Instruments in a Specific Position

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AIR SPEED 
INDICATOR ALTIMETER 
HORIZONTAL 
SITUATION 
INDICATOR 
ALL AIRCRAFT MUST HAVE CERTAIN INSTRUMENTS IN A SPECIFIC POSITION. 
THIS IS CALLED THE BASIC T. 
ARTIFICIAL 
HORIZON 
AIR SPEED 
INDICATOR 
ATTITUDE & 
DIRECTION 
INDICATOR 
ALTIMETER 
HORIZONTAL 
SITUATION 
INDICATOR 
ALL AIRCRAFT MUST HAVE CERTAIN INSTRUMENTS IN A SPECIFIC POSITION. 
THIS IS CALLED THE BASIC T. 
 
THE NAVIGATION SYSTEM GIVES NAVIGATION DATA ON 
RELATED INDICATORS ON THE INSTRUMENT PANEL. 
THERE ARE FIVE DIFFERENT TYPES OF NAVIGATION 
SYSTEMS INSTALLED. THESE SYSTEMS ARE:- 
 
FLIGHT ENVIRONMENT DATA 
 
ATTITUDE AND DIRECTION 
 
LANDING AND TAXI AIDS 
 
INDEPENDENT POSITION DETERMINING 
 
DEPENDENT POSITION DETERMINING 
NAVIGATION SYSTEMS 
FLIGHT ENVIRNOMENT DATA 
PITOT/STATIC SYSTEM 
AIR SPEED INDICATOR 
ALTIMETER 
VERTICAL SPEED INDICATOR 
ATTITUDE & DIRECTION 
HORIZONTAL 
SITUATION 
INDICATOR 
TURN 
& SLIP 
RADIO MAGNETIC 
INDICATOR 
ATTITUDE & 
DIRECTION 
INDICATOR 
STANDBY 
COMPASS 
LANDING & TAXI AIDS 
A O M
INSTRUMENT LANDING SYSTEM (ILS) 
GLIDESLOPE & LOCALISER 
 
HORIZONTAL 
SITUATION INDICATOR 
MARKER BEACON 
ATTITUDE & 
DIRECTION 
INDICATOR 
INDEPENDENT POSITION DETERMNING 
WEATHER RADAR 
RADAR ALTIMETER 
GROUND PROXIMITY WARNING 
TCAS 
DEPENDENT POSITION DETERMINING 
RADIO 
MAGNETIC 
INDICATOR 
VOR 
Distance Measuring 
Equipment 
TRANSPONDER 
ADF 
FLIGHT ENVIRNOMENT DATA 
PITOT/STATIC SYSTEM 
FLIGHT ENVIRNOMENT DATA 
This dimensional change is measured 
by a rocking shaft and a set of gears 
that drives a pointer across the 
instrument dial. 
 
An Airspeed Indicator is a differential 
pressure gauge that measures the 
dynamic pressure of the air through 
which the aircraft is flying. 
 
Dynamic pressure is the difference in 
the ambient static air pressure and the 
ram pressure caused by the motion of 
the aircraft through the air. 
FLIGHT ENVIRNOMENT DATA 
ALTIMETER. 
 
• GIVES A BAROMETRIC HEIGHT. 
• THIS IS ACHIEVED BY HAVING A STATIC 
SOURCE ACTING ON A BELLOWS. 
• AS THE AIRCRAFT CLIMBS, THE STATIC 
PRESSURE IN THE INSTRUMENT 
DECREASES, THE BELLOWS EXPANDS, 
AND THE NEEDLE INDICATES A HIGHER 
ALTITUDE. 
• AS THE AIRCRAFT DESCENDS, THE 
STATIC PRESSURE IN THE INSTRUMENT 
INCREASES, THE BELLOWS 
CONTRACTS, AND THE NEEDLE 
INDICATES A LOWER ALTITUDE. 
• TO COUNTER DAILY CHANGES IN 
PRESSURE, IT IS POSSIBLE TO ADJUST 
THE BAROMETRIC PRESSURE. A 
STANDARD DAY IS 1013 MILLIBARS. 
FLIGHT ENVIRNOMENT DATA 
VERTICAL SPEED INDICATOR. 
 
• THE VERTICAL SPEED INDICATOR USES 
THE CHANGE IN STATIC PRESSURE TO 
GIVE A RATE OF CLIMB OR DESCENT. 
 
• THIS IS ACHIEVED BY HAVING A 
CALIBRATED LEAK. 
 
• BY ALLOWING THE STATIC PRESSURE 
TO LEAK FROM THE INSTRUMENT AS IT 
CLIMBS INDICATES A RATE OF CLIMB. 
 
• BY ALLOWING THE STATIC PRESSURE 
TO LEAK INTO THE INSTRUMENT AS IT 
DESCENDS INDICATES A RATE OF 
DESCENT. 
 
AIRCRAFT COMPASS SYSTEM 
STANDBY COMPASS 
& 
GYRO COMPASS 
ATTITUDE AND DIRECTION 
ATTITUDE AND DIRECTION 
STANDBY COMPASS. 
 
• The standby or E2B compass is a direct 
indicating compass system. 
• This is normally a magnetic compass 
suspended in oil to damp out any 
overswing. 
• It can be corrected for A errors (errors 
induced by the aircraft‟s magnetic field) by 
rotating the whole assembly on its 
mounting. 
• B & C errors (earth‟s magnetic field) can be 
removed by making adjustments to the B & 
C correction pots. 
• It is highly susceptible to local magnetic 
fields and is only reliable when these fields 
are in the same state as they were when 
the compass swing was carried out (i.e.. 
the heated windows being switched off). 
ATTITUDE AND DIRECTION 
COMPASS SYSTEM 
ATTITUDE AND DIRECTION 
FLUX VALVE. 
 
 This consists of a 3 spoke device on a 
flexible mounting, damped with oil. It is 
secured to the aircraft as far from magnetic 
interference as possible. 
 
• A coil mounted on the hub of the spokes is 
fed with 400 Hz a.c. Coils mounted on 
each spoke are connected so that normally 
the EMF‟s induced in them add up to zero. 
However the earth‟s magnetic field causes 
an imbalance in this, giving an output 
proportional to the direction of the earth‟s 
magnetic field. 
 
• A errors (aircraft magnetic field errors) are 
normally removed by rotating the flux 
valve. 
ATTITUDE AND DIRECTION 
VERTICAL GYRO. 
 
• If a Vertical Gyro is used as a heading 
indicator, it will have the normal 
problems of a gyroscope used over the 
earth‟s surface. 
 
• It could drift a maximum of 360 degrees 
in 24 hours and would normally be 
corrected by reference to a compass. 
 
• Normally, the directional gyro unit 
contains the electronics required to 
update and correct the output signal. 
 
ATTITUDE AND DIRECTION 
COMPASS COMPENSATOR. 
 
