Avgeek Alert: A Look at the Cockpit’s Air Data Computer


One of the greatest revolutions in the world of air navigation came with the shift from analog to electronic technology, which is much more precise, and the mechanism that currently regulates these new systems is known as the air data computer (ADC).

In analogue flight instruments, each of the sensors sends information through a pneumatic system (the pitot-static of the altímetre, for example) or an electrical one (such as total air temperature sensor and angle of attack).

These instruments provide vital information to the pilot, but each system is isolated from the others, so in the past the crew had to calculate by hand other magnitudes derived from them – with the workload that entailed.

And not only that, but these systems are, separately, enormously complex. This requires constant revisions and maintenance to prevent measurements from being inaccurate, since we’re talking about gears and analogue mechanisms subjected to friction and wear that can cause the data to vary. Consequently, the crew had to first apply various data correction factors included in the corresponding aircraft manual.

Sensors ‘Feed’ the ADC, Which ‘Cooks’ the Data

In order for this computer to be able to interpret the information that comes to it and make new calculations, it needs power sources that feed it. These are the pitot tube, the static shot, and the total air temperature sensor (plus on some aircraft models, the angle of attack system also sends information to the air data computer),

As we’ve mentioned, information comes from various sources, but it comes raw. The air data computer “cooks” (processes) that information and turns it into useful data. In addition, since it stores a series of internal formulas in its memory, it in turn calculates a series of parameters (those previously done by hand by the crew): static air temperature, indicated, vertical and true speeds, etc. (link to aircraft speeds post)

How Data Is Tansmitted to the Cockpit

The microprocessor of the air data computer produces the data and transmits it electronically to the corresponding flight instruments that the pilot has in front of him, whether analogue or digital, including the speedometre, variometre, and altimetre.

The data is also sent to other systems which may require it, such as the autopilot system, certain types of transponders, aircraft monitoring systems, and the flight management system.

How the Air Data Computer Works

As we’ve seen, the ADC receives information through four different sensors (pitot tube, static intake, total air temperature sensor and, eventually, the angle of attack system). The information from the first two goes through a pressure transducer, which is what converts the information into an electrical signal so that the memory and processor unit can interpret it. The other two sensors don’t need any converter, so to speak, since they themselves have the ability to output their own electrical signal.

Once this information is received, the “magic” happens: the ADC calculates the missing parameters and corrects the data, eliminating errors that, as we said at the beginning, can be caused by friction, mechanical errors, etc.

And not just that. The different sensor errors that can occur in certain flight conditions are corrected in real time by the air data computer, since these values are entered into its memory.

BITE, the Test Which Confirms the Data’s Integrity

The ADC also integrates a test system called BITE (for Built-in Test Equipment), which is in charge of testing the correct operation of the air data computer both when turning on the aircraft (ignition BITE) and throughout the flight at regular intervals (continuous BITE).

What Happens if the ADC Suffers a Failure?

Throughout this post we have already seen on countless occasions that in aircraft systems there is always more than one plan. And the same thing happens here. If the circumstance should arise that the air data computer fails during the flight, there is a so-called backup system, which has the main flight tools connected to the respective sensors in analog mode.

This is in smaller aircraft, since larger ones usually have two ADCs installed, one for the pilot and another for the co-pilot, so the failure of one of them would not have any type of repercussion. They are also interconnected, so that the pilot and co-pilot can check that the data they have is the same. They may even have, in turn, their own backup system completely isolated from the previous ones.

In Airbus, the ADC incorporates an even more specialized system, called by its acronym ADIRU  (Air Data Inertial Reference Unit).