When an engine is flat rated it means that an engine of high

horsepower Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are the ...

rating is constrained to a lower horsepower rating. The engine output in this case will always remain the same, but when atmospheric conditions such as high temperatures and high altitude ("hot and high") reduce the power output of the engine it has more headroom before it falls below the limited maximum output. In some cases the total power output of an engine needs to be constrained because the airframe can only handle a certain force. This is the case with

gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...

engines. Flat rating allows airplanes to operate under more demanding conditions, without the need for extra structural strengthening due to higher peak power output of the engine. For example, the

Garrett AiResearch TPE-331 The Honeywell TPE331 (military designation: T76) is a turboprop engine. It was originally designed in the 1950s by Garrett AiResearch, and produced since 1999 by Honeywell Aerospace. The engine's power output ranges from . Design and developm ...

-5 engine originally fitted on the

Dornier 228 The Dornier 228 is a twin-turboprop STOL utility aircraft, designed and first manufactured by Dornier GmbH (later DASA Dornier, Fairchild-Dornier) from 1981 until 1998. Two hundred and forty-five were built in Oberpfaffenhofen, Germany. In 19 ...

produces . If the outside air temperature is above 20°C, the airplane's maximum speed is reduced by approximately 10 knots (19 km/h), because hotter air is less dense and thus produces less pressure inside the turbine. The Dornier 228 can also be fitted with the Garrett AiResearch TPE-331-10 conversion of the -5 engine which produces but is limited (flat rated) to only 715. In this case the airplane will be able to maintain its top speed at temperatures above 30°C without the risk of exceeding the airplane's structural limits.

External links

Honeywell AerospaceTPE 331 Engine Conversions
Aircraft engines {{aircraft-stub