Definition
A turbine engine certification practice in which the engine's rated takeoff power is set below the maximum power it could physically produce, so that the rated power can be reliably delivered up to a specified outside air temperature or altitude. The engine is held to that fixed rating rather than being allowed to produce its full thermodynamic output on cool days at sea level.
Plain English
The engine is deliberately limited to a set power figure that it can hit even on a hot day, instead of letting it produce more power on cool days and less on hot days.
Context Anchor
Seen in aircraft performance discussions, especially for turbine-powered airplanes and takeoff performance planning.
Derivation
From 'flat' (level, unchanging) and 'rating' (the certified performance figure). The term reflects how the power rating stays flat across a range of conditions where an unrestricted engine's output would normally drop off.
Why Pilots Care
It improves takeoff and climb performance in high or hot conditions and must be accounted for when calculating required runway length and climb gradients.
Intuition Check
Flat-rating does not mean the engine’s performance is unchanged in all conditions. It means the approved output is held constant only within a specified range; beyond that range, available power may decrease.
Example Sentence 1
The PT6A in this turboprop is flat-rated to 850 horsepower, so the pilot can plan on full rated power even on a hot day at a high-elevation airport.
Example Sentence 2
Because the engine is flat-rated to 750 shaft horsepower up to 25 degrees Celsius, the takeoff distance chart remained valid even at the higher density altitude.