AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

The Function of APUs in Aircraft

Auxiliary power units (APUs) are electrical power and heat sources used in mobile systems and operate independently from the main engine that is responsible for propulsion. Some of these mobile systems include passenger cars, buses, trucks, ships, and aircraft. Within aircraft, APUs generate on-board electricity for air conditioning and other related systems, especially during on-ground, and take-off procedures.     

Located in the tail of aircraft, APUs are positioned far from main engines, freeing up space for cargo and fuel in other parts of the aircraft. It is important to note that the high-pressure air that the APU supplies is not obstructed by the distance to the engine.

The APU’s independence from the main engine provides the opportunity to optimize many of its functions. For instance, traction, propulsion, and electricity supply can be made more effective, offering higher economy at reduced fuel consumption, as well as producing less noise and exhaust that impacts the environment. That being said, aircraft engineers are crafting ways to improve mobile systems, offering variations that contain environmentally friendly components and emit less harmful gasses.

APUs are considered small fuel cell units that serve as feasible alternatives to other forms of power supply that can remedy the issues associated with higher energy consumption by mobile systems. In most cases, APUs are constructed with weight, volume, and reliability metrics that suit a variety of applications. For aircraft, APUs are usually designed with certain requirements, accounting for the size of the cabin, the amount of bleed air needed to power the environmental packs, and the generator size needed to power the cockpit and cabin.

For on-ground operations, APUs serve particularly important roles. Auxiliary power units (APUs) generate electrical power for the cockpit and cabin, and they produce pneumatic pressure to operate the cabin air conditioning systems. During flight, APUs can be used for electrical power or bleed air to restart the engines in the event of engine failure. For example, the APU was critical in the unprecedented landing of US Airways Flight 1549 in the Hudson River, providing electrical power when the engine would not restart.  

With the above example in mind, the other main function of the APU is to start the main engines. The blades of the main engines turn with the use of bleed air from the APU turbine. This movement generates ample airflow through the main engine, allowing the fuel-air mixture to ignite and start the engine. Without airflow, the engine may overheat, damaging the entire apparatus. Once the engine reaches an idle power speed, the power supply from the APU is removed. Then, the other engines are started with the use of an APU or another source of high-pressure air from the already started engine. This is known as “cross bleed,” and is a common method for restarting an engine that has failed.

While APUs have become particularly integral to modern jet aircraft, they have been around for decades. In fact, many military aircraft in WWI and WWII had early versions of APUs, including the British Supermarine Nighthawk in WWI and the B-29 Superfortress in WII. Today, they find use in many other mobile systems, such as cars, trucks, trains, and various others. However, APUs are not ever-lasting components, requiring routine checks and maintenance to ensure their functionality. More than that, one must verify that all of the APU’s components are working optimally, some of which include the fuel tank, exhaust duct, system controller, and other related parts.

In the case that your APU is inactive for an extended period of time, one should carry out preservation procedures as indicated in the APU maintenance manual. This will guarantee that the APU is delivering the same level of performance. Beyond conducting regular performance checks and inspections, never put off replacing worn out parts or components. Identifying minor issues, such as a damaged sensor or a malfunctioning fuel manifold, can save you thousands in the long run. With this in mind, when you find yourself in need of top-quality replacement parts, including APU tubes, APU plug assemblies, and APU exhaust support items, rely on Stacked NSN.

Stacked NSN is a premier online distributor of aircraft parts that have been sourced from thousands of trusted global manufacturers. With over 2 billion new, used, obsolete, and hard-to-find items in our inventory, customers are bound to find exactly what they need. As we make quality the cornerstone of our operations, we subject our products to rigorous quality assurance measures, tests, verifications, and assessments. More than that, we ensure that items ship out with the proper manufacturer trace documentation, guaranteeing the integrity of our stock. Browse our ever-expanding catalogs at your leisure and take advantage of our Instant RFQ service to request a quote on any item that sparks your interest. Within 15 minutes of submission, an account manager will reach out with a customized solution. Get started today to experience the future of part procurement!


June 25, 2024


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