Definition
A reciprocating aircraft engine whose maximum allowable power output is limited by a maximum permissible rotational speed (RPM) rather than by a manifold pressure limit. The engine is certificated and operated to a specific RPM that must not be exceeded, and its rated horsepower is the power produced at that speed under standard conditions.
Plain English
An engine whose top power is set by how fast it is allowed to spin, not by how hard the air can be pushed into it. You watch the RPM gauge to stay within limits.
Context Anchor
Seen in aircraft engine descriptions, powerplant limitations, and discussions of propeller and engine operating speed.
Derivation
"Rated" comes from the Latin ratus, meaning fixed or determined. A speed-rated engine has its power rating fixed by its allowable speed, as opposed to a manifold-pressure-rated engine where the rating is fixed by allowable intake pressure.
Why Pilots Care
Determines the exact RPM needed for rated takeoff and climb power, directly affecting performance calculations and engine limitations.
Intuition Check
Do not read “speed-rated” as meaning the airplane is rated for a certain airspeed. Here, “speed” means engine turning speed, not aircraft speed.
Example Sentence 1
Because the trainer had a speed-rated engine driving a fixed-pitch propeller, the pilot used the tachometer alone to set climb and cruise power.
Example Sentence 2
During the run-up the pilot confirms the speed-rated engine reaches its specified RPM before takeoff.