Module 5: Digital Techniques and
Electronic Instrument Systems
5.15 Typical Electronic-Digital Aircraft Systems
CMS & BITE: Computer Maintenance System & Built-In Test
ACARS: Aircraft Communication Addressing and Reporting System
EFIS: Electronic Flight Instrument System
EICAS & ECAM
FMS: Fly Management System
GPS: Global Positioning System
INS / IRS: Inertial Navigation System / Inertial Reference System
TCAS: Traffic Collision Avoidance System
DFDR / CVR: Digital Flight Data Recorder / Cockpit Voice Recorder
On-board Maintenance Facilities
Data on the aircraft are acquired by:
Built-in Test Equipment:
A system is composed of LRUs, which can be computers,
sensors, probes, actuators etc. which perform specific
A part of each system is dedicated to functions such as:
monitoring, testing and troubleshooting.
This part of the system is the Built-in Test Equipment.
BITE can (a) perform error detection test (b) isolation:
identify the possible failed LRUs and give a snapshot of
the system when the failure occurred, (c) memorization:
record the error in a memory device.
The information are sent to the Centralized Maintenance
Testing with BITE
Several kinds of tests:
Ensuring compliance with safety objectives.
It is performed only on ground, because they disturb normal
They are performed after long power cuts (more than 200msec).
If the aircraft is airborne the test is limited to a few items to
enable a quick return to operation of the system:
However, when we normally refer to power-up tests, we assume
the aircraft is on ground.
Testing with BITE
Cyclic tests (also called operation test):
They are carried out permanently, because they do not disturb normal
Examples: Watchdog test (i.e. CPU reset). RAM test.
Tests available to the maintenance staff, for troubleshooting purposes.
Similar to ground power-up tests, but more complete.
Examples: Tests performed after the replacement of an LRU.
Available only to specific systems.
They are performed to generate stimuli to other devices, such as actuators
They have major effect on aircraft (such as automatic moving of flaps etc.)
They are performed only on ground by maintenance staff.
BITE Inside a Computer
Make a power up test when the computer starts-up
Operate normally and perform the cyclic tests.
If we are on ground, provide the option to do
system or specific tests. Otherwise, continue with
Aircraft Condition Monitoring System
Monitoring of engine, APU,
performance monitoring and
Collects, records and processes
aircraft system data.
ACMS data can be forwarded to
the MCDU, to a printer,
transmitted through ACARS etc.
It consists of two parts:
Data Management Unit (DMU): Handles and stores
Flight Data Interface Unit (FDIU):
Provides DMU with data from the engines, systems etc.
Electronic Library System
Collection and presentation of technical and
operational material relating to aircraft, in a digital
Can be accessed by flight crews and maintenance
staff through computers.
It’s a database of guides and manuals, stored in a
computer and accessed by an LCD touch screen.
Usually, an ARINC 744A printer is connected to the
library system, for document printing.
ARINC 744A is the standard airborne printer protocol.
Aircraft Communication Addressing and
It is used to transmit or
generated reports to
or from the ground
ACARS protocol was
designed by ARINC.
Communication can be
ground VHF stations or
Prior to ACARS development all communications
between aircraft and ground were VHF or HF voice
To reduce crew workload and ensure data integrity,
developed ACARS communication system.
AM: KHz (Not used in aviation).
HF: 3 – 30MHz (Used in aviation for longer range, e.g.
when flying above Antarctic).
VHF: Above 30MHz (Normally used in aviation).
Note: Higher frequencies are
more easily absorbed by
Automatic ACARS messages
ACARS interface with FMS (Flight Management
Systems) enables the automatic receiving of weather
reports from the ground.
Major flight phases (OOOI):
Out of the gate, Off the ground, On the ground, and Into
the gate messages.
Engine reports in real time can be automatically sent
from the airplane to the airliners.
In case of failures during flight, real time information can
be uploaded by the manufacturers associated with the
Non-automatic messages: Interaction
between the crew and the ground.
