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Instrument Systems
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Instrument Systems
• Vacuum System• Pitot-static system• Magnetic System
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Pitot-Static System
• Types of Pressure• System components
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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
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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
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Pitot-Static System
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Pitot Static Flight Instruments
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Pitot-Static Instruments
• Airspeed Indicator• Altimeter• Vertical Speed Indicator
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Airspeed Indicator
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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
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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
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Inside airspeed indicator
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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
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Airspeed Indicator
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Airspeed Definitions
• Indicated Airspeed• Value read from the indicator• indicated stall speeds remain constant• Uncorrected for installation(position)
and instrument error
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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
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Airspeed Definitions
• Ground Speed• Actual speed of the aircraft over the
ground• True airspeed adjusted for the wind• Found using the E6B
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Airspeed Definitions
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Airspeed Indicator
• White Arc• Green Arc• Yellow Arc• Red Line
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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
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Altimeter
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Altimeter
• System Operation• Types of Altitude• Markings• Errors
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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
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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
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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
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Altimeter Operations
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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
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Types of Altitude
• Indicated Altitude• Read from Indicator
• Pressure Altitude• Height above standard datum
• Density Altitude• Pressure corrected for nonstandard
temperatures
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Types of Altitude
• True Altitude• True height above
sea level• Airports and
obstruction are based on
• Absolute Altitude• Actual height above
surface
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True Altitude
Absolute Altitude
Pressure Altitude
Indicated Altitude
Pressure = 29.92" Hg Standard Datum Plane
© UND Aerospace, 1994
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Altimeter
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Altimeter Errors
• Pressure Error• High to Low-Look out below, low to high plenty of
sky• Need to set in current altimeter setting
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Vertical Speed Indicator
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VSI
• System Operation• Markings• Errors
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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
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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
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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
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VSI Operations
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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
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Markings
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VSI Errors
• Abrupt changes cause errors• Rough control and turbulent air cause
error
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Vacuum System
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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
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System Operation
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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
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Gyroscopic Principals
• Precession• When outside force is
applied to gyro it will be felt 90 degrees in rotation of spinning
• Includes friction
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Gyroscopic Instruments
• Heading Indicator• Attitude Indicator• Turn Cordinator
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Heading Indicator
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Heading Indicator
• Operation• Markings and Use• Limitations and Errors
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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
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Heading Indicator Operation
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Markings & Use
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Limitations
• Reset every 15 minutes• Pitch - 55 degrees• Bank - 55 degrees
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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
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Attitude Indicator
• Operation• Markings and Use• Limitations and Errors
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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
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Attitude Indicator Operations
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Markings & Use
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Climb
Climb and Left Bank
Level Flight
Glide
Glide and Left Bank
Level Flight and Left Bank
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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
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Turn Indicators
• Two Types• Turn and Slip
• Operation• Markings
• Turn Coordinator• Operation• Markings
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Turn & Slip Indicator
L R L R
TURN 2 MIN
BANK
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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
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Turn Coordinator
Gyro gimble (mount not shown)
Canted gyro axis
Parallel to longitudinal axis of a/c
Gyroscope rotor
Index marks
Inclinometer
Aircraft silhouette
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Turn Coordinator
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Operation
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Inclinometer
• Contains fluid and ball• Kerosene type fluid• Steel ball
• Shows the quality of the turn (Coordination)
• Shows forces acting ball in the turns
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Quality of Turn
Turn Coordinator
Turn Coordinator Skid
Slip
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Magnetic Compass
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Magnetic Compass
• Construction• Diagram and function• Markings and use• Compass Errors• Use of compass
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Magnetic Compass Construction
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The Earth’s Magnetic FieldMagnetic Pole Geographic Pole
Magnet aligns itself with magnetic force
Lines of magnetic force
The Earth's Magnetic Field
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Compass Errors
• Variation• Deviation• Magnetic Dip
• Northerly Turning Error• Acceleration/Deceleration Error
• Oscillation Error
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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.
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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
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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
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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
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Magnetic Dip
• How it works• Errors it causes
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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
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How it Works
Magnetic flux lines point downward at the poles, compass magnets dip to low side of turn
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Magnetic Dip
Aircraft Flight Path
Dip North Pole
No Dip
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Errors it Causes
• Acceleration error - ANDS• Northerly turning error
• North lags• SOS - south over shoot
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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
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• 01-185
E 6 3 N
E 6 3 N
N
N N
Constant Airspeed
Accleration
Deceleration
S15
12E
Acceleration & Deceleration Errors ~ ANDS
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Turning Errors
• Northerly Error• Initially indicate turn to opposite direction
• Southerly Error• Heading will lead the turn
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Questions?