearanceen route, you must be aware of the position of your

aircraft to terrain and obstructions. When accepting aclearance below the MEA, MIA, MVA, or theOROCA, you are responsible for your ownterrain/obstruction clearance until reaching the MEA,MIA, MVA, or the OROCA. If you are unable tomaintain terrain/obstruction clearance, you shouldadvise the controller and state your intentions.[Figure 3-35]

For all random RNAV flights, there needs to be at leastone waypoint in each ARTCC area through which youintend to fly. One of the biggest problems in creatingan RNAV direct route is determining if the route goesthrough special use airspace. For most direct routes,the chances of going through prohibited, restricted,or special use airspace are good. In the U.S., all directroutes should be planned to avoid prohibited orrestricted airspace by at least 3 NM. If a bend in adirect route is required to avoid special use airspace,the turning point needs to be part of the flight plan.Two of the most prominent long range navigationsystems today include FMS with integrated GPSand stand-alone GPS. The following example is a

Figure 3-34. Direct Route Navigation.

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simplified overview showing how the RNAV systemsmight be used to fly a random RNAV route:

In figure 3-36, you are northeast of Tuba City VOR-TAC at FL 200 using RNAV (showing both GPS andFMS), RNAV direct on a southwesterly heading toLindbergh Regional Airport in Winslow. As you moni-tor your position and cross check your avionics againstthe high altitude en route chart, you receive a companymessage instructing you to divert to Las Vegas, requir-ing a change in your flight plan as highlighted on thedepicted chart excerpt.

During your cockpit review of the high and low alti-tude en route charts, you determine that your bestcourse of action is to fly direct to the MIRAJ waypoint,28 DME northeast of the Las Vegas VORTAC on the045° radial. This places you 193 NM out on a 259°magnetic course inbound, and may help you avoiddiverting north, allowing you to bypass the more

distant originating and intermediate fixes feeding intoLas Vegas. You request an RNAV random route clear-ance direct MIRAJ to expedite your flight. DenverCenter comes back with the following amended flightplan and initial clearance into Las Vegas:

“Marathon five sixty four, turn right heading two sixzero, descend and maintain one six thousand, clearedpresent position direct MIRAJ.”

The latitude and longitude coordinates of your presentposition on the high altitude chart is N36 19.10, andW110 40.24 as you change your course. Notice yourGPS moving map (upper left) and the FMS controldisplay unit (below the GPS), and FMS map modenavigation displays (to the right of the GPS) as youreroute your flight to Las Vegas. For situationalawareness, you note that your altitude is well aboveany of the OROCAs on your direct route as you arrivein the Las Vegas area using the low altitude chart.

Figure 3-35. Off-Route Obstacle Clearance Altitude.

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PUBLISHED RNAV ROUTESAlthough RNAV systems allow you to select any num-ber of routes that may or may not be published on achart, en route charts are still crucial and required forRNAV flight. They assist you with both flight planningand inflight navigation. NACO en route charts are very

helpful in the context of your RNAV flights. PublishedRNAV routes are fixed, permanent routes that can beflight planned and flown by aircraft with RNAV capa-bility. These are being expanded worldwide as newRNAV routes are developed, and existing charted,conventional routes are being designated for RNAV

Figure 3-36. Random RNAV Route.

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use. It is important to be alert to the rapidly changingapplication of RNAV techniques being applied to con-ventional en route airways. Published RNAV routes maypotentially be found on any NACO en route chart. Thepublished RNAV route designation may be obvious, or,on the other hand, RNAV route designations may beless obvious, as in the case where a published routeshares a common flight track with a conventional air-way. Note: Since the use of RNAV is dynamic and rap-idly changing, NACO en route charts are continuouslybeing updated for information changes and you mayfind some differences between charts.

According to the International Civil AviationOrganization (ICAO), who develops standard princi-ples and techniques for international air navigation,basic designators for air traffic service (ATS) routesand their use in voice communications have been estab-lished in Annex 11. ATS is a generic ICAO term forflight information service, alerting service, air trafficadvisory service, and air traffic control service. One ofthe main purposes of a system of route designators is toallow both pilots and ATC to make unambiguous refer-ence to RNAV airways and routes. Many countrieshave adopted ICAO recommendations with regard toATS route designations. Basic designators for ATSroutes consist of a maximum of five, and in no caseexceed six, alpha/numeric characters in order to beusable by both ground and airborne automation sys-tems. The designator indicates the type of the routesuch as high/low altitude, specific airborne navigationequipment requirements such as RNAV, and the aircrafttype using the route primarily and exclusively. Thebasic route designator consists of one or two letter(s)followed by a number from 1 to 999.

