What Is an Auxiliary Power Unit (APU) and How Does It Operate

While an aircraft’s main engines provide the thrust necessary for flight and often sustain electrical functions in the air, boarding, pre-flight checks, and maintenance protocols demand electrical and pneumatic power without needing the engines to be run. For these reasons, Auxiliary Power Units were created. With this in mind, this blog will break down APU design and operation, giving you a clear understanding of the significance they have in aviation.

The Role of an APU

An APU is a compact, self-contained engine installed on an aircraft to reduce reliance on external ground support equipment by supplying electricity to onboard systems and pneumatic power for main engine starts. In most commercial and military aircraft, APUs are miniature gas turbine engines, which utilize components like:

• Compressors and Turbines: Forming the core of the system, the compressor draws in air, mixes it with fuel, and ignites it to create high-pressure, high-temperature gases. These gases drive the turbine, generating the mechanical energy that supplements other subsystems.
• Gearboxes and Generators: Mechanical energy from the turbine is transmitted through a gearbox that drives electrical generators, converting turbine rotation into stable electricity output for avionics, lighting, and other such systems.
• Exhaust Systems: Because turbine operation produces heat and gases, exhaust systems are carefully designed to safely channel byproducts away from the aircraft structure.

While turbine-based APUs remain the standard due to their ability to supply both electrical and pneumatic power, electric APUs are becoming more common in smaller and hybrid-electric aircraft. These units do not rely on combustion, offering quieter performance, lower emissions, and reduced maintenance needs.

Standard Operating Sequence

Whether turbine- or electricity-based, all APUs operate in a similar sequence:

• Step 1 — Start-Up: The APU is brought to life using the aircraft battery, or in some cases, an external power source. In turbine-based units, fuel is injected into the combustion chamber, ignited, and used to spin the turbine shaft, whereas electric APUs require stored electrical energy to start.
• Step 2 — Power Generation: Once running at a quick enough speed, the APU begins delivering electricity to aircraft systems like lighting and environmental controls. Turbine-based APUs also provide pneumatic energy, which is directed to start the main engines and support air conditioning and pressurization systems.
• Step 3 — Shutdown: After the main engines are fully operational, the APU is typically shut down to conserve resources and reduce wear. Many modern units employ automated control systems that manage shutdown procedures, keeping this transition safe and smooth.

The Functional Benefits of APUs

As we have illustrated, APUs are made to enhance aircraft operations on the ground, presenting the benefits of: