The 131-9D auxiliary power unit (APU), designed for the Boeing MD-90 aircraft, gained an immediate following. It became standard equipment on the Boeing B737NG family of aircraft, as well 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 131-9D 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.