COMPOSITION OF DESIGNATORSThe prefix letters that pertain specifically to RNAVdesignations are included in the following list:

1. The basic designator consists of one letter of thealphabet followed by a number from 1 to 999.The letters may be:

a) A, B, G, R — for routes that form part ofthe regional networks of ATS routes and arenot RNAV routes;

b) L, M, N, P — for RNAV routes that formpart of the regional networks of ATS routes;

c) H, J, V, W — for routes that do not formpart of the regional networks of ATS routesand are not RNAV routes;

d) Q, T, Y, Z — for RNAV routes that do notform part of the regional networks of ATSroutes.

2. Where applicable, one supplementary letter mustbe added as a prefix to the basic designator asfollows:

a) K — to indicate a low level route estab-lished for use primarily by helicopters.

b) U — to indicate that the route or portionthereof is established in the upper airspace;

c) S — to indicate a route established exclu-sively for use by supersonic airplanesduring acceleration/deceleration andwhile in supersonic flight.

3. Where applicable, a supplementary letter may beadded after the basic designator of the ATS routeas a suffix as follows:

a) F — to indicate that on the route or portionthereof advisory service only is provided;

b) G — to indicate that on the route or portionthereof flight information service only isprovided;

c) Y — for RNP 1 routes at and above FL 200to indicate that all turns on the routebetween 30° and 90° shall be made withinthe tolerance of a tangential arc between thestraight leg segments defined with a radiusof 22.5 NM.

d) Z — for RNP 1 routes at and below FL 190to indicate that all turns on the routebetween 30° and 90° shall be made withinthe tolerance of a tangential arc between thestraight leg segments defined with a radiusof 15 NM.

USE OF DESIGNATORS IN COMMUNICATIONSIn voice communications, the basic letter of a designa-tor should be spoken in accordance with the ICAOspelling alphabet. Where the prefixes K, U or S, speci-fied in 2., above, are used in voice communications,they should be pronounced as:

K = “Kopter” U = “Upper” S = “Supersonic”

as in the English language.

Where suffixes “F”, “G”, “Y” or “Z” specified in 3.,above, are used, the flight crew should not be requiredto use them in voice communications.

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Example:

A11 will be spoken Alfa Eleven

UR5 will be spoken Upper Romeo Five

KB34 will be spoken Kopter Bravo Thirty Four

UW456 F will be spoken Upper Whiskey Four Fifty Six

Figure 3-37 depicts published RNAV routes in theGulf of Mexico (black Q100, Q102, and Q105) thathave been added to straighten out the flight segmentsand provide an alternative method of navigation to theLF airway (brown G26), that has since been termi-nated in this case. The “Q” designation is derived fromthe list of basic route designators previously covered,and correlates with the description for RNAV routesthat do not form part of the regional networks of ATSroutes. Notice the indirect reference to the RNAVrequirement, with the note, “Navigational EquipmentOther than LF or VHF Required.”

Notice in figure 3-38 that this en route chart excerptdepicts three published RNAV jet routes, J804R,J888R, and J996R. The “R” suffix is a supplemen-tary route designator denoting an RNAV route. Theoverlapping symbols for the AMOTT intersectionand waypoint indicate that AMOTT can be identi-fied by conventional navigation or by latitude and

longitude coordinates. Although coordinates wereoriginally included for aircraft equipped with INSsystems, they are now a good way to cross checkbetween the coordinates on the chart and in the FMSor GPS databases to ensure you are tracking on yourintended en route course. The AMOTT RNAVwaypoint includes bearing and distance from theANCHORAGE VORTAC. In an effort to simplifythe conversion to RNAV, some controlling agenciesoutside the U.S. have simply designated all conven-tional routes as RNAV routes at a certain flightlevel.

RNAV MINIMUM EN ROUTE ALTITUDERNAV MEAs are depicted on some NACO IFR enroute charts, allowing both RNAV and non-RNAVpilots to use the same chart for instrument navi-gation.

