Flat rated
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When an engine is Flat rated it means that an engine of high Horsepower 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 airframe strength can only handle a certain force. this is in particulair the case with Gas Turbine 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-331-5 engine originally fitted on the Dornier 228 produces 715 Horsepower. if the outside air temperature is above 20 degrees Celsius, the airplanes maximum speed is reduced by approximately 10 knots. because the hotter air is less dense and thus produces less combustion inside the turbine. The Dornier 228 can also be fitted with the Garrett AiResearch TPE-331-10 conversion of the -5 engine which produces 1000 Horsepower but is limited (Flat rated) to only 715. in this case the airplane will be able to maintain its top speed to well over 30 degrees Celsius. Without running the risk of exceeding the airplanes structural limitations.