• This allows the compass 
system to be corrected for B & 
C errors (earth‟s magnetic field) 
ATTITUDE AND DIRECTION 
 
 
The Horizontal Situation Indicator (HSI), has a 
rotating compass card which indicates the 
aircraft heading relative to the aircraft‟s nose. 
 
The HSI compass card is driven by the output 
from the vertical gyro. 
 
 
 
The Radio Magnetic Indicator (RMI), compass 
card is also driven by the output from the 
vertical gyro. 
 
NOTE: No. 1 HSI and No. 2 RMI are driven by the 
No. 1 compass system and vertical gyro. 
 
No. 2 HSI and No. 1 RMI are driven by the No. 2 
compass system and vertical gyro. 
HORIZONTAL 
SITUATION 
INDICATOR 
RADIO 
MAGNETIC 
INDICATOR 
ATTITUDE AND DIRECTION 
ATTITUDE & DIRECTION INDICATOR 
 
 
• The Attitude & Direction indicator, displays a 
constant visual indication of the aircrafts 
lateral and longitudinal attitude relative to the 
horizon. 
 
 
 
• The pilots and co-pilots indicators are 
powered with 115-volt Ac, 400 Hz through 0.5 
ampere fuses labelled “PILOT ART HORIZ” 
and “COPILOT ART HORIZ”, located on the 
overhead fuse panel. 
 
 
 
• A symbolic aircraft reference bar in the centre 
of the instrument represents the aircraft. 
 
ATTITUDE AND DIRECTION 
STANDBY HORIZON. 
 
• A Dc-powered standby indicator 
was installed on the pilots left 
panel to fulfil European Union 
requirements for all series 
aircraft. 
 
• Power for the unit is obtained 
from the Auxiliary battery bus-
bar through a 5 ampere circuit 
breaker labelled “STANDBY ART 
HORIZ” LOCATED ON THE MAIN 
CIRCUIT BREAKER PANEL. 
ATTITUDE AND DIRECTION 
TURN & SLIP INDICATOR. 
 
• The turn and slip indicator 
consists of an electrically 
driven gyroscope which 
indicates rate of yawl, and a 
fluid dampened, ball type 
inclinometer which indicates 
slip and skid. 
 
• Each instrument has a power 
off warning flag. 
LANDING AND TAXI AIDS 
GENERAL. 
 
• The instrument landing system (ILS) is a 
radio navigation system used when the 
aircraft is in approach mode. 
 
• ILS is used to provide steering information to 
keep the aircraft approach to a runway. It 
places the aircraft in proper course and 
altitude for a landing. 
INSTRUMENT LANDING SYSTEM. 
 
An ILS facility provides guidance to an aircraft by providing signals that 
direct the pilot to a 3 degree approach angle centred along the runway. 
This is done by separating the approach into horizontal and vertical 
components. Deviation from the localiser course (left/right) would be 
displayed on an indicator, as would deviation from the glideslope 
(up/down). 
 
Marker beacons are installed along the glidepath as reference points for 
locating the aircraft along the glidepath and as reference points for 
aircraft flying at higher altitudes. 
 
Localiser and glideslope frequencies are paired, selection of the 
localiser frequency automatically selects that of the glideslope. 
LANDING AND TAXI AIDS 
LANDING AND TAXI AIDS 
CONTROL UNIT ILS INDICATOR RECEIVER ANTENNA AIRCRAFT
COMMS
ILS
AIRCRAFT SYSTEM
LANDINGAND TAXI AIDS 
CONTROL UNIT. 
 
• The control unit provides the 
necessary control and 
switching circuits for the ILS 
system. 
• The control unit may also 
provide frequency selection for 
VHF comms 
• The control unit selects the VHF 
localiser frequency which 
automatically selects the paired 
UHF glideslope frequency. 
LANDING AND TAXI AIDS 
DEVIATION INDICATORS. 
 
 
 
• The ILS signals will produce steering 
signals to indicate how much the 
aircraft is off track either up/down or 
left/right. 
 
• If the signal is unreliable a flag will 
cover the indicators or a warning flag 
will be displayed on the indicator 
 
 
 
LANDING AND TAXI AIDS 
RECEIVER 
• The receiver contains the necessary circuits 
for receiving, decoding and processing the 
bearing information from the transmitted 
VOR signal. 
 
• The receiver also contains self monitoring 
circuits that confirm the validity of the 
received signals and the reliability of the 
bearing information sent to the indicator. 
 
• Most VOR receivers also contain circuits 
required to decode and process lateral 
and/or vertical guidance information from an 
ILS ground facility. 
 
• It may also process DME and marker beacon 
information. 
LANDING AND TAXI AIDS 
ANTENNA 
• Three antennas are required for 
complete ILS operation. 
 
• A horizontally polarised, omnidirectional 
antenna operating in the 108 to 112 Mhz 
range is required for localiser operation. 
Typically the localiser receiver uses the 
same antenna as the VOR. 
 
• Glideslope operation requires a folded 
dipole antenna capable of receiving AM 
signals in the 329 to 335 Mhz range. 
 
• The marker beacon typically uses a loop 
antenna operating at 75 Mhz. 
LANDING AND TAXI AIDS 
COMMS SELECTOR 
• The ground station sends out an audio signal (morse code) every 30 
seconds. 
 
• This identifying signal is sent through the aircraft comms system to 
allow the crew to identify the VOR/ILS beacon that they are tracking. 
 
• Audio signals are sent to the aircraft system as it flies over the marker 
beacons :- 400 Hz outer, 1300 Hz middle and 3000 Hz airways tones. 
A O M
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LANDING AND TAXI AIDS 
LOCALISER
ANTENNA
GLIDESLOPE
ANTENNA
MARKER BEACON
ANTENNAS
ILS
GROUND STATIONS
LANDING AND TAXI AIDS 
LOCALIZER. 
 
• This is located at the far (departure) 
end of the instrument runway. 
• It operates between 108 and 112 Mhz 
• Its lowest assigned frequency is 108.1 
Mhz. 
• Only the odd decimal frequencies are 
localiser frequencies, i.e. 109.3, 110.7 
and 111.9. 
• The localiser is radiated to produce 
two intersecting lobes, left and right, 
directed along the length of the 
runway. The lobe on the left is 
predominately modulated with 90 Hz 
and the lobe on the right with 150 Hz. 
The two signals are equal along the 
centre line of the runway. 
LANDING AND TAXI AIDS 
GLIDESLOPE. 
 
• This is located at the near end of the 
runway to one side. 
 
• It operates between 328.6 and 335.4 
Mhz. 
 
• The correct frequency is automatically 
selected on selecting the localiser 
frequency. 
 
• The glideslope signal is radiated to 
produce two intersecting lobes one 
above the other. The upper lobe is 
predominantly modulated at 90 Hz and 
the lower at 150 Hz, with the two signals 
being equal along the glidepath. 
LANDING AND TAXI AIDS 
MARKER BEACON. 
• There are three marker beacon antennas, 
outer, middle, inner. 
 
• The outer marker transmits a 400Hz audio 
tone and when over it, illuminates the blue 
outer marker light, typically 4 to 7 miles 
from the runway. 
 
