Instrument Systems

Instrument Systems

• Vacuum System• Pitot-static system• Magnetic System

Pitot-Static System

• Types of Pressure• System components

Two types of Pressure

• Dynamic Air Pressure• Pitot or ram pressure• Supplied by pitot tube• Location varies• Needs to face directly

into the relative wind• Pressure caused by

moving air• Only linked to

airspeed indicator

• Static Air Pressure• Also, ambient static

air pressure• Supplied by static

port• Location varies• Needs to be in

undisturbed air• Pressure just outside

of the airplane• Linked to all Pitot

Static instruments

Components of system

• Pitot Pressure Lines• Connects pitot tube to airspeed indicator• Needs to run direct• Sump in lowest point collects moisture

• Static Pressure Lines• Connect to all three• Has sump in lowest lines

Pitot-Static System

Pitot Static Flight Instruments

Pitot-Static Instruments

• Airspeed Indicator• Altimeter• Vertical Speed Indicator

Airspeed Indicator

Airspeed Indicator

• Displays the speed of the aircraft through the air

• Only instrument that uses both types of pressure• Measures the

difference between the two pressures

• Greater the difference the greater the airspeed

Airspeed Indicator Operation

• Instrument is contained within a sealed case

• Pressure sensitive diaphragm• Ram pressure line is connected

directly to one side of the diaphragm

• Diaphragm expands and contracts due to ram pressure

• Inside of the case is vented to the static port

Inside airspeed indicator

Airspeed Indicator Operation• Diaphragm expands and contracts in

proportion to the difference between the two pressures• Measured by mechanical linkage• Linkage is displayed by the hands on the

face of airspeed indicator

Airspeed Indicator

Airspeed Definitions

• Indicated Airspeed• Value read from the indicator• indicated stall speeds remain constant• Uncorrected for installation(position)

and instrument error

Airspeed Definitions

• Calibrated Airspeed• Indicated Airspeed corrected for installation and

instrument error• Determine from looking in the POH

• True Airspeed• True speed of aircraft through the air• Calibrated corrected for altitude and nonstandard

temperature

Airspeed Definitions

• Ground Speed• Actual speed of the aircraft over the

ground• True airspeed adjusted for the wind• Found using the E6B

Airspeed Definitions

Airspeed Indicator

• White Arc• Green Arc• Yellow Arc• Red Line

Airspeed Errors

• Position Error• Occurs when the static port sense an

erroneous static pressure• Mainly caused by slipstream

• Error may be determined by using the airspeed calibration chart

• Instrument Error• Errors due to imperfections in the

instrument itself, imperfections with manufacturing

Altimeter

Altimeter

• System Operation• Types of Altitude• Markings• Errors

Altimeter

• Simply a barometer that measures static pressure of the air around the aircraft.

• Uses only the static pressure• Operates by the changes in pressure• Standard pressure at Mean Sea Level in

29.92 inches of mercury• Atmosphere declines 1 inch of mercury every

thousand feet

Altimeter Operation

• Aneroid wafer• Stack of hollow, elastic metal wafers• Expand and contract as pressure changes• This is shown through mechanical linkage• Each pressure setting is a definite size on

window

Altimeter Operation

• Pressure Window• Kollsman window• Small adjustable

subscale that allows the current altimeter setting to be set in

• Important to reset with current

• Above 18,000’ always set at 29.92

Altimeter Operations

Altimeter Functions

© UND Aerospace, 1994

Air moves out

Air moves in

Wafers expand

0 1 2 3

4 5

9 8 7

6

Wafers contract

0 1 2 3

4 5

9 8 7

6

0 1 2 3

4 5

9 8 7

6

Types of Altitude

• Indicated Altitude• Read from Indicator

• Pressure Altitude• Height above standard datum

• Density Altitude• Pressure corrected for nonstandard

temperatures

Types of Altitude

• True Altitude• True height above

sea level• Airports and

obstruction are based on

• Absolute Altitude• Actual height above

surface

True Altitude

Absolute Altitude

Pressure Altitude

Indicated Altitude

Pressure = 29.92" Hg Standard Datum Plane

© UND Aerospace, 1994

Altimeter

Altimeter Errors

• Pressure Error• High to Low-Look out below, low to high plenty of

sky• Need to set in current altimeter setting

Vertical Speed Indicator

VSI

• System Operation• Markings• Errors

Vertical Speed Indicator

• Provides reference to rate of change• Will show trend away from level quickly• Responds faster then the altimeter• Shows both rate and trend

• Uses only static pressure

VSI Operations

• Expandable Capsule• Directly connected to static port• Connected through mechanical linkage

• Calibrated Leak• Instrument Case’s connection to the static

port• Allows capsule to change pressure more

gradually

VSI Operation

• Pressure inside of capsule changes the same as the outside air

• Pressure in instrument case changes slower because of calibrated leak.• Gives us the rate• When pressure is equal straight and level

VSI Operations

Types of Information Portrayed• Trend Information

• Immediate indication of an increase or decrease

• First Indication

• Rate Information• Shows the stabilized

rate of change• Take 6-9 seconds

Markings

VSI Errors

• Abrupt changes cause errors• Rough control and turbulent air cause

error

Vacuum System

System Operation

•Vacuum System

•Draws air through the filter system•Moves through Attitude and Heading

indicator where it spins gyros•Spins at 18,000 RPM

•Air continues into engine driven vacuum pump

System Operation

Gyroscopic Principles

• Rigidity in Space• Remains in a fixed

plane when spinning• Gimbal instrument

around gyro to allow it remain in plane able to show changes in pitch and attitude

Gyroscopic Principals

• Precession• When outside force is

applied to gyro it will be felt 90 degrees in rotation of spinning

• Includes friction

Gyroscopic Instruments

• Heading Indicator• Attitude Indicator• Turn Cordinator

Heading Indicator

Heading Indicator

• Operation• Markings and Use• Limitations and Errors

Heading Indicator Operation• Relies on Rigidity on Space• Primary source of Heading information• Senses rotation along the vertical axis• Gyro spins in the horizontal axis

• Support gimbals drive the compass card• Works through gears and linkage

• Setting knob

Heading Indicator Operation

Markings & Use

Limitations

• Reset every 15 minutes• Pitch - 55 degrees• Bank - 55 degrees

Heading Errors

• Precession• Can be a negative in Heading indicator• Causes the heading to drift• Should check every 15 minutes

• Make sure you are in straight and level, unaccelerated flight

• Tumbling• Occurs after excessive pitch and roll

Attitude Indicator

• Operation• Markings and Use• Limitations and Errors

Attitude Indicator Operations• Mechanical Substitute for the natural

horizon• Gives immediate and direct

information of plane’s pitch and bank• Gyro spins in the horizontal plane• Self erecting mechanisms• Vacuum Driven, normally

Attitude Indicator Operations

Markings & Use

Climb

Climb and Left Bank

Level Flight

Glide

Glide and Left Bank

Level Flight and Left Bank

Attitude Indicator Errors• Usually very minor• Minor on acceleration and deceleration• Somewhat precesses on turns• Errors are maximum when rolling out of

a 180 degree or 360 degree turn• Instrument Tumbling (older AI)

• Caging mechanism• May take awhile for it to re-erect itself• After 100 degrees of bank and 60 degrees

of pitch

Turn Indicators

• Two Types• Turn and Slip

• Operation• Markings

• Turn Coordinator• Operation• Markings

Turn & Slip Indicator

L R L R

TURN 2 MIN

BANK

Turn Coordinator

• Operation• Like the Turn and Slip• But shows both rate of turn and rate of roll• Gimbal is set at a 30 degree angle

• Allows force to be felt• Allows gyro more movement

Turn Coordinator

Gyro gimble (mount not shown)

Canted gyro axis

Parallel to longitudinal axis of a/c

Gyroscope rotor

Index marks

Inclinometer

Aircraft silhouette

Turn Coordinator

Operation

Inclinometer

• Contains fluid and ball• Kerosene type fluid• Steel ball

• Shows the quality of the turn (Coordination)

• Shows forces acting ball in the turns

Quality of Turn

Turn Coordinator

Turn Coordinator Skid

Slip

Magnetic Compass

Magnetic Compass

• Construction• Diagram and function• Markings and use• Compass Errors• Use of compass

Magnetic Compass Construction

The Earth’s Magnetic FieldMagnetic Pole Geographic Pole

Magnet aligns itself with magnetic force

Lines of magnetic force

The Earth's Magnetic Field

Compass Errors

• Variation• Deviation• Magnetic Dip

• Northerly Turning Error• Acceleration/Deceleration Error

• Oscillation Error

Variation

• Variation - Angular difference between true north and magnetic north.

• Agonic line - The line where there is no angular difference.

• Isogonic - Lines showing the angular lines difference.

Variation

13 North Magnetic Pole

Geographic North Pole o

Isogonic lines Agonic

line

20 o

15 o

10 o

5 o

o 0

5 o

10 o

15 o

20 o

Compass Card

• Deviation• Error due to

magnetic interference within the aircraft

• Compensating magnets in compass help to counteract

• Called Swinging

• Error on compass correction card

Compass Card

For N 30 60 E 120 150

Steer 357 023 050 080 111 145 For S 210 240 W 300 330Steer 178 213 246 278 307 333

Magnetic Dip

• How it works• Errors it causes

Magnetic Dip

• Most significant error

• Difficult to get actual readings

• Magnet in compass tries to point 3Ds to pole

• Causes errors in turns and acceleration

How it Works

Magnetic flux lines point downward at the poles, compass magnets dip to low side of turn

Magnetic Dip

Aircraft Flight Path

Dip North Pole

No Dip

Errors it Causes

• Acceleration error - ANDS• Northerly turning error

• North lags• SOS - south over shoot

Acceleration Error

• Accelerate• Will show a turn to

the North• When speed

stabilizes compass returns to accurate

• Error greatest on headings of West and East

• Deceleration• Will show a turn to

the South• Use ANDS

• Accelerate North Decelerate South

• 01-185

E 6 3 N

E 6 3 N

N

N N

Constant Airspeed

Accleration

Deceleration

S15

12E

Acceleration & Deceleration Errors ~ ANDS

Turning Errors

• Northerly Error• Initially indicate turn to opposite direction

• Southerly Error• Heading will lead the turn

Questions?