Your Airliner Has A Hidden Engine! by AeroSavvy

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Your Airliner Has A Hidden Engine!

How many turbine engines are on an Airbus A320?

Two, right?   Would you believe three?

How about a Boeing 747?

Four engines? Wrong again. A 747 has Five turbine engines!

Hiding inside the tail of most every airliner is an extra engine called an Auxiliary Power Unit or APU. Time to find out what’s hidden in the tail of your aircraft!

Airliners are loaded with redundant systems to help make them reliable. Years ago, aircraft manufacturers and operators figured out that installing a small turbine engine in an aircraft’s tail is a great way to provide an extra level of safety, convenience, and comfort.

The auxiliary power unit is an actual turbine engine, more appropriately called a turboshaft engine. Unlike the aircraft’s main engines, the APU outputs almost no thrust. Most of the power produced is used to run an electric generator and provide pneumatic (air) pressure.

Comfort and Convenience

The auxiliary power unit is usually run on the ground during passenger boarding and deplaning. The APU turns an electric generator that powers the electrical system on the aircraft when the main engines are off. It also provides pneumatic pressure for air conditioning and, more importantly, starting the main engines. This eliminates the need for using an external power source and a noisy air compressor cart to keep the cabin comfy while boarding.

Next time you hop on an airliner (even a small regional jet), listen carefully as you board. You’ll hear the familiar whine of a jet engine, even though the main engines aren’t running. You’re hearing the APU doing its thing!


The most important job for the APU is to provide redundancy. The APU’s electric generator can be used during flight in case one or more of the aircraft’s main generators have a problem. Similarly, in the event of a pneumatic malfunction in the main engines, the APU can provide compressed air for cabin pressurization and airframe ice protection. The redundancy provided by the APU is one of the reasons twin engine jets are allowed to fly long distances over the ocean. It’s a pretty big job for a small jet engine hiding in the tail.

Small jet engine?

Auxiliary power units aren’t exactly small. Many APUs are variants of full size turbine engines used in other aircraft. Check out the classic Garrett (now Honeywell) TPE331 engine: For years it has powered large turboprops like the Jetstream 41, Metroliner, and Beechcraft King Air. A variant of this engine, the Honeywell 331, is used as an APU in many Boeing and Airbus wide body jets. Instead of a propeller, the APU has an accessory shaft that powers a generator.

Firing up the little jet engine

Operating the auxiliary power unit couldn’t be easier. The APU control panel on most airliners consists of a switch or button and a couple of lights – that’s it! Starting the unit is as easy as starting a car. APU manufacturers have done a great job designing these engines so they take care of themselves. If there is a problem, the system is smart enough to shut the APU down and let us know. It’s pilot-proof!

Where does the APU get its air?

Jet engines need a lot of fresh air to run. So, where does the APU get its air? You have to look carefully to spot the air intake. The intakes are usually covered with motorized doors that open when the APU is started. The best time to look is when the airplane is at that gate. Once the aircraft is taxiing, the APU will likely be shut down and the door will be closed, hiding its location.

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