AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

How Do Auxiliary Power Units Work?

Auxiliary power units typically comprise three main sections–the power, load compressor, and gearbox, and they are designed to operate at 115V and 400 Hz to reduce systemic inefficiency. These voltage and frequency values help make aircraft lighting and air conditioning possible for passenger comfort. Since APUs are fundamental to an aircraft’s well-being and standard operations, learning more about their functionality becomes paramount.

Working Principle of Auxiliary Power Units

The sequence of starting up APUs is similar to turning on a car’s ignition. First, a pilot tests the APU’s fire suppression system as a safety measure and then proceeds to open the flap covering the inlet for air. This step is crucial because all engines, even APUs, need air for operations, or else they will not start at all. Next, the pilot presses a button to initiate the APU’s internal electronic starting sequence, which is different from the main engine. Once the APU is running, it starts to spin a turbine for generating electricity through its connection with a generator. This electricity is then used by the aircraft’s electrical system to power up the components connected to it and the air compressor to create bleed air. This bleed air is used in the aircraft’s air conditioning and HVAC system. 

Applications of Auxiliary Power Units in Aircraft Systems

The electricity generated by APUs is used for lighting a grounded aircraft’s cabin and cockpit, alongside any other areas that may need to be lit for everyone’s convenience on board. In the APU’s absence, passengers would experience complete darkness on board before the flight takes off, as it is the primary source of power while engines are shut off. Meanwhile, the APU also powers the plane’s air conditioning system, and passengers may experience a brief disruption of the AC supply as the plane prepares to take off. This is because there is a transfer of the HVAC system’s source of power from the APU to the engines’ bleed air.

Apart from powering up the air conditioning and electrical system, the APU also acts as a source of power for the main engines’ electronic starting sequence. With the combined effect of the main engines’ electricity and bleed air being generated by the APU, the first engine starts up and provides bleed air to other engines. Overall, using the APU is far more efficient for the aircraft during initial operations than forcing its main engines to start from scratch.

Why Is the Usage of APU Kept Minimal?

When an aircraft is airborne, there is no need for it to rely on its secondary power source which is the APU. Aircraft fuel capacity is limited, and APUs consume a large amount of fuel for operations, which can be more than 100 gallons per hour. This is especially true for large commercial planes, and APU usage is kept to a minimum to ensure overall fuel efficiency. This is why APUs are not always running while flying as engine power and other resources are then relied on for the remainder of the flight.

Wrapping Up

Your search for top-quality aircraft parts stops at Emergent Aviation, the premier supplier of NSN parts in the aviation industry. We at Emergent Aviation are dedicated to providing an easy parts procurement experience to all our customers. We have an expansive supply chain network involving top manufacturers, both domestic and international. Our vast inventory of over 2 billion new, obsolete, and hard-to-find aircraft parts is streamlined through the use of relevant filters which you can utilize as you deem fit. Moreover, our 24/7x365 customer assistance services ensure that you remain updated with the progress and fulfillment of all your orders. Get started by filling out the RFQ form available on our website so that a dedicated sales rep can get back to you within 15 minutes or less of submitting the form.


Semiconductor's Certifications and Memberships
The only independent
distributor with a
  • All Orders are fulfilled in the U.S.A.
  • All shipments must comply with U.S.A export laws.
  • No exceptions.

“We Proudly Support Intrepid Fallen Heroes Fund that serves United States Military Personal experiencing the Invisible Wounds of War : Traumatic Brain Injury (TBI) and Post Traumatic Stress (PTS). Please visit website ( and help in their valiant effort”.

Thank You for Visiting.

We Hope that You Will Visit Us Again the Next Time You Need NSN Parts and Make Us Your Strategic Purchasing Partner.

Request for Quote

We use cookies to ensure that we give you the best experience on our website. If you continue to use this site we will assume that you are happy with it.