What is APU in Aircraft – Understanding Auxiliary Power Units

ByNikolas

What is an APU? – Overview of Auxiliary Power Units

The Auxiliary Power Unit (APU) is a small gas turbine engine—essentially a self-contained generator.

The APU’s primary role is to supply two types of power on the ground:

  • Electrical Power: Generates 115V AC (400 Hz) to run cockpit instruments, cabin lighting, and galley equipment.

  • Pneumatic Power: Provides high-pressure compressed air to operate the air conditioning system and start the main engines.

Typically located in the aircraft’s tail cone, the APU is easily identified by a small exhaust outlet. While this is the most common placement, some designs house it in an engine nacelle or the wheel well. Beyond its primary ground functions, the APU also serves as a critical backup power source during flight, improving operational safety and redundancy.

How Does an APU Work? – Mechanism and Functionality

An Auxiliary Power Unit operates on the same principles as a full-sized jet engine—just on a much smaller scale. The process begins as the APU draws in outside air, which is then heavily compressed, causing its pressure and temperature to skyrocket. From there, this high-pressure air enters a combustion chamber, mixes with fuel, and ignites, creating a high-energy stream of hot gas that expands with explosive force.

This expanding gas spins a turbine, converting thermal energy into mechanical force. The turbine drives two key components through a gearbox: an electrical generator for AC power and a load compressor that supplies the high-pressure ‘bleed air’ needed for air conditioning and engine starts.

An electronic control unit (ECU) controls the APU’s entire operation. After a pilot initiates start-up from the cockpit, the ECU takes over, regulating fuel flow, monitoring key parameters, and ensuring safe, efficient performance. It automatically adjusts the APU’s output to meet the aircraft’s power demands, from running cabin lights to providing the compressed air needed for a main engine start.

The Alternative: Ground Power Units (GPU)

On the ground, running an APU consumes fuel and creates both noise and emissions. To counteract this, airports provide alternatives: Ground Power Units (GPUs) for electricity and Pre-Conditioned Air (PCA) units for climate control. Plugging into these external systems allows the APU to be shut down—a simple action that cuts local noise and pollution, saves fuel, and minimizes wear on the unit.

APU Fuel Consumption – Efficiency and Impact

An APU’s fuel burn is a major operational concern for airlines. A typical narrow body aircraft, like a Boeing 737 or Airbus A320, consumes around 120 kg (about 40 US gallons) of fuel per hour just to run its APU. That figure more than doubles for larger wide body aircraft, which burn between 230 and 300 kg (75-100 US gallons) per hour to power their extensive systems on the ground.

This consumption rate is not static and fluctuates based on two main factors:

  • Power Load: The demand for electrical and pneumatic power has the biggest impact. For example, running air conditioning on a hot day consumes far more fuel than just powering cabin lights.

  • Maintenance Condition: A well-maintained unit operates more efficiently. Regular servicing is essential for managing fuel costs and reducing emissions.

What Happens if the APU Fails? – Safety Considerations

While an APU failure isn’t catastrophic, its impact depends entirely on when it happens. On the ground, an inoperative APU leaves the aircraft without its independent source of electrical and pneumatic power.

In flight, an APU failure is far less critical. With the main engines already running, their own generators provide all necessary electrical power.

However, the loss of this backup system is a serious concern, especially for twin-engine jets on long-haul oceanic routes (STOPS). In critical failure scenarios, the APU is the designated source for essential flight control power or the bleed air needed for an in-flight engine restart. Its failure removes a key layer of redundancy, narrowing the aircraft’s safety margins.

Major APU Manufacturers – Key Players in the Industry

The design and production of Apus is a specialized field dominated by a handful of key manufacturers. These companies develop the technology used in aircraft worldwide and drive innovations in efficiency, safety, and environmental performance.

Three of the most prominent APU manufacturers are:

  • Honeywell Aerospace: A leading global manufacturer, equipping a wide range of commercial and military aircraft. Honeywell’s designs focus on reliability, fuel efficiency, and weight reduction to lower operational costs.

  • Pratt & Whitney: A major engine manufacturer that uses its gas turbine expertise to produce high-performance Apus for modern airliners. Its designs emphasize durability and power density for reliable ground and in-flight backup power.

  • Saran Power Units: A specialist in aerospace power systems, producing a wide range of Apus for commercial, regional, and rotary-wing aircraft. The company focuses on enhancing reliability and developing more-electric solutions for future aviation needs.

Through continuous research and development, these industry leaders advance APU technology. Their shared goal is to create quieter, more fuel-efficient units that require less maintenance, improving the safety and sustainability of global air travel.

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