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A Lesson On Redundancy: Auxiliary Power Units

For a lot of people, flying is terrifying. People fear that planes will suddenly lose power or that engines will fail, and planes will drop out of the sky. Fortunately, that’s a very unlikely scenario since planes have utilized redundancy and been equipped with APUs since at least WWII.

An APU, or auxiliary power unit, is a smaller engine that provides energy for non-propulsion functions, generally on aircraft, ships, and larger land vehicles. In aircraft, APUs are usually located in the very back, right below the tail. Their primary purpose is to provide power, electric, pneumatic, or hydraulic, to start the main engines. APUs also start the cabin air and electric power, serve as an emergency source of electric power in the event of engine failure, and can start the aircraft engines mid-flight in the event of an emergency. But, they’re generally turned off once the aircraft is in mid-flight.

APUs are started by a battery or hydraulic accumulator. Once they’re started, APUs generate bleed air to start the turbine engines. If the APUs fail before the engine starts, the engines can’t be started without an external start cart to provide bleed air. If the APUs fail mid-flight, there’s no immediately noticeable effect. Aircraft engines can still be restarted mid-flight without an APU, if need be, either by using a working engine to generate bleed air to start a dead engine, or by having the aircraft dive and attain enough speed to spin the turbine fast enough to start the engine. That doesn’t mean that having an APU is a bad thing.

APUs are redundant. But, that redundancy is what makes flying safer and what assuages our fears of flying. So, testing APUs for reliability and functionality is really important. APU testing facilities need to coordinate with the OEM’s in order to test APUs at various conditions and circumstances to confirm that they are performing properly. High-speed data acquisition systems and performance correction software help APU manufacturers, end users, and overhaul and service agencies get the best performance from their APUs.

At Aviation Sourcing Solutions, owned and operated by ASAP Semiconductor, we care about the safety of our customers. We strive to be the premier supplier of hard to find aviation parts and MRO services because we know that offering high-quality APUs and turbine engines at an affordable price can make all the difference.


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July 15, 2019