There can be few people in this country who have not been impressed by the complexity of instruments and controls in the cockpit of a modern transport aircraft — if not the real thing, at least they will have seen pictures. This complexity, which the flight crew must master in their difficult and sometimes hazardous task of controlling the aircraft, is the display end of many elaborate sub‐systems going to make up the complete aircraft system, and the aircraft itself. All these sub‐systems and their components must be maintained in reliable operating condition by the aircraft maintenance engineer.