APU – The Hidden Engine On An Airplane

Most of us today have flown in a commercial airliner. We have seen airplanes from the outside as well as from the inside, we have all seen its fuselage, its empennage, its engines and many other parts. However, what most of us haven't seen is an important part that is usually located at the back of the airplane - the Auxiliary Power Unit or the APU. In case you've ever wondered what powers the air conditioning and basic lighting of the aircraft when it is on the ground, the APU does. 

The Auxiliary Power Unit is basically a small turbine engine, usually located at the tail of the aircraft, that is used as a source of power on the ground, when actual engines cannot be used. This lesson is a broader look at this small but crucial instrument which is sometimes referred to as "the backup powerhouse of an airplane", and aptly so!

by Nameet Shetty
1 year, 2 months ago
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Although many airplanes function perfectly well without an Auxiliary Power Unit, its importance cannot be ignored. When the APU was originally designed, it was done on the basis of three R's - Real, Rugged & Reliable. These three descriptive words go on to indicate the true nature of an APU, a bankable instrument which can take on reasonably good amounts of wear and tear while executing functions which are crucial to the safe and efficient functioning of an airplane.

Apart from this, there are other perks of using the APU. Firstly, the APU delivers a huge output compared to its small size, weight and fuel consumption. As a result, it provides ample power and takes up less space and fuel. Secondly, the APU consumes fuel from the main line system, eliminating the need for a separate fuel system. This saves a lot of space and complications which are usually involved in the installation of a new fuel system. Another reason we have APUs is because it can be used as a back-up source of energy in case the main engines fail in mid air. Although the APU performs most of its functions on the ground, it is a crucial supporting component in the air too. 

The APU is a source of bleed air and AC electrics for the aircraft, this ...
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A typical APU majorly consists of three sections - the power section, the compression section and the transfer section.The power section, as the name suggests, produces the shaft power required for operations. Next, the compression section contains a compressor which provides pneumatic power. In a few cases, the APUs also uses bleed air from the compressors located in the power section. Lastly, the transfer section, also known as the gearbox section, is mainly responsible for the transfer of power to different components of the APU. Apart from transferring the shaft power into the generator for conversion to electric power, the transfer section also transfers energy to other parts of the APU such as the fuel and lubrication unit. The controlled and synchronized working of all three sections of the APU result in efficient execution of all its functions.

Although the APU is not used for propulsion purposes, it is common knowledge that it is a turbine engine. Moreover, the APU belongs to the free turbine, turbo shaft category, which is one of the most common types of engines in the aviation industry. A turbo shaft engine usually delivers around 500 – 600 horse powers of power and the free turbine arrangement allows the engine to work flexibly at a constant speed. This is only made possible by a series of well designed components working in quick succession.
Boeing 727 APU - Auxiliary Power Unit. A Centrifugal Jet engine that starts the aircraft engines ...
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An APU is primarily controlled by the fuel system of the aircraft all throughout its start up and running procedures. Usually, it is fired up using the aircraft’s battery or a hydraulic accumulator and is then allowed to settle down at a stable RPM, temperature and pressure. Once this is done, the power from the APU is used to power up the basic instruments and air conditioning in the aircraft. However, as an APU lacks the power to support all the instruments, it can only power a few basic ones. Before the advent of the APU, the basic flight instruments had to be powered using either the main engines or a ground start-up unit. The introduction of the auxiliary power unit eliminated the need for both. In terms of controls, a lesser known fact about the APU is that it is semi automated. Even the control panel of the instrument has limited controls. The manual controls of an APU consist of four basic but crucial controls. These are the start switch, the stop switch, the emergency shutdown button and the fire suppression control. Apart from these, there are gauges that indicate the performance of the APU and there are also automatic sensors which shut down the APU in case of overheating, overloading, malfunction or usage outside the normal range.

In terms of operations, the APU usually works on the shaft priority system. In this system, the generators, provided they are operational, are given priority. Most APUs are programmed to run at 100 percent capacity, so they don’t really require a constant speed drive unit for monitoring. Some APUs also work at an idle speed of 85% when there is no load applied on the system.

However, if the load on an APU becomes high, the APU reaches its maximum permissible exhaust gas temperature (EGT) and the fuel system shuts down. The control system backs the fuel and the APU generator’s operational frequency is reduced. This causes the APU system to go offline. This is basically how the automatic shutdown system of an APU works. The auto shutdown will activate itself in the events of fire outbreak, oil pressure fluctuation or speed override.

This amateur video was created by Western Michigan University students, Class 2011.This video demonstrate ...
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Although an APU is used to carry out different functions, including the electrical, pneumatic and hydraulic functions of an aircraft, it has a few main uses. Firstly, it provides basic electrical supply to power the instruments on an aircraft, so that pilots can carry our pre-flight checks when on ground. Secondly, it provides pneumatic duct pressure in order to power the air conditioning. This makes the interior of the aircraft cool, comfortable and welcoming for boarding passengers. 

The APU also provides shaft power and pressurized air to fire up the main engines.The engine start up procedure requires a massive jolt of power and the battery alone cannot supply it. Lastly, in some aircrafts, the APU also aids hydraulic functions, allowing the pilots to operate control surfaces such as flaps and ailerons.  

Apart from this, the APU has its in-flight uses too. In case of a mid-air total engine failure, the APU can be fired up for basic electric and hydraulic assistance. However, like everything else, the APU has a few drawbacks too. Although APUs are theoretically designed to support an aircraft to its maximum permissible ceiling, it practically works best till about 15,000 to 20,000 feet. This is because air loses it density with height and less dense air prevents the APU from functioning optimally at a higher altitude.

An auxiliary power unit ( APU) is a device on a vehicle that provides energy for ...
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The APU exists to aid an aircraft in its functions other than propulsion. It is usually located in the tail of the aircraft so that its operation doesn't interfere with the movements of people working around the airplane on ground; people like freight operators, cargo crew, etc. With a better understanding about the workings of an APU, your next flight experience will be more knowledgeable  knowing that the little engine is there to make you comfortable as well as to assist the pilots in case of an emergency. 

Now, when you step into the cool interiors of a commercial aircraft and hear the warm humming noise in the background, you know whats the cause for this modern wonder!


An auxiliary power unit ( APU) is a device on a vehicle that provides energy for ...
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1. Significance
2. Components
3. Operations
4. Functions
5. Conclusion
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