Communication between the flight crew and the
ground is made through FMS (it’s similar to email).
After Air France Flight 477 it has been discussed to
make ACARS an online black-box, to retain flight
information in case of lost black-box.
How is an ACARS message propagates?
Through a VHF network:
Only applicable to land masses, where a VHF ground
network is installed.
Used for up to 200miles transmission range.
Through an HF network:
Used in areas such as the poles and oceans.
Completed in 2001.
Provides world-wide coverage.
The message passes through an ACARS network
(through ARINC’s servers) to the operator’s center.
The operators center can be either CAA or a flight operator.
ACARS service providers are used to propagate the
message to the destination.
ACARS messages categories
2 types of messages:
Air Traffic Control: Messages from / to ATC. (e.g. clearance).
Aeronautical Operation Control: Messages from / to the base
(flight operation department). (e.g. fuel consumption, engine
The message format is defined by a specific ARINC protocol.
Each message contains an address label.
ACARS mode: H Aircraft reg: D-AIRL [A321-131]
Message label: 1L [Off message] Block id: 9 Msg no: M23A
Flight id: LH3394 Org: LH06LT [Munich, Germany-Athen, Greece]
Flight distance: appr. 1511km Flight time: appr. 1.7 hours
Message content: 00016234212AN((628D8UVPCR(GKTRRUBW
ACARS vs. CPDLC
Controller–pilot data link
It is built on ACARS. The aim is to
reduce voice congestion.
It’s another communication system
between the flight crew and the flight
Similarities with ACARS:
Uses VHF, HF and Satellite.
Differences with ACARS:
Designed only for communication
between the flight crew and the
However, in Boeing 777 CPDLC
messages can be sent to the
company, as well.
The future of ACARS
An integrated network
inspired from the
while ATN uses binary
Airbus 320 Primary Flight Display
Flight Mode Annunciator
The flight mode annunciator (FMA),
AP/FD vertical and lateral modes
AP, FD, A/THR engagement
Green color is “engaged”.
Blue color is “armed”.
is related to approach indications in column 4.
Magenta are target speed, altitude etc.
in a compatible
format with the
In case of failure of
DMC1 or DMC2.
System Data Acquisition
Fly-By-Wire (FBW) is a system that replaces the
conventional manual flight controls of an aircraft with
an electronic interface.
Flight control computers determine how to move the
actuators at each control surface to provide the
The movements of flight controls are converted to
electronic signals transmitted by wires.
Fly-By-Wire allows automatic signals sent by the
aircraft's computers to perform functions without the
pilot's input, as in systems that automatically help
stabilize the aircraft
Conventional Flight Control Systems vs. Fly-
Mechanical systems are heavy, non-accurate, prone
to failures and errors. They have limited ability to
compensate for changing aerodynamic conditions.
Fly-By-Wire implies a purely electrically signaled
Computer configured controls: A computer system is
interposed between the operator and the final actuator.
The mechanical system that
controls the servomechanism,
which moves the surface is
replaced by a computer.
Advantages of Fly-By-Wire
Due to the complex calculations that computers can
make, they can make decisions without the pilot
input. e.g. Automatic stability systems.
More than one wires can be easily used to ensure the
propagation of a signal.
More than one computers can be easily used, to ensure
proper operation when one computer fails.
History of Fly-By-Wire
Tupolev ANT-20 in 1930:
The first airplane, where
long runs of mechanical
systems were replaced by
Mechanical servo valves
were replaced with
electrically controlled servo
valves, operated by an
analogue computers were
used in early versions of
In digital fly-by-wire flight
control systems, the signal
processing is done by digital
computers and the pilot literally
The programming of the digital
computers enables flight
Aircraft protection, reduced pilot’s
FADEC: Full Authority Digital
Permits control of flight surfaces
and engine autothrottles to be
FADEC contains a digital computer and a unit that controls
Allows maximum performance to be obtained from the
Protection from dangerous situations such as low-speed
stall or overstressing by flight envelope protection.
the flight control systems commands the engines to increase
thrust without pilot intervention.