• The middle marker transmits a 1300Hz 
audio tone and when over it, illuminates 
the amber middle marker light, typically 0.6 
miles from runway. 
 
• The airways marker transmits a 3000Hz 
audio tone and when over it, illuminates 
the white inner light, typically at the end of 
the runway. 
A O M
LANDING AND TAXI AIDS 
HOW ILS WORKS. 
 
• The pilot selects the frequency of an ILS ground station. 
 
• The matched glideslope frequency is automatically selected. 
 
• The pilot using conventional navigation techniques or guidance from 
air traffic control, will then manoeuvre the aircraft onto the approach 
course. 
 
• Once on the approach course, the aircraft will cross the marker 
beacons as it descends. 
 
• The aircraft, now within range of the runway and on direct course for it, 
is flown to a landing. 
 
LANDING AND TAXI AIDS 
HOW ILS WORKS. 
 
• If the aircraft is too far left then the proportion of the 90 Hz signal 
is greater than that of the 150 Hz and a fly right signal is 
displayed on the localiser steering indicator. The converse of this 
happens if the aircraft is too far right. 
 
• If the aircraft is too far up then the proportion of the 90 Hz signal 
is greater than that of the 150 Hz and a fly down signal is 
displayed on the glideslope steering indicator. The converse of 
this happens if the aircraft is too low. 
 
• If all signals are in equilibrium, then the aircraft is flying right 
down the middle of the ILS signals. 
 
RADAR ALTIMETER 
RAD ALT 
INDEPENDENT POSITION DETERMINING 
INDEPENDENT POSITION DETERMINING 
GENERAL. 
 
The radar altimeter is used to provide accurate aircraft 
height above terrain information. 
This is an airborne system used to determine the accurate 
aircraft height above terrain. 
 
The altimeter transmits a constant train of radar frequency 
pulses to the ground, receives the reflected pulses, and 
measures the elapsed time between the transmission and 
reception of each pulse. The elapsed time is processed to 
provide an analogue voltage to drive the indicator. 
INDEPENDENT POSITION DETERMINIG 
TRANSMITTER/
RECEIVER
INDICATOR ANTENNA
RADAR
ALTIMETER
INDEPENDENT POSITION DETERMINING 
TRANSMITTER/RECEIVER. 
 
• The transmitter/receiver unit 
contains all the necessary 
circuitry for the generation, 
reception and tracking of 
height determining radar 
frequency pulses. 
 
INDEPENDENT POSITION DETERMINING 
INDICATOR. 
 
• The indicator converts the output of the 
transmitter/receiver unit, to a direct scale 
readings of the aircrafts height above 
terrain. 
 
 
• On pushing the test button, the alt 
needle is driven to 50 feet, the 
on/off/failure flag comes into view and 
the low level warning light comes on if 
the low level warning system is set to 
less than 50 feet. 
 
• And a low level height warning system:- 
This consists of an adjustable bug and 
an indicator light or lights. 
INDEPENDENT POSITION DETERMINING 
ANTENNA. 
 
• Two identical horn type antennas are 
flush mounted to the underside of 
the aircraft. 
 
• They are connected to the 
transmitter/receiver by co-axial 
cables (the length of which is 
critical). 
 
• One antenna is used for transmitting 
and the other for receiving. 
 
• Bonding of the antenna to the aircraft 
skin is critical and poor bonding can 
lead to erratic readings. 
INDEPENDENT POSITION DETERMINING 
HOW RAD ALT WORKS. 
 
• The transmitter produces a train of radar 
frequency pulses to drive the transmitter 
antenna. 
• Coincident with the transmission of each 
pulse a reference pulse is supplied from 
the transmitter to the tracker. 
• The receiver receives the reply pulses 
from the receive antenna, processes 
them and sends them to the tracker. 
• The tracker takes the reference pulse and 
the received pulse measures the time 
difference and converts this to an 
analogue voltage to drive the indicator. 
• The indicator takes the voltage and 
drives the needle to show aircraft height. 
If the aircraft is above the radar altitude 
range, the needle is driven behind the no 
track mask. 
TRAFFIC ALERT AND COLLISION 
AVOIDANCE SYSTEM 
TCAS 
& 
ACAS 
INDEPENDENT POSITION DETERMINING 
INDEPENDENT POSITION DETEMINING 
GENERAL. 
 
• Traffic alert and collisionavoidance 
system (TCAS) and Airborne collision 
and avoidance system (ACAS) provide 
conflict resolution advisories in the 
form of vertical readouts. 
• It can be operated in several 
configurations to display traffic and 
resolution advisories. 
 
INDEPENDENT POSITION DETERMINING 
TCAS/ACAS 
 
This is a ground and airborne based system using the 
mode S facility of the ATC. Mode S transponder equipped 
aircraft aircraft and ground station enhance the operation 
of the ATC by adding a data link feature and a discrete 
interrogation capability, in addition to performance 
improvements in determining the aircraft location. 
 
The mode S transponder data link capabilities include bi-
directional air-to-air information exchange, ground to air 
data uplink, air to ground data downlink and multisite 
message protocol. 
 
INDEPENDENT POSITION DETERMINING 
MODE S
ATC TRANSPONDER
TCAS
COMPUTER
TCAS
ANTENNA
DISPLAY
UNIT
TCAS
AIRCRAFT SYSTEM
INDEPENDENT POSITION DETERMINING 
MODE S ATC TRANSPONDER. 
 
• As a Mode S transponder equipped aircraft 
receives an ATC Mode S interrogation, it 
sends out a reply signal that can be 
received by both ATC and other Mode S 
transponder systems. 
 
• As a mode S aircraft flies into the airspace 
served by another mode S interrogator, the 
first mode S interrogator may send position 
information and the aircraft‟s discreet 
address to the second interrogator. 
 
• Aircraft are tracked by the interrogator 
throughout its assigned airspace. 
 
• A mode S aircraft replies with its altitude or 
its ATC code,depending on the 
interrogation. 
INDEPENDENT POSITION DETERMINING 
TCAS COMPUTER. 
 
• The transmitter interrogates mode C 
and mode S transponders in nearby 
aircraft. The receiver (in the computer) 
accepts transponder replies. 
 
• The computer determines the closest 
point approach (the minimum 
separation between the TCAS equipped 
aircraft and the traffic encountered. 
 
• The computer then gives traffic alert 
(TA) and resolution advisories (RA) as 
appropriate on the TCAS display. 
 
• It also generates the appropriate audio 
response. 
INDEPENDENT POSITION DETERMINING 
TCAS ANTENNA. 
 
• The TCAS antennas mount 
on the top and bottom of 
the aircraft to give all 
round aircraft cover. 
INDEPENDENT POSITION DETERMINING 
DISPLAY UNIT. 
 
