How an Auxiliary Power Unit Works

A plane’s engines aren’t the only things that provide power to its various systems. Here’s everything you need to know about auxiliary power units.

Have you wondered how airplanes have any power while they’re sitting at the airport? Since most commercial flights start to board around 40 to 45 minutes before takeoff, it would be pretty miserable in there if there wasn’t any air conditioning or lights, for example. Without the engines running, how do these systems get power?

The auxiliary power unit (APU) of an airplane is used as a way to power the grounded plane without using the main engines. The APU runs on jet fuel and spins a turbine connected to a generator. This creates electricity for the airplane and powers an air compressor to produce bleed air.

As you can probably imagine at this point, the auxiliary power unit is an incredibly important part of an airplane. Without them, a plane’s engines would need to be running in order to generate power while it’s sitting at the airport. How inefficient would that be?! In this article, we’ll go over everything that you need to know about auxiliary power units and how they work.

The reason that SkyTough has become one of the best aviation sites on the web is because we put a lot of focus on providing nothing but the most accurate information on the web. To ensure this, we combine our own knowledge and experience with extensive research and input from other experts. This way, you’ll learn everything you want to know about airplanes, such as how auxiliary power units work.

‍What is an Auxiliary Power Unit?

Before we get into how it works and what it’s used for, it’s important to describe what an auxiliary power unit even is. While these types of power systems are not used only for airplanes, that is (of course!) going to be what we’re focusing on in this article. This information does not apply unilaterally for all auxiliary power units for any type of vehicle or machine, just airplanes. With that out of the way, let’s get into the good stuff.

In an airplane, an auxiliary power unit (APU) is a secondary power system that’s mainly used when the plane is grounded, typically while it’s sitting at the gate waiting to be boarded to take off. It’s called an auxiliary power unit because, as you most likely know, the prime source of power for all of an airplane’s systems is provided by the engines.

But when the plane is grounded and the engines aren’t running, something is needed to provide the power necessary for essential systems, such as air conditioning, electricity, and more. So what really is an APU?

An APU is basically an additional engine located at the back of the airplane that’s significantly smaller than the plane’s normal engines. Since it’s not used for propulsion like the bigger engines, the power output it produces can be redirected and used for other systems, such as those noted above. So how does it work?

How Does an Auxiliary Power Unit Work?

In the big picture, an APU is typically a small turbine engine that powers an electric generator and also an air compressor. By providing power to these two systems, the APU is able to make the plane almost self-sustainable while sitting on the ground, even without the engines running. In other words, the APU allows the plane to still have many of its functions (minus propulsion) even while grounded, which saves fuel and means the engines don’t have to run constantly.

The way this system works is that it has its own electrical startup sequence and internal power source that’s entirely different from the plane’s main engines. But before it can even start, the pilot typically has to do a few things just like with the standard engines. This starts with a big safety check — testing the APU’s fire suppression system.

Once the fire suppression system receives the go-ahead, the pilot then presses a button to open the flap covering the APU’s air inlet. Just like other engines, the APU needs air to run, so it won’t start until this flap is open. Once opened, the pilot just needs to press the button to initiate the startup sequence and the APU will fire up. This is similar to you just turning the key in your car’s ignition and the engine firing up.

Once the APU is running, it’s really just a simple transference of power from the fuel it’s burning to the various systems that we’ll get into in the next step. The APU runs and spins a turbine which is then converted into electricity through a generator. The electricity is then directed to the plane’s electrical systems and also to power an air compressor.

The electricity created by the APU is fairly self-explanatory since it’s, well, electricity. The air compressor on the other hand is mostly used for the creation of bleed air. I won’t get too deep into the weeds on bleed air in this article since we have a full article on the topic, but I’ll touch on its uses in the next section.

That’s pretty much it for how an APU works. It burns fuel and air, spins a turbine, and creates power that’s then redirected to other areas of the airplane.

What Does an Auxiliary Power Unit Do?

As I’ve mentioned a few times so far in this article, an APU is mainly used to power some of an airplane’s essential systems. These include electrical systems and air conditioning, sure, but there’s also one other major use — starting the other engines.

Electrical System

As discussed in the previous section, the APU creates electrical power by spinning a turbine that’s coupled to an electrical generator. This power is then used to turn on the plane’s main power systems, such as cabin lighting, cockpit illumination, and more. Without the APU running and no other external power source, a plane (including the cockpit) would be completely dark while it’s grounded. That’s not very good for boarding, is it?

Air Conditioning

Once the APU is running and the air compressor kicks on, bleed air valves will begin to open when necessary. When these valves open, this compressed bleed air then goes to other systems, such as the plane’s air conditioning and HVAC system. This is how the cabin of a plane is comfortable even while it’s sitting at the gate. Without the APU powering the air conditioning, it could be absolutely miserable sitting there waiting to take off.

This is also why there’s that brief moment in time when the AC seems to shut off when the plane is boarded and you get ready to start moving. As the main engines fire up and the APU starts to shut off, it takes a bit of time for the engines’ bleed air to start powering the HVAC system and pick up where the APU left off.

Engine Starting

In addition to the above, the APU is also often used to start the plane’s main engines. This is done using both of the above outputs, electricity and bleed air. With the power produced by the APU, the electronic starting sequence for the main engines can be started. In combination with this electricity, the bleed air from the APU is used to sort of kickstart the first engine.

Using the output from the APU is much more efficient than forcing the engine to start from a stop. This same general concept of bleed air is then used to start the other engines, except the bleed air comes from the compressor section of this first engine rather than the APU. But this is all described in detail in our complete guide to bleed air systems, so make sure you check out that article for more info!

Is the Auxiliary Power Unit Always Running While Flying?

No, the APU is not always running while the plane is flying. In fact, it’s almost always powered down while the plane is still grounded. Once the plane’s main engines are fired up and running, there is no need for the APU to be running since its output cannot compare to what the main jet engines are putting out.

Additionally, APUs used quite a bit of jet fuel themselves, running off of the same fuel that the plane runs on. This design means there doesn't have to be a separate fuel system installed just for the APU, but they’re not exactly fuel-savers either! Some APUs, especially those in larger commercial airplanes, can burn more than 100 gallons of fuel per hour, so they’re typically only used for as little time as possible to keep the cabin comfortable and get the main engines started.