Why Does the APU Require the Batteries to Start?

December, 24, 2020 by

It may seem logical that the APU would start without the batteries. After all, the APU is able to charge some aircraft batteries.

With a closer look to the operation of the APU, it becomes clear that this isn’t true. When you start a car (or plane for that matter), the battery is powering a starter motor to get the components of the engine turning and the cycle going. Since the APU is just a baby engine, the battery is also required to start it.

The wise guys among you may be thinking, “how come lawnmowers and generators don’t have a battery?”. These engines also require motion to start. Rather than a battery, most of these engines have a pull string to get the engine turning.

Unless you want to pull a very long pull string, the battery is the simplest way to start up an APU.

Ryan Epps is a writer for the IFATC Education Group. He is also a commercial pilot, flight instructor, and helps run the Airport Editing Team, currently as an Airport Editing Manager.