• The traffic display shows nearby traffic 
with mode C or mode S transponders 
that reply to TCAS interrogations. 
• The traffic display shows an airplane 
symbol in white in the lower centre of 
the screen. 
• A white dotted 2nmi range ring around 
the airplane symbol (units can show a 
40nmi range and a 20nmi dotted circle 
appears when 40nmi is selected). 
• Four types of TCAS traffic symbols. 
RA traffic- solid red square. TA traffic- 
solid amber circle. Proximate traffic- 
solid white or cyan circle. Other traffic 
as open white or cyan diamond 
• Various TCAS annunciators and flags 
INDEPENDENT POSITION DETERMINING 
HOW TCAS WORKS. 
• TCAS is designed to protect a volume of 
airspace around the TCAS equipped aircraft. 
The system interrogates mode C & S 
transponders in nearby aircraft and the 
computer analyses their replies to show the 
aircrafts‟ bearing, range, altitude and vertical 
speed on a traffic display. 
• The computer also analyses the replies to 
determine a straight line closure rate and the 
closest point of approach (CPA) between 
your aircraft and the traffic aircraft. 
• When the closest point of approach (CPA) 
penetrates the protected airspace around 
your aircraft and the is within 15 to 48 
seconds , the system gives appropriate aural 
and visual TA & RA on the TCAS display. 
INDEPENDENT POSITION DETERMINING 
HOW TCAS WORKS. 
 
• The TCAS system gives RA‟s in 
the form of vertical manoeuvre 
designed to increase the 
separation of the intruding 
threat aircraft and your own. 
• The vertical manoeuvres are 
shown as red and green arcs on 
the VSI indicator along the 
vertical speed scale. 
• The green arc shows the vertical 
speed to fly and the red arc 
shows the vertical speed to 
avoid. 
INDEPENDENT POSITION DETERMINING 
EGPWS. 
 
 The EGPWS uses the GPWS functions plus additional enhanced terrain 
alerting features. 
 
The EGPWS provides terrain display, situational awareness, terrain 
alerting and warning, and obstacle alerting and warning to the pilot. 
 
It is intended to give advanced alerting and warning to the pilot to help 
reduce the possibility of controlled flight into terrain. 
 
The EGPWS triggers the following warnings:- 
Aural warnings comprising aural messages heard over the flight 
compartment headsets and speakers. 
Visual warnings: illumination of TERRAIN and BELOW G/S (below 
glideslope) lamps in the pilots and co-pilots field of vision. 
 
Visual warnings on a display using colours to represent the threat. 
EGPWS 
INDEPENDENT POSITION DETERMINING 
AIRCRAFT
AUDIO
WARNING
LAMPS
EGPWS
COMPUTER
DISPLAY
EGPWS
AIRCRAFT SYSTEM
INDEPENDENT POSITION DETERMINING 
AIRCRAFT AUDIO. 
 
• The aural messages are digitally synthesized and stored in read 
only memories in the GPWS computer. 
 
• When a warning is generated, the information stored in the 
appropriate memory location is retrieved and converted to two 
audio signals. One is applied to the pilots headset and speaker, the 
other to the co-pilots headset and speaker. 
 
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INDEPENDENT POSITION DETERMINING 
WARNING LAMPS. 
 
• The visual warning of a EGPWS mode come in the form of a lamps 
in direct view of the pilot and co-pilot. 
 
INDEPENDENT POSITION DETERMINING 
EGPWS COMPUTER. 
 
• This contain the necessary circuits 
to receive data from various 
sources, process it and produce the 
appropriate warnings to the aircrew. 
• It also produces a visual warning 
onto a display. 
• Inputs to the EGPWS computer 
come from the:- 
– Rad ALT 
– Air Data and Servo Instrument 
System 
– Landing and Taxing Aids 
– Flight Instrument System 
– Flap Control 
– Landing Gear 
– Stall Warning 
 
 
INDEPENDENT POSITION DETERMINING 
DISPLAY. 
 
• This displays terrain threats, by the use of 
colours, on the aircraft flight path. 
• The display range is selectable by the pilot 
from 1 nm to 320 nm. 
 
• The colours are:- 
 
– 50% Red :- +2000 feet and over 
– 50% Yellow :- +1000 feet to +2000 feet 
– 25% Yellow :- -250 feet to +1000 feet 
– 50% Green :- -1000 feet to –250 feet 
– 16% green :- -2000 feet to –1000 feet 
– Black :- below –2000 feet 
– Cyan :- below –2000 feet 
 
INDEPENDENT POSITION DETERMINING 
HOW EGPWS WORKS 
 The EGPWS receives the following inputs 
– Radio height from the radar alt. 
– Vertical speed from the air data system. 
– Indicated air speed from the air data system. 
– Glideslope deviation and validity from the VOR/ILS/MB system. 
– A back course localiser signal if back course mode is selected. 
– A signal from the flaps, if the flaps are in the landing position. 
– A signal from the landing gear, if retraceable landing gear is fitted. 
– An input from the stall warning if triggered. 
 
The EGPWS computer processes these inputs and determines which 
of the 6 warning modes are enabled and if any of the warning 
envelopes are being penetrated. 
 
When the aircraft operation deviates into a dangerous condition 
(warning envelope penetrated) visual and aural warnings are 
generated. 
 
 
INDEPENDENT POSITION DETERMINING 
MODE VISUAL WARNING AURAL WARNING 
1- Excessive sink rate TERRAIN lamps SINK RATE 
WHOOP WHOOP PULL UP 
2- Excessive closure rate TERRAIN lamps TERRAIN 
WHOOP WHOOP PULL UP 
3- Altitude loss after takeoff TERRAIN lamps DON’T SINK 
4- Terrain clearance TERRAIN lamps TOO LOW TERRAIN 
TOO LOW GEAR 
TOO LOW FLAPS 
5- Inadvertent descent below 
glideslope 
BELOW G/S INHIBIT LAMPS GLIDESLOPE 
6-Advisory callouts BANK ANGLES 
MINIMUMS 
INDEPENDENT POSITION DETERMINING 
HOW EGPWS WORKS. 
 
• The EGPWS computer processes the data from its memory, using the 
information from its inputs work out where it is, and produce a terrain 
map of the aircrafts locale. 
 
INDEPENDENT POSITION DETERMINING 
MODE 1 
INDEPENDENT POSITION DETERMINING 
MODE 2A 
INDEPENDENT POSITION DETERMINING 
MODE 2B 
INDEPENDENT POSITION DETERMINING 
MODE 3 
INDEPENDENT POSITION DETERMINING 
MODE 4A 
INDEPENDENT POSITION DETERMINING 
MODE 4B 
INDEPENDENT POSITION DETERMINING 
MODE 4C 
INDEPENDENT POSITION DETERMINING 
MODE 5 
INDEPENDENT POSITION DETERMINING 
EGPWS INDICATORS 
INDEPENDENT POSITION DETERMINING 
WEATHER RADAR 
INDEPENDENT POSITION DETERMINING 
WEATHER RADAR 
The weather radar system is an airborne system that 
provides a moving navigational display which graphically 
shows the relationship of the pilots selected course to 
significant weather. 
 
