A term applicable to turboprop and turboshaft engines. A flat-rated engine's thrust remains practically constant during an increase in altitude up to a certain limit. A method of referring to the engine-rated thrust or engine-rated shaft horse-power (SHP) that is guaranteed to be available up to a specific ambient temperature. The term flat rating is so called because of the flat nature of thrust versus the ambient temperature curve. The curve shows that from the lowest operating temperature possible to the specified temperature, the engine produces full thrust. Above this temperature, full thrust will not be permitted. At lower altitudes, the restricting factor is the torque that the airframe can withstand, and at higher altitudes, the exhaust temperature is the limiting factor. Flat-rated thrust also applies to power settings, such as maximum continuous, maximum climb, and maximum cruise, when the manufacturer would have to specify the thrust, ambient temperature, and altitude. Also called a re-rated engine.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved