sa06 propeller safety

7/29/2019 SA06 Propeller Safety

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On takeoff, propeller tip speeds approach the speed osound. The b lades must absorb not only the punishingvibration of the engines power pulses, but also vibraticaused by the oncoming airstream. Centrifugal loadsthose forces that try to pull the blade out of the hubamount to 10 to 20 tons per blade.

The blades tw ist and flex. The stresses imposed o n thprop are more concentrated in the small areas that are

nicked or cut. These nicks and scratches act as stressrisers, which can weaken the blade enough to eventuacause blade failure.

When an engine quits, the airplane can glide to a safelanding. When a propeller blade is lost, the resultingimbalance can tear the entire engine from the aircraft,putting the center of gravity far beyond limitsandrendering the a ircraft uncontrollable.

Statistically speakingAlthough accidents and human injuries from propellerare not widespread, they are serious and most are easavoidable.

In 2000, 14 accidents occurred that w ere blamed o nthe propeller; two were prop strikes resulting in onefata lity and one serious injury. The remaining 12 w ereclassified as propeller system failures. Half of these w ehome -built a irplanes.

Five accidents (three factory two homebuilt) involvepropeller blade or hub fatigue, failure, or separation.

Two (one fac tory one homebuilt) were due to proppitch change mechanism failure. One factory built air-plane lost the propeller entirely. Two prop d rive systemfailures (one factory and one homebuilt) were recordeTwo homebuilt prop fa ilures were due to impropermaintenance or assembly.

Prop accidents break down into some general groups:

Precautionary landings because of abnormalvibration. Often caused by a missing balance weightor deteriorated spinner.

Propeller Safe

S p o n s o r e d b y H a r t z e l l P r o p e l l e r , I n c

S A F E T Y A D V I S O R

Technology No. 3

The root causeof mechanicallyinduced

accidents isalmost alwaysneglect.


2/16Propeller Safety Pg. 2

Forced landings followinga catastrophic prop failure.

Precautionary landingsdue to propeller overspeedor runaway.

Accidents from a botchedhand-propping episode.

G ro und p erso nnel,passengers, or bystanderswalking into spinningpropellers.

The root ca use of mecha nically induced accidents isalmost always neglect.

Working around the propellerNaturally, the propeller, attached as it is to a powerfulmotivator called an engine, deserves respect on theground. The single mo st important concept youshould understand is thisthe propeller must alwa ysbe treated as though the ignition has been left on andthe engine is just a ha irbread th from starting. Eventhough most pilots are careful to occasionally checkthat the mags properly shut down the enginenot alldo . Assume the worst and youll never be surprised .

Mag Check

Periodically ensuring that your magnetos p-leads havenot broken is a good defense against unexpected startson the ground. These leads, which are connected to

the ignition (or magneto) switch in the cockpit, areresponsible for grounding the mags to keep the enginefrom running. Sometimes the wires or connectionsbetween the switch and the magnetos break or comeadrift. In this case, one or both mags may be hot, orready to deliver spark whenever the prop is turned. Totest them, instead of shutting down the engine in theusual manner with the mixture control, use the key.Allow the engine to cool normally and idle down.

Move the key slowly through both Right, Left, and thento the O ff position. The engine rpm should dropslightly at both of the individual-mag positions and shutdown completely in the Off detent. Allow the prop tostop a nd then move the mixture to the idle-cutoffposition. Do not try to catch the engine before itcomes to a stop because a dangerous backfire mightoccur. If the engine does not stop when the key is inthe O ff position, shut it down w ith the mixture,prominently mark the prop as being hot, and contactmaintenance personnel immed iately.

There are a few other cavea ts to consider when in thevicinity o f the prop.

Avoid pulling the airplane around by the prop.Yes, this seems the perfect solution to a vexingproblem of how to change the airplanes positionwithout having to walk around and get the tow bar,but its worthwhile to make the extra effort. Neitherthe engine nor the prop particularly benefit from theloads imposed by horsing the whole airplane around.

Avoid pushing the airplane by the spinner. Thespinner and backing plate are built to be light, sothey re quite fragile. Pushing on them can cause thebacking plate to crack and can lead to spinner failure.

Avoid contact with prop deice boots andassociated w iring. If you w ant your hot prop to betoasty when it counts, stay clear of the boots.

The propeller

must always be

eated as though

he ignition has

een left on and

he engine is justa hairbreadth

from starting.

Treat the propeller with caution.

The spinner backingplate is fragile.



Prop Injur ies

Never attempt to load or unload the airplane w iththe engine running unless there are significantextenuating circumstancessuch as a hurricane iscoming and your starter motor has died . (That isto say there are precious few occasions thatwarrant loading passengers with the engine

running.) If you must allow passengers or crew toboa rd or otherwise approach the airplane, becertain that they understand the areas to avoid.Keep your hand on the mixture control andmonitor the movement of the individuals withoutfail. If someone begins to walk in the direction ofthe spinning prop, kill the engine immediately.Worry about how to restart it later. Remember thatpassengers are unlikely to hear your cautionaryshouts over the engine. Almost every year, thereare injuries or fa talities from someone walking intoa turning prop.

