APU = Auxiliary Power Unit
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
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
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,
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.
To learn more about turbine aircraft and how they work,
pick up a copy of The Turbine Pilot's Flight Manual
by Mark Holt and I. The material is easy for any aviation enthusiast to understand,
and I think you'll find it very interesting!