Only precipitation (or objects more dense than water) will 
be detected by X –band weather radar. Therefore weather 
radar does not detect clouds, thunderstorms or turbulence 
directly. 
 
The best radar reflectors are raindrops and wet hail. The 
larger the raindrop the better it reflects. Because large 
raindrops in a concentrated area are a characteristic of a 
severe thunderstorm, the radar displays this as a strong 
echo. 
INDEPENDENT POSITION DETERMINING 
DISPLAY
UNIT
TRANSMITTER/
RECEIVER
ANTENNA
WEATHER
RADAR
INDEPENDENT POSITION DETERMINING 
DISPLAY UNIT 
 
• The basic weather display allows the 
selection of various modes of 
operation. 
• Various ranges. 
• And by the use of colours a weather 
intensity indication. 
• Black :- nil returns. 
• Green :- weak returns, light turbulence. 
• Yellow :- moderate returns, light to 
moderate turbulence. 
• Red :- strong/very strong returns, 
severe turbulence. 
• Magenta :- intense/extreme, 
severe/extensive turbulence, hail 
lightning. 
INDEPENDENT POSITION DETERMINING 
TRANSMITTER/RECEIVER 
 
• This contains all the necessary 
electronics to generate the 
transmission pulses and receive and 
decode the replies. 
• It also controls the motors that 
control the sweep of the antenna. 
• The decode replies are transmitted to 
the display unit, and depending on 
the mode selected displayed as 
weather returns. 
• This unit can be part of the antenna 
unit to limit the use of wave guides. 
 
INDEPENDENT POSITION DETERMINING 
ANTENNA 
 
• Mounted in the nose cone 
of the aircraft. 
 
• It is a single antenna that 
transmits and receives the 
X-band (8000 to 12500 
MHz) radio pulses. 
INDEPENDENT POSITION DETERMINING 
HOW WEATHER RADAR WORKS 
 
 The transmitter generates microwave energy in the 
form of pulses. These pulses are then transferred to 
the antenna where they are focused into a beam by 
the antenna. When a pulse intercepts a target, the 
energy is reflected as an echo, or return signal back 
to the antenna. From the antenna, the return signal is 
transferred to the receiver and processing circuits 
located in the Tx/Rx unit. The echo‟s or return 
signals are displayed on a indicator. 
DEPENDENT POSITION DETERMINING 
AUTOMATIC DIRECTION FINDER 
 
ADF 
DEPENDENT POSITION DETERMINING 
INTRODUCTION 
 
• The automatic direction finder (ADF) is 
the oldest and most widely used radio 
navigation systems because of the 
availability of numerous ground 
stations. 
 
 
INTODUCTION 
The concept of ADF navigation is based on the ability of an airborne 
system to provide bearing indication with respect to the aircraft‟s centre 
line, based upon the direction of arrival of a radio wave from a selected 
ground station. 
 
I 
 
The airborne portion of the ADF consists of a receiver, control unit, 
indicator, fixed loop antennas and a sense antenna. 
 
The ground facility consists of a transmitter and antenna. A typical 
ground facility would be an AM radio station (1215 KHz Virgin) or a 
non-directional beacon (NDB). 
 
 
DEPENDENT POSITION DETERMINING 
DEPENDENT POSITION DETERMINING 
AIRCRAFT
AUDIO
INDICATOR CONTROL UNIT RECEIVER ANTENNA
ADF
AIRCRAFT SYSTEM
DEPENDENT POSITION DETERMINING 
AUDIO SYSTEM 
• On selecting an ADF beacon frequency the 
beacon identification can be confirmed either 
through the aircraft headset or speaker, 
depending on audio control unit selection, by 
its morse ident or audio output. 
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DEPENDENT POSITION DETERMINING 
INDICATOR 
 
• All indicators used with the ADF 
system indicate the bearing of the 
ground station. That is , the needle of 
the indicator always points to the 
station that the receiver is tuned to. 
 
• An ADF indicator will have a needle 
rotating against a rotating azimuth 
card, to indicate the bearing to a 
ground station, relative to the nose 
of the aircraft. (Radio Magnetic 
Indicator, RMI) 
 
• If the ADF signal is lost or ANT is 
selected on the control unit, the 
receiver will send the indicator an 
invalid signal which will park the ADF 
indicating needle at the 3 „o‟ clock 
position. 
DEPENDENT POSITION DETERMINING 
CONTROL UNIT 
• The ADF control unit provides the 
control and switching circuits to 
select the ADF receiver operating 
mode and frequency. 
 
• It allows selections in the ADF 
operating range between 190 to 1750 
KHz. 
 
• With the switch position in the ANT 
position, audio only signals are 
processed by the receiver. The 
bearing pointer will park at the 3 
„o‟clock position. 
 
• With the switch position in the ADF 
position, both audio and bearing 
information is processed by the 
receiver. 
DEPENDENT POSITION DETERMINING 
RECEIVER 
 
• The ADF receiver contains the 
necessary circuits for the reception 
and processing of radio signals to 
provide relative bearing information 
to an indicator. 
• The receiver also contains the 
circuits required to confirm the 
validity of the received signal and the 
reliability of the receiver itself. 
• If the received signal is not valid or if 
no signal is received, then it sends an 
output signal to the indicator telling it 
to park the ADF needle at the 3 
„o‟clock position. 
DEPENDENT POSITION DETERMINING 
ANTENNA 
 
• The ADF receiver requires two types of 
antenna. An omnidirectional sense antenna 
is required to help tune the receiver and a 
loop antenna is required to provide the 
bearing. Both antennas operate in the 190 to 
1750 KHz frequency range. 
 
• Characteristics of the loop antenna are used 
to determine the bearing to a selected 
ground station. Since the loop antenna is 
directional, the received signal strength is 
relative to the position of the antenna with 
respect to the ground station. 
 
• In the current ADF systems, the antennas 
are mounted in a fixed position relative to 
the aircraft. Two loop antennas are used 
and are physically 90 degrees apart. The 
sense antenna may also be contained in the 
same package as the loop antennas. 
DEPENDENT POSITION DETERMINING 
COMMERCIAL
RADIO STATION
550 TO 1660 Khz
NON DIRECTIONAL BEACONS
NDB
190 TO 550 Khz
ADF
GROUND STATIONS
190 TO 1750 Khz
DEPENDENT POSITION DETERMINING 
COMMERCIAL RADIO STATIONS 
 
• In some parts of the world, commercial 
radio stations may be the only 
navigational aid available. They often 
give a valuable cross-check on other 
navigation facilities. 
• Commercial broadcast stations are not 
limited to line of sight reception. ADF 
systems can receive signals from over 
the horizon. 
• Ground waves are the only transmitted 
waves suitable for direction finding with 
loop antennas. At commercial broadcast 
frequencies the ground wave may be 
overridden by unreliable sky waves. 
• The station selected must be of relatively 
high power and low frequency for best 
results in ADF use. 
• Frequency range550 to 1660 KHz 
DEPENDENT POSITION DETERMINING 
NON DIRECTIONAL BEACONS 
 
• The NDB is a low to medium-frequency 
navigation aid primarily intended to provide a 
broadcast signal to a mobile direction finder. 
 