Hand Propping

Were not going to tell you how to hand prop anairplane because it is NOTsomething you learn froma booklet and it is inappropriate for nose-gearairplanes.

If you alread y know how to hand prop, remember tosecure neckties, silk scarves, and loose clothing;

remove rings, wa tches, and bracelets; and have aqualified person at the controls.

Prop m echan icsPropeller technology isconsidered to be mature, likemuch of what we use inavia tion. The ba sic designshave changed little in the past30 years or so, although

incremental advances in bladeaerodynamics have improvedefficiency slightly (at best, aprop is about 85 percent efficient at converting torquto thrust). Modern production techniq ues have alsohelped reduce overall weight. Nonetheless, pilotsought to be familiar with a few basic types of props.

Fixed Pitch

Used when low weight, simplicity, a nd low cost areneeded, the fixed-pitch prop is a compromise.Beca use the pitch setting is idea l for neither cruise nclimb, the airplane suffers a bit in each performancecategory. Fixed-pitch props simplify powermanagement and cost less to overhaul than aconstant-speed version.

Constant Speed

Before there were constant-speed props, there wereadjustable-pitch models. By altering each blades

angle of attack, the prop can be better optimized forboth climb and cruise performance. Early models usemanual adjustment of the prop pitch, while a fewpioneer inventors played with automatic pitch-changmechanisms.

As soon as aircraft developed wide speed rangesthdifference between the slowest climb and the fastestcruiseit was clear that a better system was neededBy the early 1930s, the groundwork for the constant

Almost every

there are inju

or fatalities f

someone wal

into a turni

prop.

Do not attempt hand propp ing withoutpr oper traini ng. This is poor formheis leaning into the pr op.

Always shut down the enginewhen boardi ng passengers.



speed prop had been laid, inlarge part by Ercoupe andPiper Cherokee designer FredWeick.

A separate mechanism is usedto a lter each blade s angle ofattack, with the goal tomaintain a constant engine

speed. In a fixed-pitch prop,as the airplane accelerates,the engine, given a fixedthrottle position, will followsuit. With a constant-speedarrangement, the bladesangle of attack increases asthe engine tries to accelerate,loading the engine andmaintaining the set speed.This setup provides tw o mainbenefits over fixed-pitch

propellers. First, you get amore optimum blade pitchsetting, and second, theengine can be made to run ata set speed, greatly reducing

pilot work load and making precise power settingspossible.

Feathering

A feathering propeller is simply a constant-speed unitthat can rotate the blades until they are nearly alignedwith the relative wind. This provides reduced drag inthe event of an engine failure. Feathering props arefound on most twin-engine airplanes.

Blade Materi als

Today, the vast majority of props in general aviationare metal, specifically aluminum. In the days beforesophisticated meta llurgy and precision metalworkingtools, the wooden propeller reigned supreme, and itsstill popular for ultralights, small experimental aircraft,and antiq ue models. There are also a few co mposite(fiberglass, Kevlar, and graphite) props in circulation;

most of these use either a foam or wo od corewrapped with fiberglass cloth. Potentially, thecomposite prop can be lighter than a metal prop, andits stiffness-to-weight ratio is better, but so far thecertificated composite propeller has proven to be tooexpensive for the light end of the fleet.

Governing

Integral to the constant-speed setup is a device calledthe governor. It is geared to the engine and takes oilfrom the main engine supply. An internal pump

increases the pressure of this oil and directs it, throughthe hollow nose of the crankshaft, to the propeller. Insingle-engine applications, oil pressure serves toincrease the pitch o f the b lades (called co arse pitch),which in turn reduces engine rpm. Twins and someaerobatic aircraft operate so that engine oil forces theblades to fine pitch. Pilots should follow manu-facturers recommendations for feathering props andsecuring engines after failures.

orrosion pits can

cause cracks to

orm w hen the

ub is subjected

to operating

stresses. Suchincidents are

particularly

distressing

because they

re preventable

with proper

maintenanceand overhaul.Feathering pr ops are foundon most tw ins.

Constant speed p ropeller



Hubs

Of course, every propeller blade has to be fastened tothe engine; this is the task of the hub. For most fixed-pitch props, the hub is integral to the blades. Constant-speed props need to allow the blades to rotate in thehub. Many different blade retention systems have beenused throughout the years, with the latter designsintended to be long wearing and corrosion resistant.

As the fleet ages, its becomingmore common to hear of prop-hub distress in addition to themore prevalent blade maladies.For the most part, failures ofhubs are comparatively rare.Typica lly, the hub problemsinvolve cracking prior to failure.