In economy cruise modes, the flight control systems adjust
the throttles and fuel tank selections more precisely than all
but the pilots.
Further Fly-By-Wire developments
Signal is transferred by light instead of current.
Having eliminated the mechanical transmission circuits in
fly-by-wire flight control systems, the next step is to
eliminate the bulky and heavy hydraulic circuits.
Intelligent Flight Control System
In case of a failure leading to a crash, computers make
complex calculations to adjust the flight controls in a
proper position to save the aircraft.
It is believed that enchantments are mostly software upgrades
to the existing infrastructures.
Flight Management System: Introduction
FMS basic operation:
Compares the pilot selected
flight plan with the actual
horizontal and vertical aircraft
In case of difference between
them, the FMS makes a
steering and thrust command.
The FMS input and output
device is the:
CDU: Control Display Unit or
MCDU: Multifunction Control
The pilot sets the flight plan through the MCDU.
A database with airports, runways, waypoints is used.
FMS automatically selects optimal parameters
e.g. climb ration, optimal speed etc.
Shows information about the flight plan on MCDU.
Exchanges information and commands the Autopilot
/ Autothrottle Flight System AFS.
Accepts DME and VOR inputs.
Gives information to the EFIS displays.
FMS uses information form its database to automatically tune the
navaids (ILS, VOR, DME).
Database must be updated every 28 days.
The FMS calculates the shortest possible flying time at the lowest
fuel consumption. Can give predictions of fuel quantities and
arrival times at future points in the flight plan.
The FMS compares the desired position of the aircraft according
to the flight plan, with the actual aircraft position. If there is a
difference, FMS commands the AFS to bring the aircraft to the
Position and velocity are calculated using the IRS, GPS, VOR and
FMS is the primary source of information displayed on EFIS.
Setting up the FMS
Set the take off runway and destination.
Insert the waypoints.
Check or modify the flight plan.
Set flaps, weather and other information that
affects performance for each flight phase.
Flight plan is displayed on ND.
FMS block diagram
Flight Management Computer :
Performs the Navigation and performance calculations.
Stores the database and the selected flight plan.
Tunes to navaids.
Commands the AFS.
Makes EFIS display calculations
GNSS (Global Navigation Satellite System) is an
umbrella term for systems which are used to
navigate and determine current position based on
signals received from dedicated navigation satellites.
4 most important GNSS systems:
Principles of Operation
All satellite navigation systems
use the same principle as DME
(Distance Measuring Equipment):
The receivers measure the time it
takes for a radio signal (around
1.5GHz) to travel from a
transmitter in a satellite at a known
point in space.
The receiver’s computer calculates
the distance for more than one
Satellites and Space Segment
There are 6 orbital planes with 4
satellites in each plane.
Each plane is inclined 55 degrees
relative to the equator.
In the American GPS (NAVSTAR),
there are 24 satellites at 11,000nm
moving around the globe and
return to the same position after 12
Errors in Transmission
After the third
measurement, one of the
two possible points can
be discarded, since it is
far from the earth
Possible errors that
degrade the accuracy:
Noise due to sunspot
Satellite clock drift:
Variations of the clock of
the satellite transmitter
clock. (1nsec drift causes
1ft. distance error).
Calculations from a 4th
satellite are needed to
eliminate the effects of
The European GPS System
A system of 30 satellites (under
Higher accuracy that the NAVSTAR (down
to less than a meter). .
Supplementary Systems needed for aircraft
Sensors on the aircraft to detect the
quality of the GNSS data received
and correct them if necessary.
Geostationary satellites detect errors
and correct GNSS signals
transmitted to users.
They are limited to certain regions of
WAAS (USA), EGNOS (Europe).
