Auxiliary Power Unit (APU)

Auxiliary Power Unit (APU) is a small gas turbine engine commonly used in aircraft, military and commercial vehicles, locomotives, and some large maritime vehicles. The primary function of the APU is to provide electrical, hydraulic, and pneumatic power before the main engines are started or when they are shut down.

The importance of the APU in the aviation sector is significant because it performs critical functions such as maintaining cabin systems while the aircraft is on the ground, starting the engines, and providing emergency power. In commercial aircraft, it is typically located in the tail section and operates on jet fuel.

APUs are not limited to aircraft. They are also used in military vehicles, diesel locomotives, and some large trucks and trailers. For example, in military tanks, the APU is used to start the main engine and generate power with low fuel consumption during prolonged operations.

^{Auxiliary Power Unit (Credit: http://www.blackrockapu.com)}

The Auxiliary Power Unit (APU) performs important roles especially in aircraft, military vehicles, locomotives, and some large commercial land vehicles.

Aircraft engines require high-pressure air before they can be started. The APU generates pneumatic air (compressed air) to meet this requirement and enables engine start-up.

• How does it work?
• The APU produces sufficient power using an air turbine starter or an electric starter motor to rotate the main engines.

• Why is it important?
• It reduces dependence on ground support services.
• It enables rapid engine start-up.
• It allows easier engine start-up in cold weather conditions.

When the aircraft is on the ground, the main engines are turned off. However, cabin systems such as lighting, air conditioning, and avionics systems still require power. The APU generates electricity as an alternative to the aircraft’s main generators.

• Example Applications:
• Cabin lights, displays, and flight computers are powered.
• Hydraulic systems are supported.
• Communication systems remain operational.

This feature reduces dependence on ground power units (GPU) at airports and saves fuel.

The pressurized cabins and air conditioning systems of aircraft are supported by the APU, especially when the aircraft is on the ground.

• How does it work?
• The APU drives air compression systems to regulate cabin temperature and ventilation.

• Advantages:
• Enhances passenger and crew comfort.
• Maintains ideal internal temperature during extreme hot or cold weather.
• Saves fuel since the main engines are not running.

If a failure occurs in the main engines or an electrical outage happens, the APU activates as a backup power source.

• Example Scenarios:
• If one engine fails during flight, it keeps critical systems operational.
• If there is an electrical system failure, the APU generator supplies power to aircraft systems.
• In cases of fuel leaks or other emergencies, it generates the power needed for pilots to perform a safe landing.

For example, in commercial aircraft such as the Airbus A320 and Boeing 737, the APU plays a critical backup role by generating electricity independently from the engines during flight.

The APU is used not only in aircraft but also in military tanks, armored vehicles, locomotives, and large commercial trucks.

• In Military Vehicles:
• It enables communication and radar systems to operate while the main engine is off.
• It provides long-duration power with low fuel consumption for operations requiring silent operation.
• In tanks, it supports weapon systems and movement mechanisms.

• In Locomotives and Trucks:
• It generates electricity without keeping the engine idling.
• It reduces fuel consumption and lowers operating costs.
• It provides auxiliary power to prevent engine freezing in cold weather.

By supporting systems without unnecessarily running the main engines of aircraft and vehicles, the APU reduces fuel consumption and carbon emissions.

• For example:
• Fuel is saved when power is generated without running the engine while the aircraft is on the ground.
• Noise pollution at airports is reduced.
• It emerges as an environmental friendly system with lower carbon emissions.

Especially newer generation APUs are designed to be more efficient and have lower emissions.

The Auxiliary Power Unit (APU) is a system that generates electrical, pneumatic, and hydraulic energy using a small gas turbine or internal combustion engine. The operating principle of the APU used in aircraft and some land and maritime vehicles can be examined in three main phases: Start-up, Operation, and Shutdown.

1- The starter motor engages.

2- Fuel injection begins.

3- The turbine begins to operate.

4- The APU reaches steady state.

Once the APU is operational, it provides power for two main systems:

1- Electrical Generation:

2- Pneumatic Power Generation:

While operating, the APU continuously monitors temperature, pressure, and fuel consumption. If any anomaly is detected, safety systems activate and shut down the APU.

1- The APU load is reduced.

2- Fuel flow is cut off.

3- The cooling process begins.

4- The APU shuts down completely.

The operating principle of the APU varies according to its intended use:

1- Normal Mode:

2- Emergency Mode:

3- Fuel Saving Mode:

4- Cold Weather Mode:

The APU operates on the basis of a small gas turbine engine and consists of the following components:

1- Air Intake System

2- Fuel System

3- Starter System

4- Combustion Chamber

5- Turbine and Generator

6- Exhaust System

7- Control and Protection Systems

The Auxiliary Power Unit (APU) is strategically positioned in aircraft to ensure safe and efficient operation. Depending on the aircraft design, the APU is typically located in the tail section or, in some specialized designs, beneath the wings or within the fuselage.

Location of the APU in aircraft (Credit: ucaklar.org)

In most commercial and military aircraft, the APU is located as follows:

1- In the Aircraft’s Tail Section (Tail Cone / Rear Fuselage)

2- Beneath the Fuselage or Within the Wings

3- Inside the Fuselage or at the Wing Root in Helicopters

When determining the location of the APU in an aircraft, the following engineering criteria are considered:

1- Exhaust Gas Direction

2- Aerodynamic Balance

3- Maintenance and Accessibility

4- Fuel and Pneumatic Connections

• The APU emits exhaust gases during operation.
• In commercial aircraft, the exhaust outlet is typically located at the very rear of the tail.
• In military aircraft and helicopters, it can be integrated into the engine compartment.
• Proper positioning of the APU exhaust outlet provides the following advantages:
• It does not affect the cabin with hot air.
• It does not compromise aerodynamic performance during flight.
• It prevents damage to fuel lines and electrical connections.

Click here to watch a simulation explaining how the Auxiliary Power Unit works.