Corrosion pits inside the hubcan cause cracks to form when the hub is subjected operating stresses. Such incidents are particularlydistressing because they are preventable with propermaintenance and overhaul. Prior to failure a crackehub or blade retention component may provide awarning with the sudden onset of grease or oil leakaor vibration.

Preflight considera tionsMany pilots seem to take the prop for granted, so thenext time you fly, take time to carefully check it.Heres wha t to look for:

General condition: Is the prop clean or covered ingrime? (You cant te ll much about the co ndition o f thblades if you cant see them.)

Are the blades scratched, pockmarked, or nicked?This is a critical question. B lade separations start w ithsmall stress concentrato rs in the meta l. These areformed by scratches, gouges, or corrosion that allowstress to concentra te in a very small area. These stresconcentrators weaken the metal, w hich can thencrack. Constant flexing of the blade ma kes crackswider until the part fails. G enerally, a nick that is lessthan 1/32-inch wide or deep ca n be d eferred to the

Are the bla

scratched

pockmarked

nicked? This

critical quest

Corrodedsteel hub

Look carefully at the prop ellewhen you preflight.

Potenti al areas of oilor grease leakage

A cracked alumi num hub



next maintenance cycle, butanything largeror if there isa preponderance of nicks, sayfrom a recent departure froma gravel stripshould bedressed out immediately by aprofessiona l A&P mechanic.

Are the blades tight in their sockets? Constant-speed

props depend upon a certain amount of centrifugalforce to seat the blades, but there should not be mo rethan the slightest bit of movement.

With a spinner fitted, its often d ifficult to determinethe condition of the propeller hub itself, but youshould be a ble to see the mounting hardw arelookfor loose nuts or backed-out bolts. Some props on GAaircraft have oil-filled hubs to assist you in findinghairline cracks that might otherw ise go unnoticed . Anyevidence of red oil must be investigated before flight.

Is the spinner secure? Remember tha t a loose or o ff-center spinner can self-destruct in very few hours ifnot corrected. If part of the spinner departs theairplane in flight, it can feel as dramatic as if a verysmall part of a blade itself had jumped ship. Grasp thetip of the spinner firmly and try to move it in a circularpattern. Just dont overdo it. The spinner and ba ckingplate are easily damaged . Never move the aircraft orlift the nose by pushing on the spinner.

Prop Tip:Props can sustain a lot of dama ge whenoperating over loose gravel. If the airplane isparked on a gravel surface, move it to hard groundbefore starting the engine. Likew ise, w hen taxiingto the tiedown spotif it is on gravel, shut theengine down on hard ground and use a tow bar to

move the airplane.

In the cockpitPrior to flight, take a few precautions to ensure thatyour prop w ill perform as need ed. For fixed-pitchprops, listen and feel for unusual noises and vibration.Because theres so little to go wrong with a fixed-pitchpropeller, yo ure ba sically on the lookout for grossproblems like loose bolts or a tip that departed during

the start and taxi sequences.

Dur ing the Runup

For constant-speed applications, however, there areadditional considerations. Keep an eye on the oilpressure and temperature. Pressure should be in thenormal, green-arc range, and the temperature shouldbe rising according to outside conditions. Because theconstant-speed prop needs both good oil pressure todo its job and oil thin enough to be pumped throughthe smaller passages of the prop, its important to

keep these parameters in mind, particularly for cold-weather departures. A takeoff w ith cold o il will resultin a poorly governed prop and a possible overspeedevent. In subfreezing conditions, it could take 15 to30 minutes to get minimum oil temperature. Storingthe airplane overnight in a heated hangar or calling forpreheat will help greatly.

Never move the

ircraft or lift the

ose by pushing

on the spinner.

Typical gravel damage

Blade erosion: Bare metalcorrod es rapi dly.Keeping b ladesproperly painted isinexpensive rou tinemaintenance.

A prop in t he pro cess offaili ng. N ote crack.



Prop Exercise

When exercising the prop, pay attention to several

items:

Do es the prop control move freely? Excessivefriction could indicate a frayed cable or poorlubrication. It also makes precise setting of theprop lever difficult.

Does the prop respond promptly with warm oil?The prop should take no more than two or threeseconds to respond to cycling when the oil iswarm. If it fails to cycle at all, even when the oiltemperature gauge indicates green-arc conditions,

do no t attempt to take off. There may be abroken cable, failed governor, or plugged oilpassage. The latter two of these possibilities couldlead to an o verspeed event.

Do es the prop return to the set runup speedafter cycling?

Do es the oil pressure fluctuate during cycling?Oil is pulled out of the engine during cycling, sothe pressure should drop slightly w hen the prop iscycled and recover as prop rpm is restored.

On the Takeoff Roll

Takeoff is a busy time, but ensure that the prop and itsgoverning systems are functioning properly.

