Turboprop engine as a Pusher Configuration with combination of Prop-Brake can be effectively used instead of a Turbo-Shaft Engine as Auxiliary Power Unit on a Multiple Engine Powered Airplane. The Pusher Turboprop engine on ground with Prop-Brakes 'ON' can be used to power the airplane from Bleed-Air to start the Engines, Environmental system, Electrical System, Pneumatic System. The additional and primary purpose of its use will be to drive a variable pitch propeller using a reduction gear.
Turboprops are very efficient at flight speeds below 725 km/h (450 mph; 390 knots) because the jet velocity of the propeller (and exhaust) is relatively low. Due to the high price of turboprop engines, they are mostly used where high-performance short-takeoff and landing (STOL) capability and efficiency at modest flight speeds are required also catering the need as A.P.U. On Multi-Engine aircraft it will additionally add to Total Thrust effectively. To match the thrust developed from the Turboprop in form of Shaft Horse Power can be calculated with combination Turbofan engine fitted on the same airplane by Thrust Computation Equations and to design the Propeller to get the required SHP for Thrust Derivation.