For more than 50 years of jet airliner operation, the APU has been an integral part of airplane independence. The APU is a small jet engine located within the structure of the fuselage. With today’s airliners, the unit is operable both on the ground and in flight. In flight, the APU provides both a backup source of electric power and a limited amount of air pressure at lower altitudes — usually below 20,000 feet.
On the ground, the APU is capable of being the sole source of electricity and the sole source of air pressure. Air pressure from the APU is the standard method utilized in starting the engines. When air is not available, starting becomes problematic. On this particular trip, an inoperative APU became more problematic than my copilot and I could have ever imagined.