Watch the tachometer and listen for signs ofsurging or overspeed ing. Know what is normal and

what is excessive needle swingfor your aircraft. If there ismuch o f a problem, youll likelyhear it before noticing it on thetach.

Note tha t the prop hasachieved redline (or nearredline) speed.

Listen for abnormal noiseand vibration.

It is perfectly normal for aconstant-speed prop to spin upjust short of maximum rpmduring the initial takeoff roll. Asthe airplane accelerates, theprop w ill unload slightly andthe speed should come up. If itdoes not reach the redline

value at climb airspeed, havethe governor and/ortachometer checked.

If you see a drop in oil pressureor experience abnormal noise or vibration, you shoueither abort the takeoff (if theres room to safely stopor continue a round the pattern to a normal landingusing low pow er settings.

In Flight

There is no clea r pattern of propeller failurestheycan happen on the takeoff roll, in steady cruise flightor even in the landing pattern. Stay alert to signs ofprop distress in the cruise phase of flight. The suddeonset of grease or oil leakage or vibration in a failingpropeller has been freq uently reported . A timely anthorough investigation of such co nditions is prudent.

There are two ma in in-flight failure modesdepartuof part of a blade, ba lance weight, or spinner that

Keep an eye on oi l p ressur eand temperature.

If you see a d

in oil pressur

experienc

abnormal noi

vibration, y

should eithabort the tak

(if theres roo

safely stop)

continue aro

the pattern

normal land

using low posettings.

Stay alert to signsof pr op d istress.



causes strong (sometimesextreme) vibration, andgovernor maladies that cancause the prop to stay at theset rpm or to spin rapidlybeyond the redline.

Blade Failur es

Its almost too obvious tostate, but the depa rture o fany part of the prop will getyour attention right now.Equally obvious is the ad viceto stay calm.

Immediately reduce powerwith the throttle.

Slow the airplane to best-glide speedtradingairspeed for altitude if you have itand start looking

for somew here to land.

Conduct the normal power-loss troubleshooting.Did you just run a tank dry? Are the mags acting up?Usually, running a tank dry gets you a bump or two asthe fuel pressure fluctuates, then light sputteringfollow ed by silence. Similarly, a mag prob lem typicallymanifests itself in inconsistent misfiring and possiblebackfiring. A prop-blade failure, on the other hand, ismuch more rhythmic and undiminishing in amplitudeuntil the throttle is pulled back.

Unless you are sure that the engine roughness issomething other than the prop, do not ad vance thethrottle aga in. The tremendo us centrifugal forcescreated by a grossly unbalanced prop can tear theengine from its mounts. With the engine idling andthe prop windmilling, the forces will likely be smallenough to let you get the airplane on the groundbefore a major structural component comes unglued.

If you have the time and the presence of mind, shutdow n the engine if you are sure some o f the prop hasdeparted. But, first things first, fly the a irplane.

Governor Failur esOverspeed

The follow ing information applies to single-engineaircraft with loss of governor oil pressure to the prop;i.e., o verspeed . A governor failure that causes engine

overspeed or poor prop control is also a possibility.Wild changes in prop rpm in flight can signal loss ofgoverning control, forcing the blades into the fine orhigh-rpm pitch settings. When this happens, get theengine speed down before it does any damage.

Governor Failu re

If a governor fails to supply oil to a propeller, thefailure effects are different depending on whetherthe propeller is a pressure-to-decrease-pitch or

pressure-to-increase-pitch design. On multi-engine a ircraft, loss of pressure w ill cause the propto fea ther. On most single-engine aircraft, loss ofpressure w ill cause overspeed.

If a governor is not doing its job and a propelleroverspeeds, the amount of overspeed is controlled bytwo things: engine power output and airspeed.Reducing throttle and airspeed will minimize theamount of overspeed. In an overspeed cond ition on asingle-engine aircraft, it would be better to fly to an

airport while overspeeding than to shut down theengine and risk an o ff-airport land ing. If you arefaced with propeller overspeed, take the followingsteps:

Immediately reduce the throttle to idle.

Reduce airspeed, set up for best-glide airspeed,and start looking for a place to land.

Check the oil-pressure gauge. Many proprunaways result from broken oil lines or oil starvation.Dropping oil pressure and increasing tempera ture arethe classic indications.

If oil pressure is good (cross-check withtemperature if you have unusual gauge indications),then slowly advance the throttle and note the propsreactions.

Chances are good that you will be able to maintainsome pow er before the prop rpm reaches the redline.With good oil pressure and temperature, pick a throttle

Its almost too

bvious to state,

ut the departure

any part of the

op w ill get your

attention rightnow. Equally

obvious is the

advice to stay

calm.

A catastroph ic hub failure that allowed theblades to depart and puncture the nose.



setting that will allow you to maintain sub-redlineengine speeds. Better to fly to an airport with anoverspeeding engine than to land in a field because youcut the throttle.