MINIMUM IFR ALTITUDEMinimum IFR altitude (MIA) for operations are pre-scribed in Part 91. These MIAs are published on NACOcharts and prescribed in Part 95 for airways and routes,and in Part 97 for standard instrument approach proce-dures. If no applicable minimum altitude is prescribed inParts 95 or 97, the following MIA applies: In designatedmountainous areas, 2,000 feet above the highest obstaclewithin a horizontal distance of 4 NM from the course tobe flown; or other than mountainous areas, 1,000 feet

Figure 3-37. Published RNAV Routes Replacing LF Airways.

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above the highest obstacle within a horizontal distanceof 4 NM from the course to be flown; or as otherwiseauthorized by the Administrator or assigned by ATC.MIAs are not flight checked for communication.

WAYPOINTSWaypoints are specified geographical locations, orfixes, used to define an RNAV route or the flight pathof an aircraft employing RNAV. Waypoints may be anyof the following types: predefined, published way-points, floating waypoints, or user-defined waypoints.Predefined, published waypoints are defined relative to

VOR-DME or VORTAC stations or, as with GPS, interms of latitude/longitude coordinates.

USER-DEFINED WAYPOINTSUser-defined waypoints typically are created by pilotsfor use in their own random RNAV direct navigation.They are newly established, unpublished airspace fixesthat are designated geographic locations/positions thathelp provide positive course guidance for navigationand a means of checking progress on a flight. They mayor may not be actually plotted by the pilot on en routecharts, but would normally be communicated to ATC in

Figure 3-38. Published RNAV Jet Routes.

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terms of bearing and distance or latitude/longitude. Anexample of user-defined waypoints typically includesthose derived from database RNAV systems wherebylatitude/longitude coordinate-based waypoints aregenerated by various means including keyboard input,and even electronic map mode functions used to estab-lish waypoints with a cursor on the display. Anotherexample is an offset phantom waypoint, which is a point-in-space formed by a bearing and distance fromNAVAIDs, such as VORTACs and tactical air navigation(TACAN) stations, using a variety of navigationsystems. When specifying unpublished waypointsin a flight plan, they can be communicated using thefrequency/bearing/distance format or latitude and longi-tude, and they automatically become compulsoryreporting points unless otherwise advised by ATC.All airplanes with latitude and longitude navigationsystems flying above FL 390 must use latitude andlongitude to define turning points.

FLOATING WAYPOINTSFloating waypoints, or reporting points, represent air-space fixes at a point in space not directly associated

with a conventional airway. In many cases, they maybe established for such purposes as ATC meteringfixes, holding points, RNAV-direct routing, gatewaywaypoints, STAR origination points leaving the enroute structure, and SID terminating points joining theen route structure. In figure 3-39, in the top example, aNACO low altitude en route chart depicts three float-ing waypoints that have been highlighted, SCORR,FILUP, and CHOOT. Notice that waypoints are namedwith five-letter identifiers that are unique and pronoun-cable. Pilots must be careful of similar waypointnames. Notice on the high altitude en route chartexcerpt in the bottom example, the similar soundingand spelled floating waypoint named SCOOR, ratherthan SCORR. This emphasizes the importance ofcorrectly entering waypoints into database-drivennavigation systems. One waypoint character incor-rectly entered into your navigation system couldadversely affect your flight. The SCOOR floatingreporting point also is depicted on a Severe WeatherAvoidance Plan (SWAP) en route chart. These way-points and SWAP routes assist pilots and controllerswhen severe weather affects the East Coast.

Figure 3-39. Floating Waypoints.

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COMPUTER NAVIGATION FIXESAn integral part of RNAV using en route chartstypically involves the use of airborne navigationdatabases. Database identifiers are depicted onNACO en route charts enclosed in parentheses, forexample AWIZO waypoint, shown in figure 3-40.These identifiers, sometimes referred to as computernavigation fixes (CNFs), have no ATC function andshould not be used in filing flight plans nor shouldthey be used when communicating with ATC.Database identifiers on en route charts are shownonly to enable you to maintain orientation as you usecharts in conjunction with database navigation sys-tems, including RNAV.

