Air Data Computer — Page 97, Lesson 117

Let's start with the Air Data Computer itself. 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 on your flight deck. The key idea is this: the relationships between True Airspeed, Mach Number, temperature, pitot pressure, and static pressure can all be expressed as mathematical formulae. The Air Data Computer, which we call the ADC, resolves these formulae continuously. That means it's constantly solving those equations in real time. It takes pressure and temperature as its inputs, and it produces the required outputs in the form of either shaft rotations or electrical signals. So the ADC is what turns raw physical measurements into usable data for your instruments. Now, there are two ways the ADC can work internally. The first is a mechanical system, which uses shaft rotations to transmit information. The second is a digital system. In the digital system, the ADC uses digital data — that's binary data, ones and zeros — in its assessment and transmission of information. Here's where the Analogue to Digital Converters come in. These converters sit at the input side of the ADC. They take measurements of pressure, temperature, and Angle of Attack — which we call AOA — and they change them from analogue form into digital form. That digital form is what's used within the ADC itself, and it's also what gets transmitted onward to the flight deck. Now let's talk about system redundancy. This is critical for you as a pilot, because you need to know what happens when things fail. Provision for blockages or failure of an ADC is made in two ways. First, through change-over cocks, which permit an alternative static source to be connected to the computer. Second, through 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 from the other. 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. The purpose of this is to reduce the possibility of an undetected malfunction. If both instruments on one side were fed from the same computer, a failure could go unnoticed because both would show the same wrong reading. By mixing sources, you're more likely to spot a discrepancy. Now, what happens in the event of total failure of both ADCs? This could happen, for example, due to loss of power supply. In that case, the flight can be continued by reference to the standby instruments. So you always have that backup. Finally, let's look at 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. With a purely mechanical system, there's no such automatic comparison — the comparison between left-hand and right-hand instruments must be carried out visually by you, the pilot. In the digital system, a warning flag will appear on the appropriate ADS instrument if there is loss of valid data, or if an internal failure occurs. 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 flag on the instrument itself and a light on a warning panel. Let me just make sure you've got the key terms straight. The ADC is the Air Data Computer. The ADS is the Air Data System — that's the whole system including the computers. AOA is Angle of Attack. The change-over cocks are the mechanical means of switching to an alternative static source. And the comparison monitor is the automatic failure detection device. So the whole picture is: raw pressure and temperature come in, the ADC solves the mathematical relationships continuously, and it outputs either shaft rotations or electrical signals. Redundancy is built in through change-over cocks and electrical cross-feeding, and failure warning comes through comparison monitors, warning flags, and warning lights.