Operation of the CT7 Turboprop Engine as an Auxiliary Power Unit (APU)

Abstract

In response to the need of the new generation of commuter airliners, General Electric has developed the CT7 Turboprop engine so that it may be used as an Auxiliary Power Unit (APU) in addition to its normal mission as a prime propulsion unit. The General Electric CT7 Turboprop is a 1700 shaft horsepower class engine (Figure 1) developed for the new generation of 30+ passenger commuter and executive aircraft(1). Beyond this, the CT7 engine now offers the airlines a self-contained APU system to provide bleed air for the Environmental Control System (ECS) and electrical power for the aircraft during ground operation. This negates the need for a separate on-board APU with its extra cost, weight and fuel consumption and also eliminates the requirements for ground power units at the airlines’ operational terminals. The development of the engine as an APU generated a new set of technical requirements for the design and development and necessitated the development of special certification requirements as this was a new and unique operating condition for an aircraft prime propulsion system. A propeller brake had to be developed to lock the propeller and power turbine system and the engine had to be designed to operate at or near idle while producing large amounts of bleed air and electrical power. This development program was successfully completed in mid-1985 with the certification of the aircraft to operate with the CT7 Turboprop engine running as an APU.