Many of the RNAV systems available today make itall too easy to forget that en route charts are stillrequired and necessary for flight. As important asdatabases are, they really are onboard the airplane toprovide navigation guidance and situational aware-ness; they are not intended as a substitute for papercharts. When flying with GPS, FMS, or planning aflight with a computer, it is important to understandthe limitations of the system you are using, for exam-ple, incomplete information, uncodeable procedures,complex procedures, and database storage limitations.For more information on databases, refer to AppendixA, Airborne Navigation Database.

NATIONAL ROUTE PROGRAMIn the U.S., the national route program (NRP), alsoknown as “free flight,” is an example of applying

RNAV techniques. The NRP is a set of rules and proce-dures that are designed to increase the flexibility ofuser flight planning within published guidelines. Thefree flight program allows dispatchers and pilots tochoose the most efficient and economical route forflights operating at or above FL 290 between city pairs,without being constrained to airways and preferredroutes.

Free flight is a concept that allows you the same type offreedom you have during a VFR flight. Instead of aNAS that is rigid in design, pilots are allowed to choosetheir own routes, or even change routes and altitudes atwill to avoid icing, turbulence, or even take advantageof winds aloft. Complicated clearances become unnec-essary, although flight plans are required for trafficplanning purposes and as a fall-back in the event of lostcommunication.

Free flight is made possible with the use ofadvanced avionics, such as GPS navigation anddatalinks between your aircraft, other aircraft, andcontrollers. Separation is maintained by establish-ing two airspace zones around each aircraft, asshown in figure 3-41. The protected zone, which isthe one closest to the aircraft, never meets the pro-tected zone of another aircraft. The alert zoneextends well beyond the protected zone, and aircraftcan maneuver freely until alert zones touch. If alertzones do touch, a controller may provide the pilotswith course suggestions, or onboard traffic displaysmay be used to resolve the conflict. The size of the

Figure 3-40. Computer Navigation Fix.

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zones is based on the aircraft’s speed, performance,and equipment. Free flight is operational in Alaska,Hawaii, and part of the Pacific Ocean, using about2,000 aircraft. Full implementation is projected totake about 20 years.

The technology to help free flight become a reality isbeing placed into position, especially through the useof the GPS satellite system, as the FAA and industrywork together. Equipment such as automatic dependentsurveillance broadcast (ADS-B), allows you in thecockpit and air traffic controllers on the ground to “see”aircraft traffic with more precision than has been possi-ble before. The FAA has identified more than 20 waysthat ADS-B can make flying safer and can allow moreefficient use of the airspace, along with the ability toimprove your situational awareness and ability tosafely maintain aircraft spacing both en route and onfinal approach to landing. Unlike radar, ADS-B doesn’tneed to interrogate targets to display them. Rather, itrelies on the satellite-based global positioning system.Each ADS-B equipped aircraft broadcasts its preciseposition in space via a digital datalink along with otherdata, including airspeed, altitude and, whether the air-craft is turning, climbing or descending. This providesaircraft with ADS-B equipment a more accurate depic-tion of air traffic than radar can provide.

ADS-B equipment is small and light, and can be madea standard part of the equipment on board an aircraft,allowing you to see an accurate depiction of real-timeair traffic, along with controllers. Unlike conventionalradar, ADS-B works at low altitudes and on theground. It is effective in remote areas or in mountain-ous terrain where there is no radar coverage, or where

radar coverage is limited. One of the greatest benefitsof ADS-B is its ability to provide the same real-timeinformation to you in the aircraft cockpit and to groundcontrollers, so that for the first time, you can both “see”the same data.

In addition to new technologies such as ADS-B, theuser request evaluation tool (URET) helps provideenhanced, automated flight data management. URET isan automated tool provided at each radar position inselected en route facilities, and it uses flight andradar data to determine present and future trajecto-ries for all active and proposed aircraft flights. Thistechnology improves airspace efficiency and capac-ity by allowing you to select more direct routes toyour destination. A graphic plan display depicts air-craft, traffic, and notification of predicted conflicts.Graphic routes for current plans and trial plans aredisplayed upon controller request. The user requestevaluation tool can generate a predicted conflict oftwo aircraft, or between aircraft and airspace. A redalert is used for conflicts when the predicted mini-mum separation is 5 NM or less. A yellow alert isused when the predicted minimum separation isbetween 5 and approximately 12 NM. A blue alert isused for conflicts between an aircraft and predefinedairspace. URET is in use by ARTCC Centers inWashington (DC), Kansas City, Cleveland, Chicago,Indianapolis, and Memphis.

