Aviation Instrumentation Training Guide

FOR TRAINING PURPOSE ONLY
Subject Code: AFD 31202
Malaysian Institute of Aviation Technology
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AFD 31202
AV2250
ELECTRONIC FLIGHT
INSTRUMENTATION SYSTEM

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CHAPTER 1
FLIGHT INSTRUMENTATION

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Table of Contents
LEARNING OBJECTIVES
INTRODUCTION
Angle of Attack System
Pitot Static System
Central Air Data Computer (CADC)
ALTITUDE INDICATOR SYSTEM
Altimeter principle
Encoding Altimeter
Altitude Alerter
AIRSPEED INDICATOR
Airspeed Indicator principle
Maximum Allowable Airspeed / Mach Indicator
True Air Speed (TAS) / Static Air Temperature Indicator

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Table of Contents (cont’d)
VERTICAL SPEED INDICATOR
Vertical Speed Indicator Principle
Instantaneous Vertical Speed Indicator (IVSI)
COMPASS SYSTEM
Magnetic Compass
Compass Errors
Flux Gate Compass
ATTITUDE DIRECTOR INDICATOR (ADI)
TURN AND BANK INDICATOR
HORIZONTAL SITUATION INDICATOR (HSI)

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Learning Objectives
Upon completion of this chapter, you will be able to:
State the functions of the flight instrumentation.
Explain the principle of Altitude instrument system,
- Radio/Barometric encoding altimeter system
- Altitude Alerter.
Explain the principle of airspeed instrument system,
- Airspeed Indicator
- Maximum-allowable airspeed/ Mach indicator
- TAS (true airspeed)/SAT (static air temperature) indicator.
Explain the principle of vertical speed instrument system
- Instantaneous Vertical Speed Indicator

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Learning Objectives (cont’d)
Explain the principle of compass instrument system.
- Magnetic Compass
- Flux Gate (Flux Detector) Compass.
Explain the principle of Horizontal Situation Indicator.

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Introduction
There are two group of basic instruments commonly found in aircraft:
– Navigation instruments – Provide information which enables the pilot to guide
the aircraft accurately along a given course.
– Flight instruments - Aiding the pilot in controlling the altitude, attitude,
airspeed, and direction of the aircraft.
Navigation instruments consist of:
– The Automatic Direction Finder (ADF)
– Very High Omni Range (VOR)
– Instrument Landing System (ILS)
– Distance Measuring Equipment (DME)
– Weather Radar
Flight instruments consist of:
– The Barometric Altimeter (Alt)
– Airspeed Indicator (ASI)
– Turn-and-Slip Indicator / Turn Coordinator
– Magnetic Compass
– Artificial Horizon (AH) / Attitude Director Indicator (ADI)
– Horizontal Situation Indicator (HSI)

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Aircraft Instrument Panel

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Angle-of-Attack System
The angle-of-attack indicating system:
– Detects the local angle of attack of the aircraft from a point on the
side of the fuselage
– Provide reference information for the control and actuation of other
systems in the aircraft
Signals from the angle-of-attack transmitter are used:
– For continuous visual indication to pilot
– provides electrical signals to the stall warning system which
operate the stick shaker when the aircraft is approaching stall
Electrical switches are actuated at the angle-of-attack indicator at
various preset angles-of-attack.
The angle-of-attack indicating system consists of:
– An airstream direction detector / AOA transmitter
– AOA indicator

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Angle-of-Attack System (cont’d)
AOA indicator
AOA transmitter
Airstream direction detector sensing element measures local airflow
direction relative to the true angle-of-attack by determining the angular
difference between local airflow and the fuselage reference plane.
The sensing element operates in conjunction with a servo-driven
balanced bridge circuit which converts probe positions into electrical
signals.

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The operation of the angle-of-attack sensing element is based on
detection of differential pressure at a point where the airstream is
flowing in a direction that is not parallel to the true angle-of-attack of
the aircraft.
– This differential pressure is caused by changes in airflow around
the probe unit which extends through the skin of the aircraft into the
airstream.
The exposed end of the probe contains two parallel slots which detect
the differential airflow pressure.
Air from the slots is transmitted through two separate air passages to
separate compartments in a paddle chamber.

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Any differential
pressure, caused by
misalignment of the
probe with respect to
the direction of
airflow, will cause the
paddle to rotate.

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The moving paddles will rotate the probe, through mechanical linkage,
until the pressure differential is zero.
– This occurs when the slots are symmetrical with the airstream
direction.
Two electrically separate potentiometer wipers, rotating with the
probe, provide signals for remote indications.
Probe position, or rotation, is converted into an electrical signal by
potentiometers which are the transmitter component of a self-
balancing bridge circuit.
When the angle-of-attack of the aircraft is changed:-
– The position of the transmitter potentiometer is altered.
– An error voltage exists between the transmitter potentiometer and
the receiver potentiometer indicator.

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Current flows through a sensitive polarized relay to rotate a
servomotor in the indicator.
The servomotor drives a receiver/ potentiometer in the direction
required to reduce the voltage and restore the circuit to an
electrically balanced condition.
The indicating pointer is attached to, and moves with, the
receiver/potentiometer wiper arm to indicate on the dial the relative
angle-of-attack.

