Air Data Computer — Page 106, Lesson 119

Let's pick this up with the Built-in Test Equipment — the BITE. That's the system that keeps an eye on the Air Data Computer itself. There's no provision for manually entering data into the ADC if something fails. You can't just type in a correction. Instead, the Built-in Test Equipment — we call it BIT or BITE — gives you a prompt indication of any malfunction. So the ADC is self-monitoring. There are three types of BITE process in any ADC. First, Power Up BITE. This runs when power is applied to the ADC — either on start-up or after a break in power. It checks the Microprocessor, the Memory Store, and the Air Data functions. So it's a quick health check at power-on. Second, Continuous BITE. This is an automatic check of all stages of input and output, carried out throughout the entire operation of the ADC, about once every second. So it's constantly verifying that data is flowing correctly in and out. Third, Maintenance BITE. This lets the maintenance crew run checks on the ground using a Test or Test/History switch. That switch lets them look at current failures or post failures — meaning failures that have already happened and been recorded. Now, the advantages of an Air Data System compared to conventional mechanical instruments. There are five main ones. First, Improved Displays. Because the instruments are electrically-servoed, the manufacturer has complete freedom to design new displays that are easier to read and unambiguous. That includes digital displays, moving tape displays, and combined displays. Second, Reduced Instrument and Lag Errors. The major cause of instrument error in mechanical instruments is friction loss within the linkage. And the limited response rate of those linkages causes lag error. Both problems are largely overcome in an ADS by using servomotors. Third, Error Correction. Because height, airspeed, and other variables are all computed within one computer, error corrections can be applied through especially shaped cams appropriate to the particular aircraft. For example, position error correction — PEC — can be calculated within the Mach No. computer channel, and then used additionally within the height and airspeed channels. Fourth, Central Source for Other Systems. The ADC provides not just the conventional information on the instrument panel, but also air data in many forms as required by other systems. So it's a central data source. Fifth, Clean Design. Electrically-driven instruments reduce the pneumatic plumbing behind the panel to only those lines connected to the standby airspeed indicator and altimeter. That saves space, makes maintenance easier, and — importantly — shorter pitot/static lines reduce error-producing acoustic effects. So to tie it together: the ADC is self-monitoring through three BITE processes, and it gives you five clear advantages over the old mechanical instruments — better displays, fewer errors, built-in error correction, a central data source, and a cleaner installation.