How Does an APU Work in Aircraft?

Auxiliary power units (APUs) work very similarly to aircraft engines and are responsible for carrying out three separate functions. These functions include supplying conditioned air to the cabin, providing electrical power, and starting the aircraft engine. More than that, they serve as a backup supply of power in the event of a technical failure or will assist in restarting an engine in flight.

APUs are typically installed in the tail of an aircraft and operate in the same way as a jet engine. Fuel is supplied from the main aircraft fuel tanks via a series of pumps and fuel lines while exhaust exits overboard from the tail. To better understand how APUs work, this blog will provide a more in-depth explanation of their features and components.

The Various Roles of APUs

To begin, air is taken from an inlet, and it is cooled and conditioned in the AC system before being supplied back to the cabin. With warm, moist air, cooling can result in moisture condensing and appearing as fog or smoke coming from the AC vents.

As previously mentioned, the APU is able to supply electrical power to various aircraft systems via a built-in electrical generator. This mechanism is often utilized on the ground prior to pushback or when an aircraft arrives on land before any external power has been connected.

Another role that APUs take on is engine starting. While aircraft with smaller engines can utilize electrical motors to kick off the initial rotation of the engine, bigger airplanes are too heavy to be rotated by electrical motors. As such, air from the APU is used to start the engine. 

Generally, pilots shut down the APU once the engine is running normally and has taken over the supply of electricity to the aircraft and air conditioning to the cabin. There are a few cases in which the APU may continue running. For instance, engines in heavy aircraft utilize their maximum thrust during takeoff.

However, if the engine is simultaneously supplying conditioned air to the cabin, this reduces the amount of available thrust for departure. Of course, this varies based on atmospheric conditions and aircraft weight. In this case, once maximum thrust is no longer required, the APU air supply will be switched off. This can only be done once it has been confirmed that the engines have taken over the task of air supply.

How Reliable Are APUs?

Today, APUs are reliable apparatuses that rarely cause problems. However, one cannot forget that it is a source of immense power that is located in the tail. As such, these units need to be routinely monitored. Older aircraft use an overhead panel to monitor the APU, displaying indications of operating temperature and speed as well as a number of warning lights.

In modern aircraft, monitoring is more automated. For example, the APU will automatically shutdown if it detects a fault or fire, and it will discharge the fire extinguisher as a precaution. Nonetheless, one can monitor the APU by switching to the APU page on the cockpit display, allowing you to acquire real-time information.


If you find yourself in need of APUs, ground power units, or other related apparatuses, rely on Fulfillment by ASAP. Fulfillment by ASAP is the premier distributor of aviation products, NSN components, and electronic parts, all of which are subjected to various quality assurance measures, tests, verifications, and assessments. With over 2 billion items on our website, you have a vast selection of top-quality products to browse through for your operations. Initiate the procurement process with a competitive quote and see why customers rely on Fulfillment by ASAP time and time again.


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