• The NDB radiates an omnidirectional signal. 
 
• Low-powered NDB‟s are installed at some 
marker sites to assist pilots in transitioning to 
the approach aid. The low-powered NDB has an 
effective range of 15 to 20 miles. 
 
• High powered NDB‟s are used as outer marker 
compass locators at some locations. (They are 
co-located at the outer marker of the ILS) 
 
• Compass locators transmit 2- letter ID groups. 
(Morse ident). 
 
• Frequency range 190 to 550 KHz. 
DEPENDENT POSITION DETERMINING 
HOW ADF WORKS 
 
• The pilot selects the frequency of a ground 
station. 
 
• This is confirmed by its morse ident or audio 
identification. 
 
• Using charts, the pilot can plot his direction to 
the beacon. Using multiple beacons, the pilot can 
triangulate his position. 
 
• The pilot may use the bearing information to fly 
to the ground station if it is on his flight path. 
DEPENDENT POSITION DETERMINING 
HOW ADF WORKS 
 
• ADF uses two fixed loop antennas, one of 
which is perpendicular to the other, and a 
sense antenna. 
 
• The antennas are mounted in the aircraft so 
that a signal can be received in one or both 
loop antennas without manoeuvring the 
aircraft. 
 
• The relationship between direction and 
magnitude of signal of the voltages induced 
in each antenna is processed by the sense 
antenna and transmitted to the receiver. 
 
• This information is processed into a signal to 
drive the needle to the correct bearing on the 
indicator. 
DEPENDENT POSITION DETERMINING 
TRANSPONDER 
 
 
 
DEPENDENT POSITION DETERMINING 
TRANSPONDER 
 
• The airborne transponder is an 
important part of the air traffic control 
system being used today. 
 
• The safety of passengers, aircraft and 
crew depends on the ability of air traffic 
controllers to locate aircraft within 
controlled airspace. 
ATC 
 
A transponder is the airborne receiver - transmitter portion of the ATC (Air Traffic 
Control) beacon radar system. It sends an identifying coded signal, in response to 
a transmitted interrogation from a ground based radar station, in order to locate 
and identify the aircraft. 
 
Air traffic controllers use the coded identification replies of transponders to 
differentiate between the targets (aircraft) displayed on their radar screens. Being 
able to identify the aircraft aids the controller in maintaining aircraft separation, 
collision avoidance, and distinguishing types of aircraft. 
 
The airborne portion of ATC consists of a transmitter/receiver (transponder), a 
control unit, a digitiser and an antenna. 
 
The ground facility consists of a primary radar station, a secondary surveillance 
radar and a display unit (radar screen). 
DEPENDENT POSITION DETERMINING 
DEPENDENT POSITION DETERMINING 
COTROL UNIT TRANSPONDER
TRANSMITTER/
RECEIVER
ANTENNA DIGITIZER
ATC
AIRCRAFT SYSTEM
DEPENDENT POSITION DETERMINING 
CONTROL UNIT 
 
• The control unit contains the circuits 
to allow the operator to select the 
identifying code (0000 to 7777). 
 
• It also contains the controls 
necessary to select an altitude source, 
initiate a self test mode and select the 
transponder reply mode. 
 (mode A, C & S). 
 
• Indicators on the control unit will 
display the code selected. 
 
• An ident button is also available to 
highlight the aircraft on the ATC 
display. 
DEPENDENT POSITION DETERMINING 
TRANSMITTER/RECEIVER 
 
• The receiver part of the transponder 
contains the circuitry to receive, 
demodulate, amplify, and decode the 
interrogation signal. 
 
• The transmitter part of the 
transponder is comprised of the 
circuits necessary to encode , 
modulate, amplify, and transmit the 
coded reply signal. 
 
• The transponder also contains the 
circuits required for checking the 
validity of the received interrogation 
signal and for monitoring the 
integrity of the transponder. 
DEPENDENT POSITION DETERMINNG 
ANTENNA 
 
• The antenna is an L-band (radio 
frequency band from 390 to 1550 
Mhz), monopole blade type. 
 
• It is usually mounted in an area of 
the aircraft that will not be shielded 
from interrogation. This prevents the 
aircraft‟s identification from 
disappearing from the controller‟s 
radar screen. 
DEPENDENT POSITION DETERMINING 
DIGITIZER 
 
• The digitizer is a simple 
converter that converts an 
analog signal, representing 
barometric altitude, to a digital 
format. 
• The digitiser barometric 
altitude can then be encoded 
and shipped as part of the 
reply signal. 
• This may be done inside the 
barometric altimeter or 
alternately in an air data unit. 
DEPENDENT POSITION DETERMINING 
PRIMARY
RADAR
SECONDARY
SURVIELLANCE
RADAR
RADAR
SCREEN
ATC
GROUND STATION
DEPENDENT POSITION DETERMINING 
PRIMARY RADAR 
 
• The primary radar system works like 
many other radar systems. A narrow RF 
(radio frequency) type beam, 
transmitted through a rotating antenna, 
is reflected by targets in its path and 
returned to the antenna. 
 
• By calculating the elapsed time 
between transmission and reception of 
the RF beam, the distance to the target 
is determined. 
 
• The angle of the antenna is also noted 
so that the bearing to the target can be 
determined. 
 
• This information is displayed on a 
 2- dimensional radar screen. 
DEPENDENT POSITION DETERMINING 
SECONDARY SURVEILANCE RADAR 
 
• The SSR system interrogates the 
aircraft about its identity and altitude by 
transmitting two sets of pulses. The first 
set is mode A and the second mode C. 
 
• Mode A pulses are 8 microseconds 
apart and interrogates the transponder 
about the identity of the aircraft. 
 
• Mode C pulses are 21 microseconds 
apart and interrogate the transponder 
about the aircraft altitude. 
 
• There are two other optional modes 
(mode B and mode D) for transmitting 
the aircraft identification and altitude. 
DEPENDENT POSITION DETERMINING 
RADAR SCREEN 
 
• The received signal from the primary 
radar and SSR is electronically encoded 
so that it can be displayed on a 
controllers radar screen. 
 
• The type of radar screen is called a 
planned position indicator (PPI). 
 
• The images on a PPI remain on the 
screen until the next sweep of the 
screen. In this way the controller does 
not have to remember aircraft positions 
between sweeps. 
 
• The ground controller selects the 
identification codes he is interested in. If 
the controller is not interested in a 
particular aircraft its code will not be 
displayed. 
DEPENDENT POSITION DETERMINING 
HOW ATC WORKS 
• The pilot selects an identification 
code, or is instructed to select a 
code by the air traffic controller. 
 
• The SSR system transmits a coded 
interrogation signal (at 1030 Mhz) as 
the primary radar detects the 
aircraft. 
 