Plan for landing as soon as practical. Do not try tonurse the airplane home. You ma y have a broken oil-supply line to the governor (this was common for awhile on some engines that used external prop-supply

lines), w hich w ill exhaust the o il supply and seize theengine. A spun bearing or other internal leakage ma ybe restricting flow to the prop.

There a lso may be loss of o il pressure and subseq uentseizure of the engine. It may not happen, butsomething caused the prop to overspeed, and its agood bet its oil pressure related.

Prop Str ikes

Its a sad tale tha ts repeated several times a year. The

overworked pilot, perhaps coming home from adifficult day of flying terminated by a challenginginstrument approach, gets distracted and forgets to putthe wheels down. At the first sound of crunchingmetal and the strange stroboscopic blur of the proptaking chunks out of the runway, many pilots firstreaction is to add pow er and try to salvage thesituation. Two words: Do not!

As soon as the first blade tip hitsthe concrete, that prop isruined, unable to carry theaerodynamic and structuralloads imposed by the go-around. If it stays on theairplane long enough to make itaround for a wheels-downlanding, youll be beating the

odds.

In this instance, the choice,though certainly not palatableto many aircraft owners, issimply to keep the throttle atidle and ride the belly landingto the bitter end. Fatalities andinjuries from inadvertent wheels-up landings areextremely rare. However, executing a go-around wida maged blades is very risky. Even if the propellerstays intact enough not to cause serious vibration,

damage could be so severe that climb performance iseriously degraded.

Mainten ance mattersA big part of preventing in-flight emergencies is tokeep on top of propeller maintenance. Many aircraftowners believe the prop is a no-maintenance item.Not so.

Take the opportunity at the normal oil-change interv

to have a more detailed look at the prop. Have themechanic file out any nicks now, while you have thetime. This is not, incidentally, a procedure a pilot canlegally undertake, nor should you attempt to file theprop with supervision unless youve been specificallytrained. Dressing prop blades is an art, because just

As soon as the

blade tip hits

concrete, th

prop is ruin

unable to ca

the aerodynaand structu

loads impose

the go-arou

Dont t ry to salvagethi s situati on.

Mi ssing bl ade tip

Dr essing the prop

a job for an A&Pmechanic.



enough must be filed toremove the nick but not somuch that a lot of blade issacrificed to do it. The idea isto get completely to thebottom of any nick so thatthere ca n be no stress riserthat can later cause bladefailure. There a re blade

minimum dimensions, and iftoo much is filed o ff, it mustbe replaced. Any propelleroverhauler can provide thedimensions, or you canrequest service information

from the propeller ma nufacturer.

Tachometers

Have the tachometer checked annually. Mechanicaltachs are notoriously inaccurate and subject to drift

over their lifetimes. If your engine and propcombination has one or more yellow arcs or redrestricted arcs within the normal operating rangecommon to a lot of four-cylinder Lycomingsthen itsvital that the tach accurately guides you out of thesetrouble spots. These limitations are in place beca use ofvibration characteristics of the engine and propcombination and ca n lead to long-term trouble ifroutinely ignored.

Hours or Years?

Props have recommended overhaul intervals based oncalendar time and flight hours. Depending upon theprop model, this could be 1,500 or 2,000 flight hours,but theres also a calendar limit (typically five yea rs)tha t too many pilots ignore. This is a serious problemin a fleet that flies, on average, fewer than 100 hoursper year. At 100 hours per year, a typica l 2,000-hourprop might not get checked for 20 yea rs! This is

clearly imprudent, so the calenda r limit applies.

Have the prop overhauled at either the time or calendarlimit, whichever occurs first. If the engine comes up foroverhaul before the prop reaches either limit, mostshops will recommend removing the prop and governorand having them overhauled anyw ay. This will get thetimes in sync.

Corrosion Is the Culpri t

Overhaul periods deserve respect because what killsmost props are not external defects, but unseeninternal corrosion. Dissimilar metals in the prop and

hub create an environment ripe for corrosion, and theonly way to properly inspect many of these areas isthrough a teardown. Extensive corrosion candramatically reduce the strength of the blades or hub.Even seemingly minor corrosion may cause a blade orhub to fail an inspection. Beca use of the safetyimplications, this is clearly not an area in which toskimp.

verhaul periods

deserve respect

cause w hat kills

most props are

not surface

defects, butunseen internal

corrosion.

Overhaul at the tim eor calendar limitwhi chever occursfirst.

Corroded bl ade clamp

Corroded bearings



Corrosion can be internal in critical blade retentioncomponents. Such conditions present both a hiddendefect and a potential safety-of-flight issue. This is theprimary reason that calendar limits are an importantinspection requirement. Also, the overhaul need s toinclude more than just a corrosion inspection.Restoration of paint and plating are important to assurefuture co rrosion protection until the next overhaul.

