Do Aircraft Need an Auxiliary Power Unit?

From small, general aviation aircraft to large commercial airlines, modern designs are increasingly being equipped with advanced systems that bolster performance and increase safety. Among the many systems that drive flight operations, the Auxiliary Power Unit (APU) is notable for its ability to support power and thrust needs without pure reliance on the main engines. While not every aircraft requires an APU, most commercial and many military platforms integrate them due to their valuable contributions to ground operations, inflight redundancy, and overall system reliability. As such, it can be beneficial for those involved in the industry to have an understanding of what an APU does, how it operates, and why manufacturers choose to include them in various designs.

What Is an Auxiliary Power Unit?

In its most basic form, an APU is a self-contained engine that is installed in the tail section of an aircraft to supply power when the main engines are not running. Many APUs come as small gas turbines, where exact capabilities will vary based on the aircraft’s electrical needs and pneumatic architecture.

The Core Functions of APUs

1. Power Generation: APUs produce electrical power for onboard systems when the main engines are shut down, enabling lighting, avionics, communications, environmental controls, and maintenance functions to operate during preflight preparation, ground servicing, or unexpected delays at a gate.
2. Engine Starting: APUs are often leveraged to provide pneumatic power for main engine starting procedures, supplying compressed air that spins a starter turbine and initiates combustion. This function is especially valuable when ground-based start systems are unavailable or impractical.
3. System Support: APUs may also be used to support environmental control systems, enhancing passenger comfort and enabling proper cabin conditioning when reliance on airport infrastructure is unfeasible.

Together, these capabilities make APUs essential for ground autonomy and greatly improve operational flexibility for the aircraft that feature them.

Basic Operation of an APU

Though compact and serving a different role, an APU includes many of the same subsystems and functions found in full-scale jet engines. Nevertheless, their distinct operational differences and applications are important to be aware of.

1. Starting Initiation: An APU is first activated through the use of an electric or pneumatic starter, which kicks off compressor movement and airflow through the engine core.
2. Combustion and Stabilization: Fuel is then introduced into the combustor, leading to ignition and turbine rotation. As the system stabilizes, the turbine reaches a self-sustaining speed.
3. Power Delivery: Once at a suitable operating RPM, the APU provides electrical and pneumatic power to the aircraft as needed. The system’s control unit automatically regulates fuel flow, temperature, and turbine speed for optimal functionality.
4. Shutdown: Once no longer needed, the APU undergoes a controlled cooldown process before shutting down to maintain component longevity and preserve system integrity.

This autonomous, straightforward operation makes APUs invaluable during scenarios where engine power is not yet available or when redundancy is required.

Why Some Aircraft Use APUs While Others Do Not

Across the industry, APUs are most prevalent in medium to large commercial airliners, business jets, and many transport or special-mission military aircraft. While these forms of aircraft derive great benefits from the use of APUs, such systems are not essential for all designs or needs. Generally speaking, the decision to include an APU will depend on a combination of engineering, operational, and economic factors, some of which we will share below.

Aircraft Size and Mission Profile

Large commercial airliners rely heavily on APUs due to the significant electrical and pneumatic loads that are required to support preflight activities, cabin climate control, and engine starting procedures. Meanwhile, smaller general aviation aircraft may not generate sufficient operational demand to justify the weight and cost of an APU, instead using ground power units or onboard batteries as necessary.

Operational Independence and Flexibility

Aircraft designed to operate in remote regions, austere environments, or airports with limited ground infrastructure often incorporate APUs to promote more reliability and support when external assistance is scarce or unavailable. For many commercial airliners, the ability to keep cabin conditions comfortable without overreliance on runway equipment is also a major advantage.

Redundancy and Safety Requirements

Many multi-engine aircraft utilize APUs as a secondary electrical power source in the case of engine or generator failure, ensuring that critical systems and functions can continue while a safe landing or diversion is made. This redundancy enhances aircraft dispatch reliability and supports compliance with evolving operational safety rules.

Securing Quality-Assured APU Components

For aircraft equipped with APUs, maintaining the condition and airworthiness of these systems is nonnegotiable, making it necessary that professionals seek dependable distribution options when shopping for parts. Here on The Purchasing Hub, we connect customers with a wide-ranging aviation product selection that is sourced from leading manufacturers across the globe, our stock including fuel control units, starters, pneumatic valves, electronic controllers, and other in-demand APU solutions. We are fully committed to quality assurance, competitive pricing, and timely delivery, with our industry experts always being a call or email away from providing hands-on support and tailored pricing options for your benefit. With all that we have to offer, get in touch today, and see why so many customers continually depend on The Purchasing Hub for APU parts and other aviation solutions.

1. andrew paul
2. Posted on December 4, 2025
aircrafts