Expert: Ray Wilkinson - 7/16/2007

Question
Hi there.I was wondering if there is a turbine situated below the tail of a jet? Well,not really a turbine, but more of a pipe or some sort of propulsion system,allocated right at the back of the jet.It is relatively small.If there is, what is its function?

Regards
Ryan Groenewald

Answer
Hi Ryan

You are correct that there's an engine under the tail of most airliners, but it isn't propulsion. It's the auxiliary power unit (APU), which is a small gas turbine engine that generates air, electrical power and hydraulic power. Its main use is on the ground, where it can power up services such as cabin air and the electrics without needing to start one of the main engines. However, there is a safety feature as well - it isn't normally run in flight, but can be started in the event of an engine failure, to ensure continuity of electrical and hydraulic power if another engine fails. The standard joke is that if both engines on a twin jet fail, the APU lets you decide which field to die in.

Some aircraft are starting to use APUs in flight, as the air and power taken off the main engines can cause problems optimising the airflow through the engine over all flight conditions, increasing fuel burn. Having one or two APUs running continuously at their optimum settings, and allowing the main engines to produce just thrust, seems like a good idea. As more electrical power is used, for instance for producing cabin air, using the APU is becoming more attractive. For instance, the 787 Dreamliner uses electrics rather than engine bleed air for cabin air. It also means the air is less dry, which is good for throats, incidentally.

I hope this answers your question.