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!
Crews normally run the APU before flight for electrical power, air conditioning and engine start. After landing, the APU is started so engines can be shut down as soon as the aircraft arrives at the gate. If the APU is needed during flight, it can be started in about a minute. It’s a wonderful thing!
Watch an APU Start!
The video shows the start sequence of a Boeing 767’s APU (a Honeywell GTCP 331-200). When the flight crew begins the start, the air intake door opens on the aircraft’s tail and an electric motor begins to spin the engine. Fuel is added and within seconds, the APU is up-to-speed and providing electric and pneumatic pressure for various aircraft systems.
Do all airliners have an APU?
Most every modern turbojet powered airliner (including the smaller regional jets) have an APU installed in the tail. Only a few large turboprop aircraft have them due to the weight restrictions of smaller aircraft.
Look for the tailpipe!
Next time you’re at the airport, take a look at aircraft tails. You’ll notice an exhaust outlet for the aircraft’s auxiliary power unit. The APU is hiding inside the tail, just in front of the exhaust. So that’s what the little tail pipe is for!
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.
- The first airliner to have a gas turbine APU was the Boeing 727 in 1963. The 727 was designed for smaller airports with little (or no) ground support equipment. The 727’s APU was located in the main landing gear bay. The exhaust was through louvers in the top of the right wing!
- Aérospatiale-BAC Concorde didn’t have an APU. Because the aircraft was small and extremely weight sensitive, it was designed without an APU. Concorde only operated into large airports with plenty of ground support equipment.
- Two other classic airliners, the Boeing 707 and Douglas DC-8, were also built without an APU. These airplanes were widely used well into the 1990’s and beyond. A few of these aircraft were modified to include an APU in a wheel well. The KC-135 tanker (a variant of the 707) was upgraded to include an APU in its tail.