Auxiliary Power Unit — what is an APU in an aircraft?

🕗 5 minutes | July 9, 2024 | Text Mateusz Łapuć

Have you ever noticed the distinctive hum when boarding or disembarking an aircraft? Many assume it’s the sound of the main engines running, but that’s not the case. Instead, this noise comes from the APU — Auxiliary Power Unit. Let’s explore what it is and how it works.

The Auxiliary Power Unit (APU) is a small, independent engine that plays a crucial role in modern aircraft. Its primary function is to start the main engines and power essential onboard systems while the aircraft is parked at the airport.

The APU is typically located in the tail of the aircraft and operates independently of the main engines. This allows it to be switched on at any time, regardless of the status of other systems. Thanks to the APU, aircraft can run electrical, hydraulic, and air conditioning systems while on the ground without relying on external power sources. In other words, the APU allows an aircraft to be self-sufficient at the airport, eliminating the need for ground power units (GPU).

The first use of an APU in aviation dates back to 1916, when early aircraft began incorporating auxiliary power for basic systems. As aviation technology advanced, the demand for independent power sources grew.

The first transport aircraft equipped with a gas turbine-powered APU was the Boeing 727 in 1963. Since then, the APU system has evolved, becoming an essential component of nearly all commercial and military aircraft. Even space shuttles have used APU systems.

The Auxiliary Power Unit operates as a small gas turbine engine, similar to a jet engine, but without producing thrust. Instead, it burns aviation fuel to generate mechanical power, which drives an APU generator that supplies electricity to the aircraft’s systems.

Additionally, the compressed air produced by the APU is used to start the main engines. This means that the APU allows the crew to power essential aircraft systems before the main engines are turned on. The APU is especially valuable during pre-flight preparations and ground operations, where external power sources may not be available.

The APU is primarily used when the aircraft is on the ground, providing essential power for:

• Cockpit instruments
• Cabin lighting
• Air conditioning
• Hydraulic and electrical systems

One of the most critical functions of the APU is to start the main engines. At airports without Ground Power Units (GPU), the APU is the only power source available for system operations.

However, when an airport does have a GPU, it can power the aircraft’s electrical systems without the need to turn on the APU, reducing fuel consumption and noise emissions. Despite this, the GPU cannot fully replace the APU, as compressed air from the APU is essential for starting the engines.

The APU provides several important functions and benefits, including:

• Independent power supply for electrical, hydraulic, and air conditioning systems while parked.
• Starting the main engines without external assistance.
• Backup power source in case of an in-flight engine failure.
• Fuel savings by reducing reliance on ground power.

The Auxiliary Power Unit plays a crucial role in flight safety. If an aircraft experiences an engine failure, the APU can supply power to critical avionics and flight systems, allowing the flight to continue safely to the nearest airport.

Additionally, APU generators are equipped with fire detection and suppression systems, which can automatically detect and extinguish potential fires, further enhancing aircraft safety.