What is the Purpose of an Auxiliary Power Unit?

Airports · 4 min read · Dec 20, 2021
auxiliary power unit

The Auxiliary Power Unit (APU) is rarely mentioned in aviation conversations despite its value to any aircraft functioning. Generally, when people speak about aircraft, they tend to highlight things like design features, jet engine power, maximum speed, and maximum range, among other things.

Of course, these are characteristics that appeal to the mind, but the truth is that no aircraft would be as amazing as it can be without an APU. If you are wondering why just keep reading to learn the details.

What does an Auxiliary Power Unit do?

OK, saying that aircraft would not be amazing without an APU is a bold statement. Still, APUs are really important for aircraft to be able to perform as expected at each stage of the flight and even while being on the ground.

An APU is essentially a small turbine engine most aircraft have at the tail, which is why we see a small exhaust outlet there. This is normally called the APU exhaust, and it should not be confused with the main engines since it has nothing to do with them.

Auxiliary power units are designed to provide electrical power and bleed air to start the main engines and other aircraft systems without the need for external equipment such as a ground power unit.

Therefore, an aircraft auxiliary power unit (APU) allows the aircraft to independently manage both main and secondary systems, like the cabin air conditioning systems.

But how is this possible? You may wonder. Well, let’s take a closer look at the components of the APU.

An auxiliary power unit (APU) outlet on an Embraer ERJ 135.
Image source: Olivier Cleynen

Electrical power generation

The APU is generally composed of an electric generator that is capable of creating the electric power required by the aircraft’s main engines.

Moreover, this generator can keep functioning after the main engines have started to provide electrical power for the aircraft’s lighting and other systems requiring electrical power, such as galley equipment and avionics.

Compressor: also key for air conditioning

While the compressor of the APU is vital to draw enough high-pressure air to start at least one of the main engines, this high-pressure air can also be key for the aircraft’s air conditioning. The often called bleed air is used to drive the Environmental Control System (ECS), a system that mainly controls the following:

These are the main aspects that the ECS controls, thanks to the bleed air provided by the APU. However, thanks to the compressor of the APU, other systems can also be controlled.

Other aircraft systems

Since the compressor of the APU provides bleed air to help with the different functions of the ECS, other aircraft systems that are covered include:

  • Avionics cooling.
  • Smoke detection systems.
  • Fire suppression systems.

Clearly, the APU helps a lot to keep the crew and the passengers safe.

What is the difference between an auxiliary power unit and a ground power unit?

Taken from the names, the main difference between these two is the fact that a ground power unit can provide power to aircraft that are on the ground only. Still, the small turbine engine called the auxiliary power unit will be in the aircraft at all times. This fact takes us to the next difference.

Although an APU is not a jet engine that will provide thrust for the aircraft in case of a main engine failure, if a jet engine fails during flight, the APU can provide enough power and air pressure to reignite any of the aircraft’s main engines in the air. This is obviously impossible with a ground power unit.

The fact that an APU can be crucial in case of emergency makes it a must-have for any large commercial aircraft, especially those flying on non-ETOPS routes or those with a long diversion rating. You can learn more about this by reading our ETOPS blog post here.

Can a plane fly without an APU?

The simple answer to this question would be yes, they can. In fact, some small jet engine aircraft do not carry an APU as it would be a weight problem that would reduce the useful load of the aircraft.

However, the operation of this type of aircraft will see several functions with limitations. For example, it will always require a ground power supply to start at least one small jet engine to make it possible to provide electrical power for the rest of the systems and high-pressure air to operate the air conditioning. And this comes with added drawbacks.

Having to start any of the main engines to operate secondary systems usually results in a higher cost due to extra fuel consumption, maintenance required more often fr the engines, among other aspects that will not be necessary by having an APU.

Also, if an engine fails during flight, the pilot would have to fly with only one engine, as there will be no generator to provide the power to reignite that failed engine. This is a clear reason only to see small airplanes without an APU.

Therefore, most airplanes will carry one while the flight is possible without an APU. If you want to ensure whether your airplane has the APU installed or not, remember to look at the tail cone. You will see the exhaust, whether at the rear of the airplane’s tail cone or at one side of it.

A central view of the auxiliary power unit (APU) on an A320.
Image source: https://www.reddit.com/r/aviation/comments/6kmg98/auxiliary_power_unit_apu_on_the_a320/

How much does an Auxiliary Power Unit cost?

There are plenty of APU manufacturers and models nowadays, so the prices may vary. However, the average price is usually in the range of $7,000 to $12,000. Although it may seem like a lot of money, it is not really much when considering the improvement provided to the operation of the aircraft.

Moreover, the safety and comfortability provided for the passengers make the companies that operate airplanes with APU more competitive and profitable. So, they can easily see the ROI of having the APU installed in their airplanes.

Final conclusions

APUs consume less fuel than a regular engine, and they can be shut down during flight which reduces the time it is in operation and makes it require less maintenance. Therefore, they are a great solution, and we should keep seeing them in the near future.

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Jet pilot @NASA

1 comment


    My answer to the question “Can an airplane fly without an APU?”: 1. YES if the airplane was designed to be airworthy without an APU. #2. YES even if it is designed to carry an APU onboard but was not installed, if the location of the APU in the aircraft is at the landing gear bay, close to the center of gravity such as the Boeing 707 and Boeing 727. #3. YES with an unserviceable APU (not running) if the destination has GPU for both electric and pneumatic outputs. #4. NO, if not installed at the location of the APU is at the tail section. It affects the center of gravity. #5. NO if the destination does not have GPU for both electric and pneumatic outputs.

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