Things to Know about Auxiliary Power Unit

Posted on January 20, 2020 James Hinds Aerospace Engine

An auxiliary power unit (APU) is a small turbine engine installed near the rear of the fuselage and serves as an additional energy source in starting one of the main engines on an airliner or business jet. Though a number of airports in the United States restrict the operation of APUs (especially at night) due to the noise they produce, the majority of an APUs usage is on the ground, before take-off and after landing. In addition to its starting capabilities, APUs are equipped with an extra electrical generator that creates enough power to operate onboard lighting, galley electrics and cockpit avionics.

Auxiliary power unit manufacturers like Honeywell Aerospace, United Technologies and Safran lead the market in aircraft APU production. These aircraft manufacturers, and others alike, determine APU requirements by taking into consideration cabin size, the amount of bleed air required to power environmental packs, and which size generator is needed to start an engine and power aviation components located in the cockpit and cabin.

Flight route projectories that place an aircraft more than 60 flying minutes away from a suitable emergency airport, negate the need of an APU systems to be tested  -- using a cold-start procedure -- to confirm reliability prior to flight, in case an aircraft’s main engines fail while in the sky. Like all modern turbine engines, APUs are equipped with aerospace hardware parts such as fire-extinguishing systems that operate automatically in newer aircraft. When on the ground and an aircraft’s APU is not automatic, at least one pilot is required to remain near the aircraft to manually discharge an APU’s fire bottle incase of a fire. Unlike an aircraft’s main engine TBO (time before overhaul), many APUs are treated more like a pass/fail item, versus an overhaul that requires a regular tear-down for maintenance. This allows operators to run their APU until something breaks.   

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