The Role of Auxiliary Power Units in Aviation

While it may not always be in the spotlight, the auxiliary power unit (APU) plays a significant role in the complex world of aviation. Simply put, the APU contributes to the seamless operation of aircraft by ensuring power is available during ground operations or emergency scenarios. In this blog, we will delve into the design and function of APUs, as well as what happens in the event that they encounter issues.

An APU is a small turbine engine typically located in the tail section of an aircraft. Its primary function is to provide electrical power to operate various onboard systems while the main engines are not on, including lighting, avionics, and cabin systems. It also offers bleed air, which is used for starting the main jet engines and for powering the air conditioning and pressurization systems. The use of the APU means that the main engines do not need to be running for these tasks, thus reducing fuel consumption and emissions during ground operations.

When activated, the APU's turbine engine begins to spin, generating electrical power through an onboard generator. Simultaneously, the compressor unit draws bleed air from the turbine, which is then routed through a series of ducts and valves to air conditioning and pressurization systems. The fuel system, which includes a fuel pump and fuel control unit, keeps a steady and regulated supply of fuel to the turbine for consistent operation. Additionally, the integration of these components ensures that the APU provides both electrical power and bleed air effectively.

When discussing the applications of an APU in aviation, it is important to understand its role during different phases of flight. While the APU is very useful for ground operations, its operation during flight is not required. Once an aircraft's main engines are running, they generate sufficient electrical power and bleed air to support all systems, so as a result, the APU is generally shut down during flight. This practice helps to conserve fuel and minimize wear and tear on the APU, ensuring it is in optimal condition for when it is needed most.

Should a malfunction occur with the main engines, however, the APU can be activated to provide essential electrical power and bleed air. Its ability to act as a backup source of power and air is a key safety feature, allowing an aircraft to continue functioning and maintaining vital systems like communications and navigation. Although they are not intended for continuous use during flight, they can operate for several hours if required, depending on the specific model and its operational conditions. This duration is generally sufficient to handle most emergency situations, allowing ample time for pilots to make a safe landing.

This may lead one to wonder what happens if an APU fails in flight. In most cases, the failure of the APU during flight does not pose a significant risk aside from the loss of a critical backup system. The primary impact is usually limited to the loss of some auxiliary functions that it might have been providing, such as powering ground support equipment or offering additional power for specific onboard systems. In these instances, the flight crew will still rely on the main engines.

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