ADVANCED AREA NAVIGATION ROUTESOff-airway direct routings are available in some areasof the U.S. You can file these preplanned routes ifyou are an advanced navigation equipment systemuser. Most of these advanced navigation routes(ANRs) currently are in the Northwest and Western-

Pacific FAA regions, andinvolve RNAV direct routesegments to/from the airportsof the Los Angeles Basin,Portland/Seattle, San Fran-cisco/Oakland, San Jose,Phoenix, and Las Vegas.

IFR TRANSITION ROUTESIn order to expedite the han-dling of IFR overflight trafficthrough Charlotte ApproachControl Airspace, severalRNAV routes are published inthe Airport/Facility Directoryand available for you whenfiling your flight plan. AnyRNAV capable aircraft filingflight plan equipment codesof /E, /F, or /G may file forthese routes. Other aircraftmay request vectors along

Figure 3-41. Free Flight.

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these routes but should only expect vector routes asworkload permits. Altitudes are assigned by ATCbased upon traffic. [Figure 3-42]

IFR transition routes through Class B airspace for gen-eral aviation aircraft en route to distant destinations arehighly desirable. Since general aviation aircraft cruiseat altitudes below the ceiling of most Class B airspaceareas, access to that airspace for en route transitionreduces cost and time, and is helpful to pilots in theirflight planning. Establishing RNAV fixes could facili-tate the implementation of IFR transition routes,although every effort should be made to design routesthat can be flown with RNAV or VOR equipment. IFRtransition routes are beneficial even if access is notavailable at certain times because of arriving or

departing traffic saturation at the primary airport. Forthese locations, information can be published to advisepilots when IFR transition access is not available.

REQUIRED NAVIGATION PERFORMANCE As RNAV systems grow in sophistication, high tech-nology FMS and GPS avionics are being phased in asNDBs, VORs, and LORAN are being phased out. As aresult, new procedures are being introduced, includingRNP, RVSM, and minimum navigation performancespecifications (MNPS). In addition to route designa-tors, RNAV routes may also be appended with RNPvalues. ICAO defines required navigation perform-ance as a statement of required navigation accuracy inthe horizontal plane (lateral and longitudinal positionfixing) necessary for operation in a defined airspace.

Even such terms as gross navi-gation errors (GNEs) arebeing introduced into the naviga-tion equation. If you commit agross navigation error in theNorth Atlantic oceanic region ofmore than 25 NM laterally or 300feet vertically, it has a detrimentaleffect on the overall targeted levelof safety of the ATC airspace sys-tem in this region. This applies tocommercial operators, as well asPart 91 operators, all of whommust be knowledgable on proce-dures for operations in NorthAtlantic airspace, containedin the North Atlantic MNPSOperations Manual.

RNP types are identified by asingle accuracy value. Forexample, RNP 1 refers to arequired navigation perform-ance accuracy within 1 NMof the desired flight path atleast 95 percent of the timeflying. Required navigationperformance values are beingestablished by countries aroundthe world. For example, Japanwas the first to declare all theirairways with RNP 4. Europeis beginning with RNP 5, oth-erwise referred to as B-RNAV.Since all the airways in theU.S. are 4 NM wide on eitherside of the airway center-line, the U.S. airways havean equivalent RNP of 2.Establishing RNP values isan evolving process that

Figure 3-42. IFR Transition Routes in the Airport/Facility Directory.

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guarantees to impact navigation procedures nowand for years to come. RNP parameters are slowlybecoming more stringent. For example, Europe’s goalis to reach RNP 1 by the year 2005. Figure 3-43 showsICAO RNP containment parameters, including refer-ence to lateral and longitudinal total system errors(TSEs).

The U.S. standard RNP values or levels are for routessupporting RNAV operations based on GPS or earth-referenced navigation systems such as the inertial nav-igation system (INS) and the inertial reference system(IRS). RNP levels will be depicted on affected aero-nautical charts and procedures, and these referencesmay include a notation referring to eligible aircraft byspecific navigation sensors.