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Some of the Flight instruments that operate using Pitot and Static
pressure are:-
– Altimeter
– Airspeed Indicator
– Rate-of-Climb Indicator
Static air pressure is the ambient air pressure around the airplane
Pitot air pressure is the air pressure in the Pitot probe tube as a result
of the forward motion of the airplane.
ICAO standard atmosphere state that at Sea Level (0 ft):-
– Press 29.92 in Hg / 1013.25 Mb / 14.7 psi
– Temp 15°C
Atmospheric pressure and temperature decreases as altitude
increases.
Pitot-Static System

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Pitot pressure provides impact air pressure;
– The pressure of the airstream against the aircraft as it flies
through the air.
– Is called dynamic pressure [(Q = ½ ρV²), Q - Dynamic Pressure,
ρ (rho) - density of air, V – velocity of aircraft]
The head is mounted :
– On the outside of the aircraft in a forward direction parallel to the
aircraft's line-of-flight.
– In an area where the air is least likely to be turbulent, such as the
leading edge of the wing or the nose section.
Pitot-Static System (cont’d)

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Figure above show a pressure head which sense pitot and static
pressure.
Baffle plate - to prevent moisture and debris from entering the pitot tube.
Drain hole – remove moisture and located at the bottom of the unit.
The pitot tube leads back to a chamber near the rear of the assembly.
A riser, or upright tube, routes the air from this chamber through tubing in
the aircraft to all the instrument that required it.
Small openings on the top and bottom surfaces of the pressure head
allows still (static) air to enter into the static air chamber.

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A riser tube in the static chamber provides a sample of this atmospheric
pressure to all the indicator that required
Heating element is located within the head to prevent the unit from
becoming clogged due to icing conditions experienced during flight.
The heater is controlled by means of a switch in the cockpit.
An alternate method of deriving pitot and static pressure may be
obtained from separate pitot tubes and static vents installed in different
locations on the aircraft.
– left and right static ports are paralleled together to compensate for
any variation in static pressure due to erratic changes in the aircrafts
attitude.
Pitot-Static System must be tested for leaks upon installing equipment
that requires connection to pitot or static lines.

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The method of testing consists basically of:
– Applying pressure and suction to the pressure heads and static
vents respectively,
– Using a leak tester and coupling adapters
– Measuring the rate of leakage to within the prescribed tolerances for
the system.

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When performing leak test,
pressure and suction should be
applied and released slowly to
avoid damage to the instruments.
Should an electric flight
instrumentation system be
installed, standby altitude and
airspeed instruments must be
provided
– It operated from the pitot-static
source in the event a failure
should occur in the ADC or
aircraft electrical system.

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Aircraft that operate at high speeds and high altitudes can have
significant errors in the pitot-static system instruments if it uses a simple
system.
– An Air Data Computer (ADC) is used to operate:
Airspeed indicator
Altimeter
VSI
Any other systems that require this data
The air data computer is placed in the system between the sensor ports
and the instruments to automatically apply corrections in order to
increase accuracy.
Central Air Data Computer (CADC)

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The air data computer has three inputs;
– Pitot pressure from the pitot tube
– Static pressure from the static ports
– Total air temperature (TAT) from a special
TAT probe
TAT measurement is needed to:
– correct the instrument indications for friction
heating of the air at high speeds.
– TAT probe also permits the calculation of
SAT (static air temperature) which is used to
apply corrections for non-standard
temperatures for any flight altitude.
The outputs of the air data computer is supplied
to a number of aircraft systems.
Central Air Data Computer (cont’d)

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High speed jet airplanes require a
machmeter; this could be a separate
instrument or included with the
airspeed indicator.
Aircraft with an air data computer
usually have a digital display on the
instrument panel which gives:
– Calculated true airspeed
– Total air temperature or static air
temperature.
TAT includes the heating effect of the
friction at high speed whereas SAT is
just ambient outside air temperature.
An air data computer provides more
accurate readings on the pitot-static
instruments for high performance aircraft

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The temperature indications are especially important for turbine
engines
– They are affected by the temperature of the intake air.
The air data computer system automatically compensates for both
temperature effects and the compressibility of the air at higher Mach
numbers.
– This helps to ensure accurate instrument readings throughout a
wide range of altitudes and airspeeds.
The altitude outputs supplied by the CADC are corrected for
instrument scale error and static defect correction is provided as a
function of mach.

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A typical CADC supplies the following outputs in accordance with
ARINC specification 565:
1. An encoded digit pressure altitude output, in increments of 100 feet,
for use by an ATC transponder for automatic altitude reporting.
2. A dual-synchro coarse and fine altitude output for driving an electric
servo altimeter.
3. Altitude rate output for driving an electric vertical speed indicator.
4. Outputs for operating electric mach, true airspeed, and static air
temperature indicators.
5. Altitude, airspeed, and mach hold functions for use by the flight
director system in computing vertical guidance steering commands
for driving the autopilot elevator servo.
6. A failure monitor signal to actuate a malfunction indicator flag in the
systems receiving information from the CADC.

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The Kollsman model 621 is a
typical example of a standard
ARINC CADC unit presently
being used on large corporate
and commercial transport aircraft.
The principle functional
components of the Kollsman
CADC consist of
– two servo-operated
diaphragm displacement
transducers,
– a mach follow-up servo, and
– the associated servo
amplifier/failure monitor
cards.
Central air data computer

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The altitude transducer servo drives:
– Fine and coarse output synchros
– An altitude digitizer encoder
– An altitude scale error and function potentiometer used in computing
mach.
The altitude hold functions are also geared to this servo shaft.
The airspeed transducer servo positions an airspeed scale error
potentiometer and an airspeed function potentiometer, also used in
computing mach.
The airspeed function potentiometer provides the signal for the mach
servo.
The mach servo shaft positions the potentiometer which provides the
static source error correction as a function of mach.
– It also provides the drive for the mach hold and the mach output
synchros.

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A change in static pressure produces an error signal in the altitude
transducer.
In repositioning the transducer to null the error signal, there are
components to be driven to provide the desired outputs such as:
– The altitude potentiometer
– Coarse and fine synchros
– Two clutched synchros
– The digitizer
The potentiometer provides an output for the computation of mach to:
– The altitude scale error,
– Linear altitude, and
– Altitude function potentiometers.