Notification

Its vitally important to keep up-to-date w ithairworthiness directives (ADs) or service bulletins (SBs)for your prop. Compliance with ADs is, of course,required to make the airplane legally airworthy, butits also good form to follow the SBsparticularlythose marked mandatorybecause there may beprop maladies that manifest themselves only undercertain conditions, such as aerobatic flight or basingthe airplane in a salt-intensive environment. All work

performed on the propincluding AD and SBcomplianceshould be noted in the propellerlogbook.

Airwort hiness Di rectives

and Service Bulleti ns

Aircraft owners are seemingly besieged by ADsand SBs. Although some ow ners see only anoutlay of cash, paying attention to ADs and SBs,particularly where propellers are concerned, mightjust save your life.

For Part 91 o perators, only ADs are ma nda tory.An a irplane is not considered airwo rthy unless allADs have been complied with, either by provingby model or serial number that the AD does notspecifically apply, or by show ing that an inspectionor replacement of parts has taken place.Compliance with ADs is required to be noted inthe aircraft, engine, and propeller logbooks.

SBs, even those marked bythe manufacturer asmanda tory, are purelyoptional for Part 91opera tors. Does this meanyou should ignore them?Ha rdly. Many ADs are simplyrewritten SBs, and in many

cases, compliance with apreviously o ptional SB willcover the requirements of anew AD. SBs can alsoprovide useful serviceinformation.

A list of SBs can be o rdered from the propellersmanufacturer. Current AD information can bedownloaded from the AOPA Web site,www.aopa.org.

Overhaul: What Do They Do?

Upon receipt for overhaul, a document is preparedthat will track the propeller components throughoutthe overhaul process. All applicable ADs, currentspecifications, and manufacturers SBs are researchefor incorporation during the overhaul process. Theserial number is double-checked, and notes are madon the work order regarding the general condition inwhich the propeller was received.

As the unit is disassembled a nd c leaned , a preliminainspection is accomplished on a ll related pa rts. Thosrevealing discrepancies req uiring rework or replace-ment are recorded in the overhaul record by partnumber, along with the reason for the required actioAll threaded fasteners are d iscarded during disassembly and, w ith a few exceptions permitted by the manufacturer, are replaced with new components. Manspecialized tools and fixtures are required in the disa

Its vita lly

important to

up-to-date w

airworthine

directives (A

or service bul(SBs) for

your prop

A badly corroded blade

This prop actuating cylinder bur st from pressur ecreated by a rusted governor pr essure r elief

valve that stuck in t he closed position.



sembly and proper reassemblyof propellers. These too ls aregenerally model specific andrange from massive 15-foottorque adapter bars and 100-ton presses down to tinydowel pin alignment devices.

The Hub

Nonferrous hubs and compo-nents are stripped of paintand anodize and inspected forcracks using a liquid penetrantinspection (LPI) procedure.The pa rts are etched, rinsed,dried, and then immersed in afluorescent penetrant solution.After soaking in the penetrant,

they are rinsed aga in and blown dry. Developer isthen applied, which draw s any penetrant caught in

cracks or defects to the surface. Und er an ultravioletinspection lamp, the penetrant clearly identifies theflaw. Certain models of hubs are also eddy-currentinspected around critical, high-stress area s. Edd y cur-rent testing passes an electrical current through a con-ductive material that, when disturbed by a crack orother flaw, causes a fluctuation on a meter or CRTdis-play. This method of inspection can detect flaw s thatare below the surface of the ma terial and not exposedto the eye.

Magnetic particle inspection (MPI) is used to locate

flaw s in steel parts. The steel parts of the propeller aremagnetized by pa ssing a strong electrical currentthrough them. A suspension of fluorescent iron oxidepow der and solvent is spread over the parts. Whilemagnetized, the particles within the fluid on the partssurface immediately align themselves with the discon-tinuity. When examined under black light, the crackor fault show s as a bright fluorescent line.

Components that a re subject to w ear a re dimensional-ly inspected to the manufacturers specifications.After passing inspection, aluminum parts are ano dizedand steel parts are cadmium plated for maximum pro-tection against corrosion.

The Blades

The first step in blade overhaul is the precise mea sure-

ment of blade width, thickness, face alignment, bladeangles, and length. The mea surements are thenrecorded on ea ch blades inspection record andchecked against the minimum acceptable overhaulspecifications established by the manufacturer.

Blade overhaul involves surface grinding and re-pitch-ing, if necessary. Occasionally, blad e straightening isalso req uired. The manufacturers specification dic-

tates certain allowable limits within which a damagedblade ma y be cold straightened a nd returned to air-worthy cond ition. Specialized tooling and precisionmeasuring equipment permit pitch changes or correc-tions of less than 1/10 of 1 degree. To ensure accura-cy, face alignment and angle measurements are takenrepeatedly during the repair process.