RNP requires you to learn new procedures, communi-cations, limitations, and to learn new terminology thatdefines and describes navigation concepts. Some ofthese terms include, RNP Airspace, a generic termdesignating airspace, routes, legs, operations, or pro-cedures where minimum RNP has been established.P-RNAV represents a 95 percent containment valueof ±1 NM. B-RNAV provides a 95 percent con-tainment value of ±5 NM. RNP is a function ofRNAV equipment that calculates, displays, and

provides lateral guidance to a profile or path.Estimated position error (EPE) is a measure of yourcurrent estimated navigational performance, alsoreferred to as actual navigation performance (ANP).

RNP RNAV is an industry-expanded specificationbeyond ICAO-defined RNP. Some of the benefits ofRNP RNAV includes being an aid in both separationand collision risk assessment. RNP RNAV can furtherreduce route separation. Figure 3-44 depicts routeseparation, that can now be reduced to four times theRNP value which further increases route capacitywithin the same airspace. The containment limit quan-tifies the navigation performance where the probabil-ity of an unannunciated deviation greater than 2 xRNP is less than 1 x 10-5. This means that the pilotwill be alerted when the total system error can begreater than the containment limit. Figure 3-45 showsthe U.S. RNP RNAV levels by airspace controlregions, including RNP 2 for the en route phase offlight, and figure 3-46 illustrates the U.S. standardRNP (95%) levels.

REDUCED VERTICAL SEPARATION MINIMUMS RVSM airspace is any airspace between FL 290 andFL 410 inclusive, where airplanes are separated by1,000 feet vertically. In the early 1980’s programs

{{ { {

Distance to Waypoint

TrueAircraftPosition

Lateral TSE= RNP Type

Lateral TSE= RNP Type

Longitudinal TSE= RNP Type

Longitudinal TSE= RNP Type

Nav SystemIndicatedPosition

DesiredFlight Path

Inside the box95% of TotalFlight Time

Figure 3-43. ICAO RNP Containment Parameters.

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2 X RNPRNP: 95%

RNP

Containment Limit

Containment Limit: 99.999%

Defined Path

Desired Path

RNP RNAV is referenced to the airplane defined pathICAO RNP is referenced to the airspace desired path.

Figure 3-44. RNP RNAV Containment.

U.S. RNP RNAVLEVELS

ADS-B RNP 1

RNP 2RNP 0.3

RNP 0.3

Airport Surface

Final Approach/Initial Departure

Approach/Departure Transition

Arrival/Departure

Enroute

Figure 3-45. Airspace Control Regions.

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were established to study the concept of reducedvertical separation minimums (RVSM). RVSMwas found to be technically feasible without impos-ing unreasonable requirements on equipment.

Reasons for implementingRVSM primarily centeraround significant air traf-fic increases worldwide.RVSM is the most effectiveway to increase airspacecapacity to cope with traf-fic growth. Reduced verti-cal separation minimumsairspace is quickly becom-ing a standard around theworld. By the year 2005most of the preferred inter-national and domesticflight routes will be underboth RVSM and RNPRNAV rules.

The history and backgroundof RVSM includes the year1960, when vertical separa-tion between airplanes wasofficially increased to 2,000feet above FL 290. The rea-son at that time was due tounreliability of accurate

pressure sensing barometric altimeters at high alti-tudes. In 1966, the FL 290 change-over altitude wasadopted globally by ICAO. [Figure 3-47]

Figure 3-46. U.S. Standard RNP Levels.

FL 330

FL 310

FL 290

FL 280

FL 270

2000 ft

1000 ft

Figure 3-47. History of RVSM.

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By 1997, however, the nature of traffic flow allowedimplementation of the first RVSM 1000 feet sepa-ration between FL 330 and FL 370 over the NorthAtlantic. In 1998, RVSM was further implementedfrom FL 310 to FL 390. Today RVSM from FL 290to FL 410 is being implemented by States (govern-ments) around the globe. There are many require-ments for operator approval of RVSM. Each aircraft

must be in compliance with specific RVSM criteria.A program must be in place to assure continued air-worthiness of all RVSM critical systems. Flightcrews, dispatchers, and flight operations must beproperly trained, and operational procedures, check-lists, etc. must be established and published in the OpsManual and AFM, plus operators must participate in aheight monitoring program.