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Summed into the input network of the altitude servo is a static source
error correction signal which is supplied by the mach servo.
The airspeed transducer has a follow-up servo similar to the altitude
servo.
– This servo produces a shaft output linear with airspeed.
The shaft positions the potentiometer, accepts the airspeed scale error
correction and supplies an output to the linear airspeed potentiometer
and the airspeed function potentiometer used in computing mach.
– Also geared to this shaft is the airspeed hold clutch servo.
Mach is computed by altitude and airspeed functions to provide the shaft
position for the static source error correction signal, which is used in the
correction of altitude.
– Also geared to the mach shaft are the mach output synchro and the
clutched synchro for mach hold.

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Altitude Indicator System
Although there are many different types of Altimeters in service, all
pneumatic altimeters operate on the principle of an aneroid capsule,
which expands or contracts with changes in atmospheric pressure.
As the aircraft altitude increases, the static source senses the
decrease in atmospheric pressure and causes the aneroid to expand.
– This movement of the aneroid actuates mechanical linkages
which drive the pointer to the proper altitude scale reading on the
indicator.
A bi-metallic yoke surrounding the aneroid compensates for
inaccuracies caused by variations in temperature.
Altimeter Principles

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The most common source of altimeter error is caused by the scale not
correctly oriented to the standard pressure conditions.
Since the atmospheric pressure continually changes in level flight, the
Altimeter scale must be calibrated to the local barometric setting
before the Altimeter will indicate the correct altitude of the aircraft
above sea level.
When the barometric correction knob on the face of the instrument is
turned, the pointer and aneroid mechanism move to the new altimeter
setting.
Two other types of error are also common:
– Hysteresis error
– Static port alignment error
Altimeter Principles (cont’d)

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Hysteresis error is induced by sudden changes in altitude which results
in a lag in altimeter response due to the elastic properties of the
materials which comprise the instrument.
– This error will eliminate itself after a new altitude has been
maintained.
Another source of error is caused by the change in alignment of the
static port with regard to the relative wind.
The magnitude of error caused by static port misalignment will vary with
the aircraft's speed and angle-of-attack.
An unpredictable erroneous indication will be caused by installation of a
static port on the fuselage where a disturbed air pressure field exists.
Altimeter Principles (cont’d)

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Typical pneumatic altimeter mechanism

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The Kollsman RADBAR (RADio-BARometric) Electropneumatic
Encoding Altimeter System is designed to provide both barometric and
radar altitude information in a single instrument.
RADBAR Encoding Altimeter System
RADBAR altimeter system

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The barometric section of the Altimeter:
– Measures and displays static-defect-corrected pressure altitude
– Encodes that altitude into a binary code output
It is sent to the onboard radio beacon Transponder for
automatic altitude reporting (when operating in Mode C).
Synchro outputs of the baro-corrected altitude are provided for:
– Altitude Alerter
– Flight Management System
– Long-Range Navigation System
An altitude rate signal is provided to drive an electric Vertical Speed
Indicator.
RADBAR Encoding Altimeter System (cont’d)

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The basic mechanism of the
barometric section of the
RADBAR indicator is:
– a servo operated,
– diaphragm displacement,
– follow-up transducer.
By sensing the diaphragm
motion through an inductive
pickoff, very accurate repeatable
altitude measurements are made
and presented on the altimeter
counter-drum pointer display.
RADBAR altimeter

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The pointer makes one revolution per 1,000 feet and graduations are
at 20-foot increments.
The counter reads altitude in thousands and hundreds of feet.
The knob located in the lower right-hand corner of the RADBAR
Altimeter permits setting of the local barometric pressure in both inches
of mercury and millibars.
Setting of the barometric scale knob does not affect the reported
altitude information sent to the Transponder, but only the indicated
altitude reading on the Altimeter.
Reported altitude is always the altitude relative to standard barometric
pressure at sea level (29.92 inches of mercury).

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Depressing the decision height (DH) knob on the top right-hand comer
of the RADBAR indicator will cause the radio altitude display to readout
the preset decision height.
Depressing and rotating the knob allows the new DH to be set into the
system.
Whenever the DH knob is depressed to display the DH altitude, the
visual and aural DH warning circuit is self-tested.
A self-test button is also included to test both the barometric and radio
altitude sections of the altimeter.
Depressing the self-test button displaces the pointer approximately 300
feet, and causes the digital readout to display all “8s".
Releasing the button initiates a test of the complete radio altimeter
system.
Decision Height and Self-Test

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The optical encoder in the barometric section of the altimeter uses
light-emitting diodes as light sources and photo-Darlington transistors
as light sensors.
An encoding disk with nine concentric tracks of clear and opaque
sections is positioned between the LEDs and photo sensors.
As the disk is rotated by the altimeter aneroid mechanism, the light
passing through the clear sections of the disks tracks the altitude
changes.
For every 100 feet of altitude, a new combination of light patterns
reaches the sensors to produce the coded altitude information which is
processed into a binary code output and sent to the Transponder.
Altitude Digitizer

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The radio altimeter section of the RADBAR altimeter is connected to a
radio altitude digital-to-analog converter to provide:
– a three-digit,
– seven-segment,
– incandescent digital readout of the absolute altitude, or
– height of the aircraft above the terrain.
The radio altitude converter receives:
– analog signals from the radio altimeter RT, and
– glideslope signals from the ILS navigation receiver.
The radio altitude signals are converted to digital form to:
– drive the radio altimeter display in the RADBAR altimeter, and
– provide voice terrain advisory outputs of radio altitude.
Radio Altitude Readout with Voice Terrain Advisory (VTA)

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The localizer and glideslope signals are also converted to provide
voice output warnings of excessive localizer and/or glideslope
deviation from center on ILS approaches.
In actual operation, when the aircraft is on the ground:
– the radio altitude section will display zero feet, and
– a placard will show the legend "RADIO ALT'.
After take-off, the radio altitude display will indicate the height of the
aircraft above the terrain up to 990 feet.
The voice function remains silent during take-off unless a negative
radio altitude rate is sensed.
The illuminated placard, "RADIO ALT", will remain on up to an
altitude of 2,500 feet above ground level (AGL) and then turn off.
Radio Altitude Readout with Voice Terrain Advisory (cont’d)

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As the aircraft descends to the approach altitude,
– the placard will illuminate at 2,500 feet AGL, and
– the radio altitude converter synthesized voice output will announce
"TERRAIN" at 2,000 feet AGL.
Radio Altitude will be displayed in 100-foot increments down to 1,000
feet AGL.
The "TERRAIN" announcement will be repeated at the 1,000-foot level,
and shortly thereafter, the radio altitude numerical display will
illuminate at 990 feet above the terrain and begin to count down in 10-
foot increments.
At 900 feet AGL, the VTA will announce "NINE HUNDRED";
At 800 feet AGL, the announcement "EIGHT HUNDRED" will be heard.
The announcements will continue at each 100-foot interval (700, 600,
500, etc.) until touchdown.

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When the aircraft reaches the minimum decision height (DH), or the
altitude at which the pilot must declare a missed approach (“go-
around") and abort the landing if the runway is not yet visible:
– the external DH warning light will illuminate,
– the aural warning from the external tone generator will sound, and
– the VTA announcement "MINIMUM" will be heard over the cockpit
speaker.
The system also provides voice advisories for reminding the pilot to
extend the flaps and lower the landing gear.
If the aircraft deviates below the glideslope less than one dot on the
CDI or HSI, the announcement "GLIDESLOPE" will be heard at a
nominal voice level at two second intervals.
Should the aircraft deviate below the glideslope more than one dot, the
announcement "GLIDESLOPE" will be heard at above nominal voice
levels at one second intervals.

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The altitude alerter is an optional
instrument used to alert the pilot of an
approach to, or departure from, a preset
flight level by means of automatic audio
and visual signals.
The input signals required to operate the
altitude alerter consist of:
– a two-speed, baro-corrected,
synchro altitude signal from a
servoed altimeter
– a 26-volt AC 400-Hz reference
voltage from the aircraft electrical
bus instrumentation transformer.
Altitude Alerter
Altitude alerter/preselector

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The altitude alerter incorporates a five-digit display to indicate the
preset flight level.
– It can be adjusted by means of a knob located in the lower right-
hand corner of the instrument.
A three-digit counter displays thousands and hundreds of feet.
– Fixed zeros are placed in the tens and units digits.
An altitude alert light is located on the left side of the instrument to
indicate approach to or departure from the preset flight level.
In addition to altitude performance data, the alerter displays an "OFF"
flag to indicate a loss of either the altitude valid signal from the
altimeter or 26-volt AC power.
The alerter also contains dual coarse and fine differential resolver
synchros to provide an altitude preselect output for the flight control
system.
Altitude Alerter (cont’d)

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Typical altimeter/alerter wiring diagram

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In actual operation, as the aircraft approaches within 1,000 feet of
the preset flight level,
– the altitude alert light comes on and an aural tone is
momentarily heard (for approximately two seconds) through an
external tone generator.
The light, however, remains on until the aircraft crosses 300 feet
of the preset altitude, at which time it extinguishes.
Should the aircraft deviate from the preset altitude by more than
plus or minus 300 feet, the aural tone and alert light will again be
activated.
The light will then remain on until the aircraft returns to within 300
feet of the preset altitude, or until the pilot selects a new altitude
setting on the alerter indicator.
Altitude Alerter (cont’d)
Flight Test Procedures

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Airspeed Indicator System
The airspeed indicator is basically a
differential pressure gage which
measures and indicates the
difference between pitot and static air
pressures.
The airspeed indicator consists
primarily of a sensitive metallic
diaphragm that responds to pitot-
static pressure differential and
multiplies this movement through
mechanical linkages to indicate the
aircraft's speed on the dial face in
terms of knots or miles per hour.
Airspeed Indicator Principles

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The majority of airspeed
indicators are marked to
indicate speed limitations at
a glance.
– The never-exceed
velocity is designated by
a red radial line;
– A yellow arc designates
the cautionary range;
– A white arc is used to
indicate the range of
permissible limits for flap
extension.
Airspeed Indicator Principles (cont’d)

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VSO – stall speed / minimum steady
flight in landing configuration
(lower limit of white arc)
VFE – max. flap-extended speed (upper
limit of white arc)
VS1 – stall speed in specified
configuration (lower limit of green
arc)
VNO – max. structural cruising speed
(top of green arc, bottom of
yellow arc)
VNE – never exceed speed (upper limit
of yellow arc, marked in red)
VLE – max. landing gear-extended
speed.

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Another type of airspeed indicator
commonly used is the maximum-
allowable airspeed indicator.
– This indicator includes a maximum-
allowable needle, which shows a
decrease in maximum-allowable
airspeed with an increase in altitude.
The needle operates from an extra
diaphragm,
– It senses changes in altitude, to
provide an indication of the
maximum-allowable airspeed at any
altitude.
Maximum Allowable Airspeed Indicators

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Machmeters:
– installed onboard high-performance aircraft
– indicate the ratio of aircraft speed to the speed of sound at the
particular altitude and temperature existing at any time during
flight
A machmeter usually consists of:
– a differential pressure capsule which senses pitot-static pressure
– an aneroid capsule which senses static pressure.
 The actions of these capsule are mechanically resolved to
display mach numbers on the face of the instrument.
Machmeters

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Combined airspeed/mach indicators:
– usually provided where instrument
space is at a premium
– it is desirable to present this information
on a combined indicator
These instruments display indicated
airspeed, mach, and limiting mach by
means of differential pressure and aneroid
diaphragms.
The combined airspeed/mach indicator
uses a single needle which indicates
airspeed on a fixed scale and mach on a
rotating scale.
A knob, located on instrument is provided
to set a movable index marker to reference
a desired airspeed/mach setting.
Combined Airspeed/Mach Indicators

