What is an APU on a Plane – Understanding Auxiliary Power Units

What is an APU – Definition and Overview

The answer lies in a compact, self-contained engine called the Auxiliary Power Unit (APU). This small gas turbine provides all the energy an aircraft needs on the ground—from cabin lights to air conditioning—ensuring passenger comfort and operational readiness without firing up the powerful and fuel-hungry main engines.

This compact powerhouse is crucial for the smooth operation of modern commercial aircraft, bridging the gap between external ground services and the immense power of the main engines.

The acronym APU should not be confused with its counterpart in computing, where it stands for Accelerated Processing Unit. In the context of aviation, the APU is exclusively the Auxiliary Power Unit—a device critical to an aircraft’s daily operations.

How Does an APU Work – Operational Principles

An APU is essentially a gas turbine. The process begins simply: an inlet door on the aircraft’s fuselage, usually near the tail, opens to draw in outside air. This action begins the process to generate both electrical and pneumatic (air pressure) energy for the plane.

Once inside, the air is channeled into a compressor section, where a series of rotating blades squeezes it, dramatically increasing its pressure and temperature.

Functions of an APU – Powering Aircraft Systems

The APU performs several critical functions, ensuring an aircraft’s self-sufficiency on the ground and providing essential backup power in the air.

Powering Onboard Systems at the Gate

• Electrical Power: Supplies electricity to all onboard systems, including cockpit instruments, cabin lighting, galley equipment, and navigation systems.

• Pneumatic Power: Provides high-pressure “bleed air” to operate the environmental control systems, ensuring cabin comfort before departure.

Starting the Main Engines

A critical function of an APU is starting the aircraft’s main jet engines. A jet engine is too large to be started with a simple electric motor like a car. Instead, it requires a massive volume of high-pressure air to spool up its internal components. The APU supplies this essential bleed air through a network of ducts directly to the engine starters. Once the engine reaches a self-sustaining speed, the bleed air from the APU is shut off, and the engine runs on its own power.

Providing Backup Power In-Flight

While the main engines power the aircraft during flight, the APU serves as a vital backup. In the rare event of an engine failure, it can be started to provide supplementary electrical and pneumatic power, ensuring that critical flight systems remain operational. This redundancy is fundamental to modern aviation safety.

APU Fuel Consumption – Efficiency and Usage

A key reason for an aircraft’s reliance on an APU is its remarkable fuel efficiency. Main engines, designed for immense thrust, are highly inefficient when idling on the ground. The APU, a much smaller turbine, is optimized to provide electrical and pneumatic power while consuming significantly less fuel, making it the ideal power source for ground operations.

For example, the APU on a narrow-body jet like an Airbus A320 or Boeing 737 typically consumes 110-150 kg (approx. 240-330 lbs) of fuel per hour. By comparison, running just one of the main engines at idle on the ground could burn more than five times that amount. These significant fuel savings make the APU an economic necessity for ground operations.

APU vs Ground Power Unit – Key Differences

While an aircraft is parked at the gate, it has two primary options for sourcing power without running its main engines: its own onboard Auxiliary Power Unit (APU) or an external Ground Power Unit (GPU). Although they serve a similar purpose, they are fundamentally different in design and application.

A Ground Power Unit is a piece of airport ground support equipment—either a mobile cart or a fixed installation at the gate—that connects to the aircraft via a large electrical cable. Its sole function is to supply electrical power from the airport’s grid or its own generator, allowing the plane’s lights, avionics, and other systems to operate while parked.

• Location and Autonomy: The APU is an onboard engine, giving the aircraft complete autonomy. In contrast, a GPU is external ground equipment that tethers the plane to a fixed power source.

• Type of Power: The APU generates both electricity and pneumatic pressure (bleed air) for air conditioning. A standard GPU supplies only electricity, often requiring a separate ground unit for climate control.

Importance of Apus in Aviation – Reliability and Safety

The Auxiliary Power Unit is more than a convenience for ground operations; it is fundamental to modern aviation safety and reliability.

This role is especially critical during flights over vast oceans or remote terrain. For twin-engine aircraft to be certified for long-distance STOPS (Extended-range Twin-engine Operations) routes, the reliability of their backup systems, including the APU, is rigorously tested. The APU must be able to start and provide power at high altitudes to guarantee a safe diversion if an engine fails.

APU Failure During Flight

In contrast, an APU failure during flight is rarely a critical event. At cruising altitude, the main engines provide all necessary power, with the APU serving only as a backup. Should it fail, the flight can typically continue safely to its destination.