• The interrogation signal is received, 
detected and decoded by the 
airborne transponder. 
• The transponder then encodes and 
transmits a set of reply signals. 
(depending on the mode and code 
selected) 
 
• The reply signal is the received, 
decoded and displayed at the ATC 
ground station. 
DEPENDENT POSITION DETERMINING 
VHF OMNIDIRECTIONAL RANGE 
 
VOR 
DEPENDENT POSITION DETERMINING 
INTRODUCTION 
• The VHF omnidirectional range (VOR) 
is a radio navigation system. VOR is 
used for position-fixing, maintaining 
course track and navigating along 
established airways. Basically, it 
provides the ability to follow a roadway 
in the air. 
VOR 
This is a system used to determine the relative bearing from the aircraft to a 
ground based transmitter (with respect to the aircraft centre line). 
 
The VOR transmitter produces a carrier wave and a variable signal which is 
shifted in phase.The VOR navigation receiver detects the VOR radial signal and separates the 
reference and variable signals. The phase of the variable signal is then compared 
to the phase of the reference signal. 
 
The phase difference is proportional to the radial angle from the VOR station. The 
bearing is then determined from this phase difference. From the determined 
bearing and the compass input to the indicator, aircraft heading and ground 
station bearing are displayed on the indicator. 
 
 
DEPENDENT POSITION DETERMINING 
DEPENDENT POSITION DETERMINING 
CONTROL UNIT VOR INDICATOR RECEIVER ANTENNA AIRCRAFT
COMMS
VOR
AIRCRAFT SYSTEM
DEPENDENT POSITION DETERMINING 
CONTROL UNIT 
• The control unit provides the 
necessary control and switching 
circuits for a VHF navigation 
system. 
• The control unit may also 
provide frequency selection for 
VHF comms and distance 
measuring equipment (DME). 
• The control unit selects the VHF 
localiser frequency which 
automatically selects the paired 
UHF glideslope frequency and 
DME frequency (if co-located 
with a NAV system). 
DEPENDENT POSITION DETERMINING 
INDICATOR 
• There are two basic types of indicator used 
in the VOR system. The RMI (Radio magnetic 
indicator) and the HSI (Horizontal situation 
indicator). 
• The RMI will have a needle rotating against a 
rotating azimuth card, to indicate the bearing 
to a ground station, relative to the nose of 
the aircraft. 
• The HSI will have a movable course pointer, 
a steering bar and a to-from arrow. The 
steering bar indicates the direction to be 
steered to bring the aircraft in track with the 
beacon. If the steering bar is central then 
that is the course to the beacon. The to-from 
arrow indicates whether the aircraft is flying 
towards or away from the beacon. 
 
DEPENDENT POSITION DETERMINING 
RECEIVER 
• The receiver contains the necessary 
circuits for receiving, decoding and 
processing the bearing information 
from the transmitted VOR signal. 
• The receiver also contains self 
monitoring circuits that confirm the 
validity of the received signals and the 
reliability of the bearing information 
sent to the indicator. 
• Most VOR receivers also contain 
circuits required to decode and 
process lateral and/or vertical 
guidance information from an ILS 
ground facility. 
• It may also process DME and marker 
beacon information. 
DEPENDENT POSITION DETERMINING 
ANTENNA 
• The typical antenna used by a VOR 
navigation system is a bat-wing 
type antenna, with an 
omnidirectional, horizontally 
polarised radiation pattern capable 
of receiving VHF signals in the 108 
to 118 MHz range. 
DEPENDENT POSITION DETERMINING 
COMMS 
• The ground station sends out an audio signal (morse code) every 30 
seconds. 
 
• This identifying signal is sent through the aircraft comms system to 
allow the crew to identify the VOR beacon that is being tracked. 
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DEPENDENT POSITION DETERMINING 
VOR
ANTENNA
VOR
GROUND STATIONS
DEPENDENT POSITION DETERMINING 
ANENNA 
• The VOR ground station 
transmits continuously and is 
capable of handling all aircraft 
within the limits of the ground 
station transmitter and the 
capability of the aircraft‟s 
receiver. 
• The ground station provides 
voice transmission and an 
identifying code to ensure that 
the desired VOR station is being 
monitored. 
• The identification signal (in 
morse code) is a 2 or 3 letter 
word repeated every 30 
seconds. 
DEPENDENT POSITION DETERMINING 
HOW VOR WORKS 
• The pilot selects the frequency of a 
ground station. 
• This is confirmed by its morse ident. 
• The phase difference between the 
carrier wave and the variable signal is 
computed and an output signal is sent 
to the indicator. 
• Using charts the pilot can plot his 
direction to the beacon. Using multiple 
signals the pilot can triangulate the 
position of the aircraft. 
• The pilot may use the bearing 
information to fly to the ground station if 
it is on the aircraft flight path. 
DEPENDENT POSITION DETERMINNG 
DISTANCE MEASURING 
EQUIPMENT 
 
DME 
DEPENDENT POSITION DETERMINING 
INTRODUCTION 
• Distance measuring equipment (DME) is a 
system combining ground based and 
airborne equipment to measure the distance 
of the aircraft from a ground facility. DME is 
used primarily for position fixing, enroute 
separation, approach to an airport, avoiding 
protected airspace, holding at a given 
position or figuring ground speeds 
DEPENDENT POSITION DETERMINING 
DME 
 
This is an airborne and ground based system, that measures the slant 
range of the aircraft from the ground station. The DME frequency if not 
manually selected, is automatically selected when an ILS/VOR 
frequency is selected, (the DME station being co-located with the 
ILS/VOR beacon). 
 
Since the speed of a radio wave is a constant and known factor, the 
amount of time the signal travels is proportional to the distance. The 
airborne portion of the DME measures the amount of elapsed time and 
converts this to the distance (slant range) between the aircraft and the 
station. 
 
DME indicators may also show time to station (TTS) and/ or computed 
ground speed. 
DEPENDENT POSITION DETERMINING 
CONTROL UNIT TRANSMITTER/
RECEIVER
INDICATOR ANTENNA
DME
AIRCRAFT SYSTEM
DEPENDENT POSITION DETERMINING 
CONTROL UNIT 
• The control unit provides the necessary switching circuits for the 
airborne DME. 
 
• The control unit may also provide the frequency selection for VHF 
comms. 
 
• Control units that provide frequency selection for more than the DME 
automatically select the DME operating frequency for the NAV receiver 
selected. 
DEPENDENT POSITION DETERMINIG 
TRANSMITTER/RECEIVER 
• The transmitter section of the unit 
contains all the necessary circuits to 
generate, amplify and transmit the 
interrogating pulse pairs. 
 
• The receiver section contains the 
circuits required to receive, amplify 
and decode the received reply 
pulses. 
 
• This information is then sent to the 
indicator. 
DEPENDENT POSITION DETERMIING 
INDICATOR 
• The distance indicator displays the 
aircraft distance in nautical miles 
from the ground station (slant 
angle). 
 
• The indicator will also display, in the 
form of a flag or dashes (on digital 
indicators), as a warning that the 
system is either malfunctioning or 
not locked on to the reply signal. 
 