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The Kollsman maximum-
allowable airspeed/mach
indicator is intended for
use in subsonic aircraft
to provide:
– airspeed
– mach
– maximum allowable
airspeed indications
in a single instrument
Maximum-Allowable Airspeed/Mach Indicator

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The indicator comprises:
– two separate capsule
mechanisms
– a stationary airspeed dial
– a moving mach sub-dial
– a single airspeed pointer
– a maximum-allowable
airspeed indication
– an index marker which can
be rotated by means of a
knob in the lower left-hand
corner of the instrument
Maximum-Allowable Airspeed/Mach Indicator (cont’d)

Issue No : 000
A white pointer indicates:
– Airspeed against a fixed
subdial calibrated from 60
knots to 400 knots
– Mach number against a
moving mach subdial
calibrated from 0.3 mach to
1.0 mach.
Maximum-allowable airspeed is
indicated on the dial by a yellow
and red marker.

Issue No : 000
The index marker is basically a manually-set
command bug that:
– permits the pilot to select a desired approach
speed
– provides signal outputs to drive the fast/slow
speed command needle on the attitude
direction indicator
The speed command output signals are
determined by the difference between the airspeed
pointer and index marker.
The indicator also provides:
– airspeed hold
– mach hold
– AC and DC airspeed
– mach analog outputs for use in the flight
control system

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The Kollsman maximum allowable airspeed/mach indicator
comprises primarily of two mechanically independent diaphragm-
operated mechanisms;
– one responding to the differential pressure between pitot and
static pressures
– the other to static pressure only
The airspeed mechanism of the indicator is driven from a diaphragm
capsule with pitot pressure introduced into its interior,
– while the exterior of the diaphragm is subjected to the static
pressure introduced into the instrument case.
The capsule expands approximately linearly with the indicated
airspeed.
Functional Description

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The motion of the centerpiece of the capsule is transmitted to a jeweled
rocking shaft by a dual link and lever arrangement.
One link is connected to a short lever on the rocking shaft, which is at
nearly right angles to the link.
– This lever is lengthened or shortened to provide the correct
calibration of the pointer on the spread-out linear section of the dial
up to 200 knots.
At 190 knots, the second link comes into contact with its lever.
The second lever is longer than the first, and at 190 knots, is more
nearly in line with the link.
– This angular relation between link and lever converts the
diaphragm's linear motion into rotational movement, linear with the
log of differential pressure.
– This nonlinear portion of the dial above 190 knots is required in
order to correctly indicate mach.
Functional Description (cont’d)

Issue No : 000
Both links are slotted so each can over travel while the other is
active.
A hairspring on the rocking shaft provides tension on the active link
at all times.
The rocking shaft motion is amplified by a step-by-step gear pair to
turn the jeweled pointer shaft.
Damping of the airspeed pointer is accomplished by a capillary
tube through which the pitot pressure is fed into the capsule.
The indication is compensated for ambient temperature changes
by selection of materials for the frame, linkage, and capsule.
Static balance is achieved through an adjustable counterweight on
the rocking shaft and a balanced pointer shaft.

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The mach subdial is driven from an evacuated diaphragm capsule
through:
– a link
– lever
– rocking shaft
– gearup stage
The capsule expansion is linear with pressure altitude, and when
modified by link and lever, the subdial is rotated in proportion to the
log of static pressure.
In the manner of a circular slide rule, this causes the correct Mach
number for any altitude to appear opposite the corresponding
airspeed value.
The airspeed pointer indicates both mach and airspeed over the
range where the sub-dial is visible.

Issue No : 000
Modern commercial aircraft instrument panels often have
installed a true airspeed (TAS) indicator to supplement the
information provided by the airspeed/mach indicator.
True airspeed is indicated airspeed corrected for variations
in pressure altitude and temperature.
TAS is always greater than indicated airspeed (IAS),
except in extremely low temperatures at low altitudes.
TAS/SAT Indicator

Issue No : 000
A typical pneumatic TAS indicator consists of:
– an aneroid
– differential pressure diaphragm
– bulb temperature diaphragm
A typical pneumatic TAS indicator responds to changes in
– barometric pressure
– impact pressure differential
– free air temperature
The actions of the diaphragms are mechanically resolved to
show true airspeed in knots on the dial face of the indicator.
TAS/SAT Indicator (cont’d)
Pneumatic TAS Indicator

Issue No : 000
The Kollsman TAS/SAT indicator
is an electric flight instrument that
provides a digital incandescent
display of both true airspeed and
static air temperature (SAT):
– from analog signals received
from a TAS/SAT computer, or
– from an independent airspeed
network contained within the
static defect correction
module
TAS/SAT Indicator (cont’d)
Electric TAS/SAT System
TAS/SAT indicator

Issue No : 000
In poor weather, SAT indicates the potential for icing to occur.
During takeoff, it affects the amount of thrust available from the
engines and available lift due to air density.
The outside air temperature input for SAT computation is derived from
a fuselage-mounted total air temperature probe that contains a
thermistor.
The thermistor has a negative temperature coefficient, or in other
words, its resistance increases upon sensing a decrease in outside air
temperature.
At 0oC, the resistance of the temperature sensing element in the probe
is typically 500 ohms.
Within the TAS/SAT computer, a matrix of trimming potentiometers,
connected to pots on the altitude and airspeed servos, compute the
function of mach.
The combination of temperature and mach produces the required true
airspeed output.
Electric TAS/SAT System (cont’d)

Issue No : 000
Another circuit within the TAS/SAT computer uses the same
temperature probe input signal to provide a static air temperature
output.
The TAS and SAT output signals from the computer are provided to
the TAS/SAT indicator.
The TAS output is also provided to the long-range navigation system
or flight management system for computation of ground speed.
The DC SAT voltage from the computer is applied to the indicator
where a squaring operation is performed in order to linearize the
SAT voltage with temperature.
– The computer outputs the square root of SAT.

