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AIR DATA COMPUTER
Copyright of Airspace Insider 2024
OCTOBER 2024
Copyright 2023 Airspace Insider
What is an ADC?
The Air Data Computer (ADC) is an electronic device (computer)
that converts mechanical inputs, such as static pressure or total
pressure, etc. into a digital data format.
ADC
This digital data can then be displayed on various glass cockpit
applications, as well as be distributed to other aircraft’s systems.
What is the purpose of using ADC?
The main purposes of using an Air Data Computer are:
Data digitalization
Aircraft weight reduction (eliminating the need for long pressure
lines)
Ability to display data, such as CAS (Calibrated Airspeed) or TAS
(True Airspeed), without manual calculation
Ability to provide air data to other systems, such as FDR (Flight
Data Recorder), FMS (Flight Management System), Autopilot, etc.
Boeing 747 was one of the first
airliners to incorporate an Air
Data Computer (ADC) in its
avionics.
ADC inputs and outputs
The Air Data Computer (ADC) typically has 4 inputs:
Total pressure - supplied from the pitot tube
Static pressure - supplied from the static port
Total Air Temperature (TAT) - supplied from the TAT sensor
Angle of Attack (AOA) - from the AOA sensor
Apart from the primary inputs, an ADC, in order to correct e.g.
IAS for the position error when calculating CAS, needs to access
some additional data. This data can be:
Flap/slat settings
Measured angle of attack
Aircraft's performance data
When it comes to ADC outputs, apart from the obvious ones (such
as electronic displays for altitude, airspeed and other readouts), its
data is supplied to other systems, mainly the autopilot, flight director,
transponder, ACARS, and Flight Data Recorder (FDR).
Electronic Displays
Total pressure
Autoflight Systems
Static pressure ADC Flight Management
Systems
TAT
(Total Air Temperature) Flight Data
Recorders
AoA
Other systems
(Angle of Attack)
ADS-B
Datalink comm.
TCAS
EGPWS
ADC interactions
ADC system’s layout
In a smaller, GA aircraft equipped with an ADC providing data
for a glass cockpit, a standby set of essential, analog instruments must
be installed. The sensors (pitot tube, static port, etc.) feed both the
ADC and the analog instruments, but in case of an ADC failure, the
standby instruments are usable.
Total pressure
Static pressure
TAT
ADC
(Total Air Temperature)
AoA
(Angle of Attack)
Standby Instruments
ADC scheme
In larger aircraft, equipped with two primary sets of instruments
(for the first officer and the captain), usually two (or even more) ADCs
are installed, each with a separate set of sensors.
Moreover, the most modern standard usually inlcudes the Air Data
Computers as part of even more complex, integrated system. For
example, ADC can be integrated with an IRS (Inertial Reference
System - discussed later in the book). This integrated computing unit is
called ADIRU (Air Data Inertial Reference Unit). Usually, there are
more than one unit mounted on one aircraft, and the whole system
which consists of these units is called ADIRS (Air Data Computer
Inertial Reference System). ADIRS can even further be integrated with
GNSS, which e.g. in Airbus A320 is known as “GNADIRS”.
For example, provided True Airspeed (TAS) enables the ADIRS to
compute wind direction, aircraft’s track (TK) and Ground Speed (GS)
ADIRS Panel in A320 (left) and B737NG (right)
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ADC failure
Pilots have to keep in mind that in case of an ADC failure, all
instruments currently supplied by the ADC unit will be lost. This
situation is different from a conventional analog set of instruments,
where the failure of one instrument does not affect the other
instruments and can be easily identified and disregarded.
The file you’ve just read was a small part of my new ebook about
flight instruments. The whole book has over 170 pages, and is
designed to let you learn fast what you truly need, without the
unnecessary academic jargon, straight to the point.
There are some example topics from the book, explained in a
similar manner:
Aircraft’s instruments
Types of airspeed and altitude
Engine parameters (N1, EPR, etc.)
Mach number
Airliner’s climb procedure
Total air temperature
Inertial systems (INS/IRS)
and much more...
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Happy reading and clear skies,