Precision hand grinding of the blade airfoil is done toremove all corrosion, scratches, and surface flaws.When all stress risers and faults have been completelyremoved, final blade measurements are taken andrecorded on each blade s inspection record. The pro-peller blades are balanced to ma tch each other andare anodized and painted for long-term corrosion pro-tection.

Prop Reassembl y

When both the hubs and the b lades have completedthe overhaul process, the propeller is ready for finalassembly. Part numbers are re-checked w ith the man-ufacturers specificat ions. The parts are lubrica ted

iss im ilar m etals

n t h e p ro p a n d

h u b c r e a te a n

nv ironm ent r ipe

r corro sion , and

he on ly way torop er ly insp ec t

m a n y o f t h e se

eas is th rou gh a

t e a r d o w n .

A cracked hub

Disassemblywor k stand



and installed per each units particular overhaul man-ual. After fina l assembly, both high- and low -pitchblade angles on constant-speed propellers arechecked for proper operation and leaks by cycling thepropeller w ith air pressure through its blade anglerange. The assembled propeller is then checked forstatic ba lance. If necessary, weights are placed on thehub areas of ea ch light blade socket to bring aboutits proper balance . These weights should be consid-

ered part of the basic hub assembly and should not bemoved during subsequent dynamic balancing to theengine. As with most aircraft compo nents, all of thehardware on the propeller assembly must be safetywired unless secured by self-locking devices. Mainte-nance release tags reflecting the work accomplished,applicable ADs, and all incorporated service docu-ments are then filled out and signed by the finalinspector. These documents certify that the ma jorrepairs and/or alterations that have been mad e meetestablished standards and that the propeller isapproved for return to service.

Accident BriefsFrom the NTSB Fil es

The pilot of the Cessna 150 stopped at the terminalafter a local flight. A person standing on the rampcame over to the airplane a nd ta lked to the pilotthrough the open do or. The engine wa s left idling.After the conversation had concluded, the bystanderwaved goodbye and walked into the propeller,sustaining serious injuries.

It s always good fo rm to shut dow n the engine in the

vicinity of onlookers, even if they are experienced

pilots.

From the NTSB Fil es

The Taylorcraft was in cruisewhen a sudden severe vibrationwent through the airframe. Thepilot shut down the engine andnoted that the brass abrasionstrip and part of one woodenblade had departed. Although

the pilot landed the airplane ina field, it could not be stoppedbefore plunging down a ravine.A mechanic who recovered theaircraft reported seeing dry rotin part of a remaining blade.

Constant inspection and

maintenance is a necessity,

particularly for the older

wooden props. The pilot and

flight instructor in this accident

are to be lauded for keepingtheir cool.


During the takeoff roll, the Piper Super Cubs rightmain gea r hit a hole. The pilot said he heard thepropeller strike a rock. He continued the takeoff, buafter liftoff, the a irplane began to vibrate. The pilotreduced engine power and landed in a river.Examination o f the propeller disclosed approximatelthree inches of one blade missing.

Anytim e you hit a solid object with the prop

whether it s during a takeoff on a rough strip or the

m isguided attempt to salvage a gear-up land ingyo

shou ld immediately discontinue the takeoff. This pil

was lucky that the imbalanced prop did not do

serious secondary damage to the engine or airframe

In t he worst case, the engine can be shaken from its

moun ts; the loss of engine weight w ill , in m ost case

make the airplane uncon tro llable.


The pilot of the Air Tractor noticed serious enginevibration during a spraying run. He reduced power,then reapplied power, but the vibration remained.The a ircraft was landed in w et terrain and nosed o veInspection revealed that a bolt in the propellercounterweight had broken.

Something as seemingly sim ple as a counterweight

caused th is pilot to sit up and t ake notice. The mora

here is that quick action is the key to salvaging a pro

Anytim e you

so lid ob ject w

th e p ro p

w he the r it

du r ing a tak

on a rough sor the m isgu

a t te m p t t

salvage a

gear-up landi

y o u sh o u l

immedia te

d iscon t inuthe takeo fThe older wooden pro ps need thorough inspection.



problem ; the pilot walked

away without injuries.


A factory-overhauled enginehad been installed in theCessna Cardinal RG. The pilotdeparted on a post-

maintenance test hop. Elevenminutes into the flight, the pilot reported a runawayprop. The engine seized shortly therea fter. A forcedlanding on a soccer field ended in a collision with twovehicles on a n adjoining road ; the pilot w as notinjured. Later inspection revealed that one of tworequired gaskets at the base of the prop governor padhad been o mitted, allow ing the engine oil to bevented overboard.

Good work on the part of the pilot fo r keeping this

dead-stick Cardinal under cont rol . It serves as a

reminder, though, that any flight after majormaintenance shou ld be conducted as close to a

suitable runway as possible.