Issue No : 000
Kollsman TAS/SAT indicator functional block diagram

Issue No : 000
The 20-volt DC SAT reference signal is also squared, and then
subtracted, to provide 0 volts at 0oC.
The SAT voltage is then applied to a scaling amplifier and
measured by an analog-to-digital (A/D) converter.
The output results in a seven-segment digital display of plus or
minus SAT in degrees centigrade.
The DC TAS voltage from the computer is applied to the indicator
where a scaling function is performed, and then applied to an A/D
converter in conjunction with a 20-volt DC TAS reference signal.
– This output results in a seven-segment digital display of TAS in
knots.

Issue No : 000
Vertical Speed Indicator System
The pneumatic vertical speed indicator is a sensitive differential
pressure instrument that measures and displays the rate at which an
aircraft is ascending or descending in altitude.
The VS indicator is connected to the static system and senses the
rate of change in static air pressure.
The rate of altitude change, as shown on the indicator dial, is
– positive (clockwise rotation of the pointer) in a climb
– negative (counterclockwise pointer rotation) in a dive
In level flight, the pointer remains at zero.
Vertical Speed Indicator (VSI) Principles

Issue No : 000
The case of the instrument is airtight, except for a small connection
through a restricted passage to the static line.
Inside the sealed case is a diaphragm with connecting linkage and
gearing to the indicator pointer.
Both the diaphragm and the case receive atmospheric pressure from
the static line.
When the aircraft is on the ground or in level flight, the pressures
inside the diaphragm and the instrument case remain the same and
the pointer indicates zero vertical speed.
When the aircraft climbs, the pressure inside the diaphragm
decreases but, due to the metering action of the restricted passage,
the case pressure will remain higher which causes the diaphragm to
contract.
Vertical Speed Indicator (VSI) Principles (cont’d)

Issue No : 000
The diaphragm movement actuates the mechanism,
causing the pointer to indicate a rate of climb.
In a descent, the pressure conditions are reversed.
The diaphragm pressure becomes greater than the
pressure in the instrument case.
– This causes the diaphragm to expand which drives
the pointer mechanism to indicate a rate of descent.
When the aircraft levels off, the pressure in the
instrument case becomes equalized with the pressure in
the diaphragm causing the diaphragm to return to its
neutral position and the pointer to return to zero.

Issue No : 000
When the aircraft is ascending or
descending at a constant rate, a definite
ratio is maintained between the
diaphragm pressure and the case
pressure through the calibrated restricted
passage.
Since it requires approximately six to
nine seconds equalizing the two
pressures, a definite lag persists before
the proper vertical speed is indicated.
Also, any abrupt changes in the aircraft's
attitude may cause erroneous indications
due to the sudden change of airflow over
the static ports.

Issue No : 000
The instantaneous vertical speed
indicator is a more recent refinement
of the conventional pneumatic VS
indicator.
The lVSI incorporates acceleration
pumps to eliminate the limitations
associated with the calibrated leak.
For example, during an abrupt climb,
vertical acceleration causes the
pumps to supply extra air pressure
into the diaphragm to stabilize the
pressure differential without the
usual time lag.
During level flight and steady-state
climbs and descents, the instrument
operates on the same principle as a
conventional vertical speed indicator.
Instantaneous Vertical Speed Indicator (IVSI)

Issue No : 000
The aircraft magnetic direction indicator or compass is a completely
independent instrument.
– It does not require any electrical or tubing connections.
– It contains a compass card with magnets that line up with the
magnetic flux lines of the earth.
The following basic parts of a magnetic compass:
1. A compass card or float which is mounted on jeweled bearings. It
has numbers and direction markings so that the magnetic heading
of the aircraft can be read from the instrument.
2. The case is filled with a light oil (usually refined kerosene) which
dampens float motion and lubricates the bearings.
Magnetic Compass
Compass Systems

Issue No : 000
The parts of a liquid-filled aircraft magnetic compass

Issue No : 000
3. A diaphragm or bellows
accommodates thermal
expansion and contraction of
the fluid.
4. The compensator is two small
moveable magnets used to
adjust the compass for
deviation error.
5. The lubber line is a marker
against which readings are
taken.
Magnetic Compass (cont’d)
The face of a liquid-type magnetic compass
The indicated magnetic heading is 035o

Issue No : 000
The magnetic compass is subject to a number of errors which affect its
operation.
– These include variation, deviation, acceleration error, northerly
turning error and oscillation error.
Variation error is simply the fact that a magnetic compass will give
indications based on the magnetic north pole and not the north
geographic pole.
The normal grid lines on an aeronautical chart are in true directions
based on the geographic poles and the equator.
The north magnetic pole is hundreds of miles from the north geographic
pole.
In most locations, there will be a difference between true and magnetic
directions.
Compass Errors

Issue No : 000
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

Issue No : 000
TN MN
VAR
12
E
o 48
60
o
o
Easterly Variation
TNMN
VAR
12
W
o
72
60
o
o
Westerly Variation

Issue No : 000
Lines of variation are drawn on aeronautical charts

Issue No : 000
In Figure above shown how the amount of variation is
drawn on aeronautical charts for pilots to use.
There are some locations where true and magnetic
directions are the same.
– This would be along the line called the agonic line.
Everywhere else the pilot would consult the variation
markings on the map and add or subtract the
appropriate number of degrees to convert from true to
magnetic headings.
Acceleration error and north turning error are both a
result of compass dip.
Compass Errors (cont’d)

Issue No : 000
The earth is round so that at high latitudes in the northern
hemisphere, the compass card will tilt downward toward the
north magnetic pole.
– This compass dip causes both of these errors.
If an airplane is flying east and it accelerates, the compass will
momentarily indicate a turn to the north.
If it decelerates, it will indicate a turn to the south.
North turning error occurs when the aircraft is flying north or
south.
If a turn is made from a north heading, the compass will
indicate a turn in the opposite direction momentarily and then it
will lag behind the actual heading during the turn.