Ground stations around the airports
enhance positioning accuracy.
They are considered a long term
replacement to ILS.
Example: Differential GPS: A
ground-station propagates the GPS
error to GPS receivers.
Acceleration moves the
strings to the opposite
direction of the
The acceleration of
indication can be
integrated once to give
velocity and once more
to provide distance.
On the aircraft, induction
is are used.
By knowing the starting
position (IRS alignment)
an aircraft can calculate
the distance covered.
The Gyroscope principle
When the rotor spins, no
matter how the plane
rotates on the yaw, the
plane at which gyro
rotates remains the
The gimbal will move so,
the spin axis remain the
same, no matter how it
Inertial Navigation System
A gyro along with
the gyro spin axis
and the gimbal
Inertial Reference System
Mechanical gyros are
replaced with laser gyros,
for greater accuracy.
Movement of gimbal is
measured with the
difference between arrival
times of two laser beams.
When rotation takes place,
the orientation of the
mirrors changes, thus the
beams reach at different
times the detector.
TCAS: Traffic Alert Collision Avoidance
System designed to
reduce the incidence
of mid-air collisions
aircraft appears in
the vicinity an
negotiation is being
made between the 2
aircrafts to avoid
displayed in EHSI
A number next to each
aircraft shows the height
of each aircraft in
comparison to this one.
Traffic Advisory (TA):
Pilots are instructed to
initiate a visual search for
the traffic causing the TA.
If the traffic is visually
acquired, pilots are
instructed to maintain visual
separation from the traffic.
Resolution Advisory (RA):
Pilots are expected to
respond immediately to the
This means that aircraft will
at times have to maneuver
contrary to ATC instructions
or disregard ATC
Clear of Conflict (CC):
Pilots shall promptly return
to the terms of the ATC
Type Text Meaning Required action
TA Traffic; traffic.
Intruder near both horizontally and
Attempt visual contact, and be
prepared to maneuver if an RA
RA Climb; climb. Intruder will pass below Begin climbing at 1500–2000 ft/min
RA Descend. Descend. Intruder will pass above. Begin descending at 1500–2000 ft/min
RA Increase climb. Intruder will pass just below Climb at 2500 – 3000 ft/min.
RA Increase descent. Intruder will pass just above. Descend at 2500 – 3000 ft/min.
RA Reduce climb. Intruder is probably well below. Climb at a slower rate.
RA Reduce descent. Intruder is probably well above. Descend at a slower rate.
RA Climb; climb now.
Intruder that was passing above,
will now pass below.
Change from a descent to a climb.
RA Descend; descend now.
Intruder that was passing below,
will now pass above.
Change from a climb to a descent.
Maintain vertical speed;
Intruder will be avoided if vertical
rate is maintained.
Maintain current vertical rate.
RA Adjust vertical speed; adjust.
Intruder considerably away, or
weakening of initial RA.
Begin to level off.
RA Monitor vertical speed.
Intruder ahead in level flight, above
Remain in level flight.
Passing through the intruder's
level. Usually added to any other
Proceed according to the associated
CC Clear of conflict. Intruder is no longer a threat.
Return promptly to previous ATC
Flight Data Recorder (FDR)
Flight Data Recorders store snapshots
of the following information:
Thrust on each engine
Use of Autopilot
Angle of attack
These information are from the same
sources that supply the flight crew.
Recording begins with the start of the
first engine and ceases at shut-down of
the last engine.
Must survive impact velocity of
Each snapshot is taken 1-2 times per
Can record from 17 – 25 hours
30 days up to
Cockpit Voice Recorder (CVR)
Often referred as “black box”.
Records all the communication
transmitted or received by to /
from the flight deck.
Voice communication between
the flight crew.
All sounds in the cockpit, e.g.
audio signals, sound alarms.
Must be capable of recording
for at least 2 hours.
Recording begins with the start
of the first engine and ceases
at shut-down of the last engine.