331 Series Auxiliary Power Units

The 331-200 introduced several new technologies to the world of auxiliary power. It was the first electronically controlled APU and the first to employ the same internal design as an existing turboprop engine, dramatically reducing development time and manufacturing costs.

The 331-500 series became the first Honeywell APU to use an eductor for oil cooling, eliminating the need for oil cooling fans. The success of this design led to its use on both the Honeywell 131-9B and RE200 series.