Issue No : 000
6
N 33
30
25
21 15
12
Turn Errors
Compass
briefly gives
indication of
turn in the
opposite
direction
Compass
indicates turn
in correct
direction but at
a faster rate
than actually
being turned
Aircraft heading NORTH Aircraft heading SOUTH
Magnetic
Dip
Magnetic
Dip

Issue No : 000
Turns from south will cause the compass to lead the actual heading or
indicate a higher than actual turning rate.
Oscillation error is caused by the very delicate bearings in the
compass.
In rough air, the compass will oscillate back and forth 40o, 50o or more.
The compass may never settle down as long as the turbulence
persists.
– This forces the pilot to have to estimate the actual compass
reading.
Deviation error is the most important one for maintenance technicians
because they usually perform the checks and adjustments for
deviation error.
– This error is also called magnetic influence error since it is caused
by magnetic influences within the aircraft.

Issue No : 000
All aircraft have some steel parts that may have some permanent
magnetism.
Most aircraft also have electrical circuits that can produce
electromagnetic fields.
Both of these can affect the magnetic compass and cause errors.
The compensator magnets in the compass are used to adjust this
error to a minimum.
This process is called swinging the compass.
It should be performed whenever equipment is installed that could
cause a change or when a problem with the compass accuracy is
suspected.

Issue No : 000
Earth magnetic field distorted by aircraft

Issue No : 000
Swinging the compass-the basic
procedure is:
1. Locate a compass rose on a
ramp area which is accurate
and can be used as a
reference. A compass rose is a
circle with magnetic directions
indicated as shown in Figure
on the right side.
2. Configure the aircraft for the
checks by turning on electrical
equipment and radios, running
the engines and establishing a
level attitude.
A compass rose contains the markings and
numbers needed for magnetic direction references

Issue No : 000
3. Set the compensators to zero
(there are two little screws
labeled N-S and E-W)
4. Point the aircraft north on the
compass rose and adjust the
N-S screw to zero error or as
close to zero error as possible.
5. Point the aircraft east and
adjust the E-W screw to zero
error or as close to zero error
as possible.
6. Point the aircraft south and
remove half the error.
7. Point the aircraft west and
remove half the error.

Issue No : 000
The process so far has averaged
the error for all headings. Now you
are ready to record the error.
8. Point the aircraft on all
headings every 30o, and
record the compass heading
for each.
9. Prepare a placard which lists
the deviation error at least
each 30o. Place it on or near
the compass and make a
logbook entry.

Issue No : 000
The compass correction card
is used to record the deviation
error for the aircraft's compass.
An example is shown.

Issue No : 000
A newer type of compass is
called the vertical card compass.
It operates like the other types of
magnetic compass but the
presentation on the face of the
instrument is a full compass card
which is easier to read.
It sometimes eliminates the use
of oil and employs eddy current
damping.
The vertical card compass displays a complete compass
card and is easier to read than the older type

Issue No : 000
The full compass card
presentation of the vertical card
compass makes it easier to
read.
If the pilot wants to turn to a
heading of 180o, the
presentation makes it easier to
determine if it is quicker to turn
left or right to reach that
heading.
The compensator screws can be
seen at the bottom of the vertical
card compass.
The vertical card compass displays a complete
compass card and is easier to read than the older type

Issue No : 000
The flux gate compass is a special
type of remote mounted compass
– It is more stable than a standard
magnetic compass
– It usually eliminates the problems
of acceleration and north turning
errors.
The sensor used with a flux gate
compass system is called a flux
valve or flux gate.
– It is a wheel shaped device made
of a ferrous material with three
spokes and the rim cut into three
equal parts.
Flux Gate Compass
The flux valve sensor

Issue No : 000
The excitation coil is in the
center and the pick up coils
or output coils are installed
with one on each spoke of
the flux valve core.
The excitation coil is
supplied with AC current
with a frequency of 400 Hz.
It is designed so that when
the current flow in the
excitation coil is at peak
value, the core material is
saturated.
Flux Gate Compass (cont’d)

Issue No : 000
When the current falls below
peak value, the earth's magnetic
flux lines cut across the pick up
coils and produce an output
signal in each one.
The excitation coil in effect
alternately blocks out the earth's
magnetic field and then allows it
to move across the output coils.
– This produces an AC output
signal from each of the three
output coils.

Issue No : 000
Since the angle of the earth's flux lines to the flux valve changes for
each different heading, the relative values of the three output
voltages will be different for each different heading.
The changing angle of the earth's flux lines

Issue No : 000
An electronic component measures the three output signals and derives
the magnetic heading of the aircraft.
In order to give accurate readings, the flux gate sensor must normally
be maintained in a level, horizontal position with respect to the Earth's
surface.
– This leveling can be accomplished in one of two ways.
In the first type of flux gate sensor found on aircraft, the sensor is
suspended by a pendulous mechanism so that it can remain level when
the aircraft attitude is changed.
– This type of flux gate has a housing filled with a light oil to dampen
the motions of the moving parts.
In the second type of flux gate compass system, the flux gate sensor is
stabilized by a gyro system to keep it level.

Issue No : 000
The output signals from a flux gate sensor are sent to an electronic unit
which amplifies the signals and calculates magnetic heading.
The output of this electronic unit is sent to cockpit indicators that require
magnetic heading information and sometimes to navigational systems that
require heading information.
The two common cockpit instruments that receive signals from the flux gate
system are the HSI and the RMI (radio magnetic indicator).
The flux valve sensors of HSI and RMI are normally installed near the wing
tips to keep them away from magnetic influences in the aircraft.
The location of the vertical card magnetic compass is also shown in Figure
below.

