Ever wondered about that little exhaust pipe protruding from the tails of many jets and turboprops? Looks like another engine exhaust, doesn't it? Well, your eyes aren't deceiving you _ in many cases those are indeed jet engine exhausts, from small "extra" jet engines known as "APUs."

An "APU" (Auxiliary Power Unit) is a "small" turbine engine installed to provide supplementary power. APU's are often found in the tails of aircraft ranging from larger turboprops to jets. They serve a number of useful purposes.

APU's are installed with dedicated generators, allowing them to provide auxiliary electrical power, in addition to that provided by aircraft engines. This is valuable for running aircraft systems on the ground at facilities where no ground electrical power is available, without powering up the engines. Applications include powering environmental systems, such as air conditioning, supporting maintenance of major electrical systems, and providing power for crew functions such as preflight, cabin cleanup, and galley (kitchen) operation. APU's on many (but not all) aircraft may be operated in flight, providing backup power for engine generators.

APU's on larger aircraft are also plumbed to provide an auxiliary "bleed air" source, referring to pneumatic pressure drawn from the engine's compressor section. You may be aware that large jet engines used on airline aircraft must be started using pneumatic power. Unless a ground pneumatic source is available, there is no way to start a large turbine engine without an operating APU (unless another engine is already running, of course!)

For starting a large aircraft's main engines, the small APU turbine engine is first started using an electric motor. Once up and running, APU bleed air is routed to pneumatic starters on the plane's main engines. Those, in turn, spin up the engine compressors for starting.

Along with starting the main engines, APU's on many aircraft can provide backup pneumatic power for pressurization in flight, and to back up environmental systems on the ground and in the air. Here's a schematic showing some of the many parts of a typical APU installation.