Auxiliary Power Unit (APU)

The Auxiliary Power Unit (APU) is a gas turbine engine commonly used in aircraft, military and commercial vehicles, locomotives, and some large sea vehicles. The primary function of the APU is to provide electrical 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 on the ground, starting the main 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)

Primary Functions of the APU

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

1. Starting the Aircraft’s Main Engines

Aircraft engines require high-pressure air before they can be started. The APU generates this demand by producing pneumatic air (compressed air) to enable engine start-up.

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

• Why is it important?
• Reduces dependence on ground support equipment.
• Enables engine start-up.
• Facilitates easier engine starting in cold weather conditions.

2. Providing Electrical Power While on the Ground

When the aircraft is on the ground, the main engines are turned off. However, cabin systems such as lighting, climate control, avionics systems and others require electrical power. The APU generates electricity as an alternative to the main generators.

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

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

3. Enabling Air Conditioning Operation

The pressurized cabins and air conditioning systems of aircraft are supported by the APU, especially when 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 conditions.
• Saves fuel by avoiding operation of the main engines.

4. Providing Emergency Power

If a failure or power interruption occurs in the main engines, the APU activates as a backup power source.

• Example Scenarios:
• If one engine fails during flight, the APU keeps critical systems operational.
• In the event of an electrical system fault, the APU generator supplies power to aircraft systems.
• During fuel leaks or other emergencies, it provides the necessary power for pilots to execute a safe landing.

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

5. Supporting Military and Commercial Vehicles

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

• In Military Vehicles:
• Maintains operation of communication and radar systems while the main engine is off.
• Provides extended power with low fuel consumption during silent operations requiring stealth.
• Used in tanks to support weapon systems and movement mechanisms.

• In Locomotives and Trucks:
• Generates electricity without keeping the engine idling.
• Reduces fuel consumption and lowers operating costs.
• Acts as a supplementary power source to prevent engine freezing in cold weather.

6. Improving Fuel Efficiency and Environmental Benefits

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 electrical power is generated on the ground without running the engines.
• Noise pollution at airports is reduced.
• It stands out as an environmentally system solution with lower carbon emissions.

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

Operating Principle of the APU

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 sea vehicles, can be examined in three main stages: Start-up, Operation, and Shutdown.

A) Start-up (Starting Phase) 

1- The starter motor engages.

2- Fuel injection begins.

3- The turbine begins operation.

4- The APU reaches steady state.

B) Operation (Running Phase) 

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

1- Electrical Generation:

2- Pneumatic Power Generation:

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

C) Shutdown (Shutdown Phase) 

1- The APU load is reduced.

2- Fuel flow is cut off.

3- The cooling process begins.

4- The APU shuts down completely.

Different Operating Modes of the APU

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

1- Normal Mode:

2- Emergency Status Mode:

3- Fuel Saving Mode:

4- Cold Weather Mode:

Components of the APU

The APU operates on the principle of a small gas turbine 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

Location of the APU in Aircraft

The Auxiliary Power Unit (APU) is installed in a specific location on aircraft to ensure safe and efficient operation. Although its position varies depending on aircraft design, it is typically located in the tail section or, in some specialized designs, beneath the fuselage or within the body.

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

Typical APU Location

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- In Helicopters: Within the Fuselage or Near the Wing Root

Factors Influencing APU Location Selection

The following engineering criteria are considered when determining the APU’s location on an aircraft:

1- Exhaust Gas Direction

2- Aerodynamic Balance

3- Maintenance and Accessibility

4- Fuel and Pneumatic Connections

 APU Location by Aircraft Model

Location and Importance of the APU Exhaust Outlet

• The APU emits exhaust gases during operation.
• In commercial aircraft, the exhaust outlet is typically located at the rear of the tail.
• In military aircraft and helicopters, it may be integrated into the engine compartment.
• Proper positioning of the APU exhaust outlet provides the following advantages:
• Hot air does not affect the cabin.
• It does not compromise aerodynamic performance during flight.
• Fuel lines and electrical connections are protected from damage.

Advantages of the APU

Click to view the simulation explaining the operating principle of the Auxiliary Power Unit.