Issue No : 000
The location of the various components of a flux valve compass system

Issue No : 000
The HSI and the RMI both have a compass card which indicates
the magnetic heading of the aircraft.
The heading information comes from a flux gate compass system.
The compass cards on both instruments are driven by a remote
mounted directional gyro.
The DG receives signals from the flux gate compass that
automatically reset it to the correct magnetic heading.
The remote DG is slaved to the flux gate compass and the compass
cards on the instruments are slaved to the remote DG.

Issue No : 000
The face of an RMI is shown in Figure below.
The compass card in this instrument indicates the magnetic heading
of the aircraft as previously described.
The RMI can be identified by the two pointers that have a common
pivot point in the center of the instrument.
– These pointers are connected to radio navigation systems so that
they point toward the location of the ground transmitter.
The selector switches allow each pointer to be connected to an ADF
or VOR radio receiver.

Issue No : 000
The radio magnetic indicator (RMI) has a compass card which indicates
the magnetic heading of the aircraft

Issue No : 000 The compass cards in the aircraft HSI and RMI instruments

Issue No : 000
1. Must be installed to prevent influence by airplane vibrations or
magnetic fields.
2. Maximum deviation in level flight is 10 degrees on any heading.
3. Magnetic non-stabilized may deviate more than 10 degrees due
to electric heated windshield etc. if either a stabilized magnetic
direction indicator or DG is installed. Deviation over 10 degrees
requires a placard.
Fares for Compass Systems
FAR 23.1327 Magnetic Direction Indicator

Issue No : 000
1. Placard must be installed on or near the MDI (compass).
2. Placard must list calibration for level flight with engines running.
3. Placard must state if calibration is for radios on or off.
4. Calibration increments must be 30 degrees maximum.
5. More than 10 degrees deviation for electric heated windshield
etc. must be placarded.
FARs for Compass Systems (cont’d)
FAR 23.1547 Magnetic Direction Indicator Deviation Placard

Issue No : 000
Attitude Director Indicator
The artificial horizon indicates the aircraft's orientation relative to the
horizon.
– The instrument is based on a gyro with a vertical axis.
– This provides a reference to the vertical and a measurement for two
aircraft attitudes; pitch and roll.
A basic artificial horizon contains the gyro inside of the panel
instrument.
The gyro is mechanically connected to the brown disk (earth) and a
blue disk (sky) representing the horizon.
Held in a gimbal, the gyro remains vertical for both yaw and pitch.
The vertical gyro is designed with a tendency to align with gravity
An improvement over the artificial horizon is the Attitude Director
Indicator, or ADI (Figure on the right side).

Issue No : 000
Primarily an artificial horizon, the
ADI is the obvious place to put
glide slope and localizer pointers.
The ADI also contains the
indicator for the radar altimeter
and DME distance.
A specialized version of the ADI,
when combined with an HSI, is
called a flight director and
contains other indicators.

Issue No : 000
The Turn-and-Bank indicators in
Figure on the right shows the
correct execution of a bank and
turn and indicates the lateral
attitude of the aircraft in level
flight.
The turn needle is operated by
an electric gyroscope.
A gyroscope consists of a rotor
mounted on moveable frames.
Turn-and-Bank Indicator

Issue No : 000
When the rotor spins at high speed, the spin axis on which it turns
continues to point to the same direction, no matter how the frames
are moved.
The spin axis is automatically positioned parallel to the earth's axis.
A gyro turns over slowly as the earth rotates.
– This turning, called precession can cause the spin axis to
change direction.
The turn needle indicates the rate, in number of degrees per
second, at which an aircraft is turning about its vertical axis.
It also provides information on the amount of bank.
Turn-and-Bank Indicator (cont’d)

Issue No : 000
The gyro axis is horizontally mounted so that the gyro rotates up and
away from the pilot.
The gimbal around the gyro is pivoted fore and aft.
Gyroscopic precession causes the rotor to tilt when the aircraft is
turned.
Due to the direction of rotation, the gyro assembly tilts in the opposite
direction from which the aircraft is turning.
– This prevents the rotor axis from becoming vertical to the earth's
surface.
The linkage between the gyro assembly and the turn needle, called
the reversing mechanism, causes the needle to indicate the proper
direction of turn.

Issue No : 000
A single needle-width deflection shows when the aircraft is turning at
one and one half degrees per second or half standard rate (four
minutes for a 360o turn).
The slip indicator (ball) part of the instrument is a simple
inclinometer consisting of a sealed, curved glass tube containing
kerosene and a black agate or a common steel ball bearing, which is
free to move inside the tube.
The fluid provides a damping action, ensuring smooth and easy
movement of the ball.
The tube is curved so that in a horizontal position the ball tends to
seek the lowest point.
During coordinated turns and straight-and-level flight, the force of
gravity causes the ball to rest in the lowest part of the tube, centered
between the reference wires.

Issue No : 000
It was developed to assist pilots to interpret and use aircraft
navigational aids
It displays information obtained from combinations of the heading
indicator, radio magnetic indicator (RMI), Course Deviation Indicator
(CDI). It may also display VOR, DME, ILS or ADF information.
Aircraft heading is displayed on a rotating compass card under the
heading lubber line.
Course pointer provides magnetic bearing information from the aircraft
to the selected ground station (VOR or ADF).
Fixed aircraft symbol and course deviation bar display the aircraft's
position relative to the selected course (VOR or ILS localizer).
Compass card is positioned by the DG, which uses a synchro-based
servo system in the same fashion as employed in the RMI.
Some HSI also display Glideslope and Localiser.
Horizontal Situation Indicator (HSI)

Issue No : 000
HSI display

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