Air Data Computer — Page 106, Lesson 119

Right, so we've already covered how the Air Data Computer takes its raw inputs—pitot pressure, static pressure, total air temperature—and turns them into the computed values the crew sees. Now I want to walk you through what happens when that system goes wrong, and why the whole air data concept is such an improvement over the old mechanical instruments. First, the failure side. There is no provision made for the manual input of data into the ADC in the event of any failure. That's a deliberate design choice—you, the pilot, cannot hand-type a value into the computer if something goes wrong. Instead, the system watches itself. That's the Built-in Test Equipment, abbreviated BIT or BITE. Its job is to give prompt indication of any malfunction that might occur. So if something fails, you get a warning quickly, not after the fact. Now, within any ADC there are three distinct types of BITE process, and I want you to know each one by name and function. The first is Power Up BITE. This functions when power is applied to the ADC on start-up or after a break. So every time the computer is energised, it runs a check on three things: the Microprocessor, the Memory Store, and the Air Data functions themselves. Think of it as the computer checking its own brain, its memory, and its core calculations before it declares itself fit for duty. The second is Continuous BITE. This is an automatic check of all stages of input and output, and it runs throughout the operation of the ADC about once every second. So while you're flying, roughly every second, the computer is verifying every input and every output stage. It's a constant, ongoing self-audit. The third is Maintenance BITE. This one is for the ground crew, not for you in the air. It enables maintenance crew to carry out checks on the ground using a Test or Test/History switch. That switch lets them look at current failures or post failures—meaning they can see what's failing right now, or what has failed in the past. That history is gold for troubleshooting. Now, let's move to why the whole Air Data System—the ADS—is worth having at all. Compared with conventional mechanical instruments, there are several clear advantages, and I'll take them one by one. First, Improved Displays. Electrically-servoed instrumentation allows the manufacturer complete freedom to design new displays that are easier to read and unambiguous. Because the display is driven electrically rather than by mechanical linkages, the designer isn't constrained. That gives us digital, moving tape, and combined displays. So instead of a needle swinging over a dial, you might see a digital readout, or a vertical tape that moves, or a display that combines several parameters into one. Second, Reduced Instrument and Lag Errors. The major cause of instrument error in conventional mechanical instruments is friction loss within the linkage. All those little pivots and gears rub against each other, and that friction corrupts the reading. The limited response rate of such linkages gives rise to lag error—the instrument lags behind the actual value because the mechanical parts can't keep up. Both problems are largely overcome with ADSs by the use of servomotors. A servomotor drives the display precisely and quickly, so you don't get that friction-induced error or that lag. Third, Error Correction. Computation of height, airspeed, and other variables within one computer permits error corrections to be applied through especially shaped cams appropriate to the particular aircraft. Because everything is computed in one place, the computer can correct for known errors using cams shaped for that specific aircraft type. Here's the key example: position error correction, abbreviated PEC, can be calculated within the Mach No. computer channel for additional use within the height and airspeed channels. So the Mach number channel does the PEC calculation, and that correction is then shared with the height and airspeed channels. One correction, computed once, applied in multiple places. Fourth, Central Source for Other Systems. The ADC provides not only the conventional information displayed on the instrument panel but also air data in many forms as required for other systems. So it's not just feeding your primary instruments—it's a central data source feeding air data to autopilot, flight management, pressurisation, and so on. And fifth, Clean Design. The use of electrically-driven instruments reduces the amount of pneumatic plumbing required behind the instrument panel to only those lines connected to the standby airspeed indicator and altimeter. So instead of a web of pipes behind the panel, you only keep pneumatic lines for the standby instruments. In addition to space saving and easier maintenance, the use of shorter pitot/static line reduces error-producing acoustic effects. Shorter lines mean less acoustic noise in the pressure signal, which means less error. That figure shows you the combined air data system layout—how the ADC sits at the centre feeding all those outputs. So to tie it together: the ADC is a self-monitoring computer with three layers of BITE, and it replaces the error-prone mechanical instruments with servoed displays, built-in error correction, and a clean, centralised architecture.