Modern transport airplanes use precooled engine bleed air as a source for cabin pressurization, air conditioning, engine cowl and wing deicing, cross-engine starting, air-driven hydraulic pumps and other pneumatic demands. The Boeing Model 767 airplane air supply system consists of separate sets of equipment for each of the airplane's two engines and a remotely located built-in-test equipment (BITE) module. Each set of equipment independently regulates the pressure and temperature of the extracted engine bleed air. The normal control functions are completely pneumatic, not relying on electrical signals or interconnecting wires to provide their control or protective features. Electrical signals are used only for failure conditions using latching-type shutoff solenoids and for the BITE provisions. The digital BITE module monitors inputs from dedicated BITE elements throughout the system on a time-shared basis to isolate 95% of all failures to the LRU (line replaceable unit) level.