The APU, or auxiliary power unit aboard an aircraft has a specific purpose, but also a pretty big drawback. Here’s what you need to know about it.
If you’ve ever noticed a little exhaust hole at the rear of a large passenger plane and wondered what it was, that’s the location of the plane’s APU, or auxiliary power unit. The APU is found on multiple types of aircraft, including commercial airliners, small private jets, and some military aircraft serving as an extra power source. The B-29, one of Boeing’s biggest achievements, used one of the earliest versions of an APU that looked similar to a motorcycle engine. The Boeing 727 passenger jet was the first plane by the manufacturer to have an APU installed. Pilots will often use the APU to start one of the plane’s main engines. If an aircraft has one, it’s typically found in the tail cone, an engine nacelle, or in the wheel well.
The APU might resemble a jet engine and producing enough energy on its own to power a variety of electronics within the aircraft, including onboard lighting, galley electronics, and cockpit avionics, but it doesn’t provide any sort of propulsion like the main engines.
