Air Data Computer — Page 97, Lesson 117

Let’s start with the heart of this: the Air Data Computer, or ADC. I want you to think of it as the brain that sits between the raw sensors on the outside of the aircraft and the instruments you read on the flight deck. The key idea is that the relationships between True Airspeed, Mach number, temperature, pitot pressure, and static pressure can all be written down as mathematical formulae. The ADC’s job is to solve those formulae continuously. It takes in pressure and temperature inputs and produces the required outputs — and it does that in the form of either shaft rotations or electrical signals. So it’s not just one calculation; it’s a constant, ongoing process that keeps feeding the instruments. Now, there are two important terms here. First, the Digital system — that means the ADC uses digital data, which is binary data, in how it assesses and transmits information. Second, the Analogue to Digital Converters, which sit at the input side of the ADC. These converters take measurements of pressure, temperature, and Angle of Attack — AOA — and change them from analogue form into digital form. That digital form is what the ADC uses internally, and it’s also what gets transmitted onward to the flight deck. So the flow is: analogue measurements come in, get converted to digital, the computer works on them, and the results go out to your instruments. Now let’s talk about System Redundancy, because in professional flying we always plan for failure. Provision for blockages or failure of an ADC is made in two ways. One is through change-over cocks — those permit an alternative static source to be connected to the computer. The other is electrical switching, which enables the Captain’s instrument to be fed from the First Officer’s ADC, and vice versa. So if one side fails, you can cross-feed. There’s a subtle but important design point here. In some aircraft, the Air Data System — the ADS — is designed so that the outputs from each computer are not directed exclusively to instruments on one side of the panel. Instead, the sources of air data are mixed to each side. Why does that matter? Because by mixing the sources, the possibility of an undetected malfunction is reduced. If both sides were fed from the same source, a single fault could go unnoticed because both instruments would show the same wrong value. And then there’s the worst case: total failure of both ADCs, perhaps due to loss of power supply. In that event, the flight can be continued by reference to the standby instruments. So you always have a fallback. Finally, Failure Warning. A comparison monitor can be incorporated into the system. Its job is to compare the outputs of the ADCs and give automatic warning to the pilot of a malfunction. Now contrast that with a purely mechanical system — there, comparison between left-hand and right-hand instruments must be carried out visually. You, the pilot, have to look and compare. On the ADC instruments themselves, a warning flag will appear on the appropriate ADS instrument if there is loss of valid data, or if an internal failure occurs. And in addition to that flag, a light will illuminate — either on the instrument warning panel or on the central warning system indicator. So you get both a local flag on the instrument and a system-level light. Let me pull that together for you. The ADC is a continuous calculator turning pressure, temperature, and AOA into usable air data. It’s digital, so it converts analogue inputs first. Redundancy is built in through change-over cocks, electrical cross-feeding, and mixing of sources, with standby instruments as the final backup. And failure detection comes from comparison monitoring, warning flags, and warning lights.