Honeywell - Model 131 Series -Auxiliary Power Unit (APUs)

The 131-9 auxiliary power unit (APU), previously designed for the Boeing MD-90 aircraft, gained an immediate following. With the introduction of the 131-9B it became standard equipment on the Boeing B737NG family of aircraft, as well as the with the introduction of the 131-9A as airline selectable on the Airbus A320 family. It was then also chosen for the Bombardier C-Series.

It’s easy to understand its popularity. The 131-9 introduced a two-stage axial turbine for longer engine life and a single starter/generator that uses electrical power to start the APU, and then becomes a generator once it’s running. This has led to the 131 Series becoming one of the most successful APUs with more than 100 million hours of in-service use.