• On some types of indicators, the 
information displayed also includes 
a computed ground speed and the 
time to station (TTS). These are only 
accurate if the aircraft is flying on a 
radial from the ground station. 
DEPENDENT POSITION DETERMINING 
ANTENNA 
• The antenna is a single L-band 
(radio frequency band from 
390 to 1,550 Mhz) transmit and 
receive antenna with an omni-
directional pattern. 
DEPENDENT POSITION DETERMINING 
ANTENNA RECEIVER TRANSMITTER
DME
GROUND STATION
DEPENDENT POSITION DETERMINING 
GROUND STATIONS 
• There are several different types of 
ground station (eg VOR/DME, ILS/DME). 
 
• VOR/DME is DME located with a VOR 
station. 
 
• ILS/DME is DME located with an ILS 
station. 
 
• Ground stations are capable of handling 
100 interrogations at one time. If more 
than 100 aircraft interrogate the ground 
station, the ground station limits its 
sensitivity and responds to the strongest 
interrogations. 
 
DEPENDENT POSITION DETERMINING 
HOW DME WORKS 
• The pilot selects an ILS/VOR frequency. 
This automatically selects the DME 
frequency paired with that frequency. 
• The receiver/transmitter of the airborne 
DME transmits interrogating pulse pairs. 
• The ground facility receives these pulse 
pairs, delays 50 microseconds, and then 
transmits reply pulse pairs back to the 
airborne DME. 
• The airborne receiver/transmitter 
receives the reply pulse pairs and 
verifies that they are valid. 
• Then it calculatesthe distance 
• This is sent to the indicator for the pilot 
• This cycle continues until the frequency 
is changed or the aircraft is out of range. 
AUTOPILOT 
INTRODUCTION 
• The autopilot system, when selected, 
reduces the workload of the pilot. It 
controls and physically flies the 
aircraft. 
AUTOPILOT 
 
The autopilot system comprises two main systems, the flight 
director and the aircraft control system. 
 
The flight director side of the system takes the selected 
operational mode and, using the output of the autopilot 
computer, displays the steering information required to fly 
the aircraft. 
 
The aircraft control system, takes the output from the 
autopilot computer and, using a set of servos, moves the 
aircraft controls, physically flying the aircraft in the mode 
selected. 
 
 
 
 
 
AUTOPILOT 
FLIGHT
DIRECTOR
AUTOPILOT
COMPUTER
FLIGHT DIRECTOR
INDICATOR
HORIZONTAL SITUATION
INDICATOR
SERVOS PITCH/TURN
CONTROL
AUTOPILOT
FLIGHT DIRECTOR 
• The basic flight director uses modes 
selected by the pilot to display steering 
information. If the selected mode is 
flashing, it is in standby, becoming 
steady when it is operational. 
• HDG :- using the heading bug on the HSI 
then the steering information from the 
computer will be outputted to intercept 
and fly along that heading . 
• NAV :- the nav selection will flash until 
the selected VOR beacon is found. Then 
the steering information from the 
computer will be outputted, to intercept 
and fly along that heading . 
• APPR :- the appr selection will flash until 
the selected ILS beacon is found. Then 
the steering information from the 
computer will be outputted, to intercept 
and fly along that heading . 
 
FLIGHT DIRECTOR 
• ALT :- when selected, will try and 
maintain the aircraft at the altitude it 
was selected. This is a barometric 
altitude selection. 
 
• IAS :- will try to maintain the aircraft‟s 
indicated air speed, by changing the 
aircraft‟s attitude. 
 
• B/C :- back course, disabled on many 
autopilot systems, allows the aircraft to 
be flown at the ILS beacon from the 
reverse direction. Only localiser 
information is given. It will flash until 
the selected ILS beacon is found then 
the steering information from the 
computer will be outputted, to intercept 
and fly along that heading . 
 
FLIGHT DIRECTOR 
• ENG/DIS :- this switch engages or 
disengages the autopilot. The green 
triangle next to the switch indicates the 
autopilot is engaged and the amber that 
it is disengaged. 
 
• Trim UP/DOWN :- flashing amber lights 
indicate when the autopilot is trimming 
the aircraft either up or down. If the 
system is run in flight director mode 
only or if there is no trim output 
function to a servo, then these lights 
will come on (not flashing) to indicate 
that a manual trim is required. 
 
• DIM :- this is a rotary switch to dim the 
display lighting. 
AUTOPILOT COMPUTER 
• The autopilot computer contains 
the necessary circuits to take 
information from navigation 
sources, process it and then 
produce steering information to 
the flight instruments and 
control servos. 
 
• If the system only produces 
steering information to the flight 
instruments, then it is a flight 
director system, not an 
autopilot. 
ATTITUDE & DIRECTION 
INDICATOR 
• The command bars on the 
attitude & director indicator will 
show the angle of bank required 
to pick up the selected nav 
source. 
 
• When in true autopilot, the 
aircraft‟s attitude will match that 
of the command bars. 
 
• The glideslope and the localiser 
pointers will be displayed during 
appr mode and indicate the 
aircraft‟s position relative to the 
ILS signal. In appr mode in a true 
autopilot system it will fly down 
the centre of the glidepath. 
HOROZONTAL SITUATION INDICATOR 
• This allows the heading to be flown by manually selecting the 
heading bug to the desired heading. 
SERVOS 
• There are various servos to control 
the aircraft‟s flying control surfaces. 
• Aileron servo :- controls the aircraft 
roll. 
• Elevator servo :- controls the 
aircraft pitch. 
• Trim servo :- controls the aircraft 
trim 
• Rudder servo :- controls the aircraft 
rudder. 
• The servos can all be overridden by 
manual inputs into the system. This 
is achieved by having a capstan 
fitted with a breakout torque. 
PITCH/TURN CONROL 
• This allows pitch and turn information to 
be manually inputted into the autopilot 
system without disengaging the autopilot. 
• Pitch control :- this is used to command 
the a pitch rate proportional to knob 
displacement. Rotating the control up or 
down produces a pitch command. The 
aircraft will hold the pitch last selected. The 
pitch control is spring loaded to centre, 
giving a pitch hold mode. 
• Turn control :- this is used to command the 
roll rate proportional to knob displacement. 
Rotating the control left or right produces a 
roll command. The aircraft will hold the roll 
last selected. The pitch control is spring 
loaded to centre, giving a roll hold mode. 
 
HOW AUTOPILOT WORKS 
• The pilot selects the autopilot mode required. 
 
• The autopilot computer takes the relevant steering information or 
nav source and computes the steering information required by the 
servos to make the aircraft fly in the selected mode. It also sends 
out the steering information to the aircraft instruments giving the 
pilot a visual indication of the autopilot commands. 
 
• As the autopilot nears its desired track/ heading it automatically 
reduces its rate of turn, so that it can intercept the course without 
over swing. 
 
 DHC-6 TWIN OTTER AVIONICS 
 FOR DUMMIES