During cruise flight, about half of one prop bladedeparted the Beech Musketeer, resulting in substantialengine and a irframe da mage. The airplane land eduneventfully at a nearby airport. Inspection laterrevealed that the prop blade had failed from fatiguethat started from a single corrosion pit on the camber

side of the blade. There w ere no records in theaircrafts logbooks that the prop had been servicedsince it was manufactured in September 1963.

Wine may improve wi th age, but metal does not . That

the prop had apparently been untouched for 33 years

when the accident occurred is not hing short of

amazing. Again, most prop makers specify, in

addition to the hours-in-service lim its, a calendar

lim itation of, on average, five years between

overhauls. This one was way overdue.

From the NTSB Files

The pilot o f a Piper Comanche (PA-24-250) was struck

in the head and elbow by the propeller as he wasattempting to start the a ircraft in cold weather. Thepilots o perating handbook suggests pulling the propthrough four to six times before attempting a coldweather start, but in this case the pilot had alreadytried to start the airplane without success. Leaving hisnon-pilot passengers on board, he exited the aircraftto pull the prop through but neglected to ensure thatthe magneto switch had been returned to the offposition.

A couple of thoughts here: Be sure that any time you

touch a propeller the magnetos are in the off posit ion preferably with the key in your pocket. Even then you

must assume the engine could start. That means keep-

ing all parts of your body out of the prop arc. Had the

engine started, the non-pilots in the airplane would be

unlikely to help. Having a qualified pilot at the controls

would have been the safest opt ion.

Keep on t opof propellermaintenance.

Publisher: Bruce Landsberg; Editors: Kathy Dondzila, John Steuernagle; Consul tan t : Marc Cook

Special thanks to:

Hartzell Propeller, Inc.Aircraft Owners and Pilots Association

Aircraft Propeller Service, Inc.

Worldw ide Aircraft Propeller Association

Frederick Aviation

Copyright 2002, AOPA Air Safety Foundation

421 Aviation Way Frederick, Maryland 21701

800/638-3101 www.asf.org

Train ing: If you

do nt ha ve it ,

do nt t ry han d-

p ro p p in g th e

airplane.


15/16

General Aviation

Safety?Weve got that .Keeping up with the chang es in airspace, regulations, and safety informa -

tion can be toug h. The AOPA Air Safety Foun da tion m akes it easier for

you. Visit our Web site or call us. H ere's wha t we o ffer:

You like face-to-face interaction?We offer live seminaran d tra ining courses all over the U .S. Ch eck out ASF's

seminar sched ules, as well as schedu les for upcom ing

educational opp ortun ities like the Flight I nstructorRefresher Clinics and Pinch-H itter G roun d Schools.

No time to go places?Renew your CFI online, or participate in onlineinteractive training courses.

Need hardcopies?Obtain Safety Advisors and H ighlights on a varietyof topics and aircraft types by order ing online or by phone. These

publications ar e also ava ilable on our Web site.

How do we do it? We do nt you d o. Tax d edu ctible contribut ionsfrom d onor pilots like you, and a nu mber of corpor ate sponsors fund

the free and low cost prog ram s that are m ad e available to ALL pilots.Shou ld you consider a d ona tion to ASF, please visit our Web site or call

us at 800/638-3101.

Take advantage of the quick and easy access to information thatcan help make your flying safer, more fun, and as inexpensiveas possible.

AOPA Air SafetyFoundation

421 Aviation WayFrederick, MD 21701

800/638-3101www.asf.org

A

SF

w w w . a s f . o r g


16/16

Is your propeller airworthy?

When was it last inspected?

Propeller Maintenance:

How important is it?Propeller maint enance cannot be neglected without risking accidentand injury.

I t s far more com mon now than 1 0 years ago to have a blade fai l the overhaul guidelines, simply as aresul t of the aging f leet.

Do you know:

What the root cause of most mechanica l ly induced acc idents is? What s i ze n ick o r sc ra tch needs to be d ressed out immed ia te ly? Ho w t o c he ck t h e ma gs t o se e if t h e p ro p i s h ot ?

The d if fe rence be tween a f ixed-p it chand constant-speed propeller?

How a go ve rno r work s?

Find the answers to these questions in this Safety Advisoron Propeller Safet y from the AOPA Air Safet y Foundation.

Education is the best insurance a pilot can have.

Learn with the general aviation safety experts.

AOPA Air Safety Foundation421 Aviation Way, Frederick, MD 21701

For information about other AOPA Air Safety Foundation safety materials or seminars, call us at

1-800-638-3101, e-mail us at [email protected], or visit our Web site at www.asf.org

In the interest of greater aviation safety,

thisSafety Advisorhas beenunderwritten by Hartzell Propeller, Inc.

SA06-3/02

Cracked hub

Ni cked and cracked blade

sa06 propeller safety

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