Most commercial jetliners have their primary engines under the wings, but there’s also an extra motor hidden under the tail. Unlike the primary engines, these auxiliary power units don’t actually work to propel the plane. Instead, they provide energy to get the primary turbines moving, power some electronic systems, and contribute to the comfort of the passengers. Below are answers to some common questions you might have about these vital aircraft parts.

FAQ About Aircraft Auxiliary Power Units Answered

What is an auxiliary power unit?

The auxiliary power unit in the tail is actually a turbine engine, similar to those that actually propel the plane. However, this motor is used to power an electric generator, which supports several crucial functions during flight.

Why is an auxiliary power unit necessary?

During the flight, the aircraft’s primary engines produce most of the electricity the plane needs to power the navigation and other vital systems. The primary purpose of the auxiliary power unit is to supply the energy to start the main engines, which are typically too large to be fired up with a battery.

What else do they do?

When the plane is on the ground, the auxiliary power unit helps create air pressure that makes the cabin more comfortable and keeps the air conditioner running. If aircraft parts fail during flight, the auxiliary power unit will also kick in to provide additional energy the system might need.

Do all aircraft have auxiliary power units?

All large jetliners have auxiliary power units to power secondary functions and start the engines. Smaller private planes can usually rely on batteries to provide the same functions without sacrificing power or comfort.

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