Definition
The maximum horsepower an engine is certified to produce continuously at a specified rpm and manifold pressure under standard atmospheric conditions, as established by the manufacturer and approved during certification.
Plain English
The official top power output the engine is approved to make, measured at a specific rpm and manifold pressure setting.
Context Anchor
Seen in engine and propeller discussions, especially when comparing engine power, propeller RPM, and the performance numbers in the airplane’s handbook.
Derivation
Rated' comes from the Latin 'ratus,' meaning fixed or determined. In aviation it refers to a value set and approved by the manufacturer, not one measured on the day. So 'rated horsepower' means the officially approved power figure, not whatever the engine happens to be producing right now.
Why Pilots Care
Pilots use this value to set manifold pressure and RPM without exceeding engine limits, protecting performance calculations and engine life.
Intuition Check
Rated does not mean someone merely judged the engine as good or bad. Here it means the approved power value assigned to that engine under specified conditions.
Example Sentence 1
The Lycoming O-360 has a rated horsepower of 180 at 2,700 rpm.
Example Sentence 2
Operating below rated horsepower in cruise extends engine life while still providing adequate performance.