Definition
A condition in which a piston engine's propeller rotates faster than its certified maximum RPM limit. It typically occurs in constant-speed propeller systems when the governor fails to restrict blade pitch, when oil pressure to the propeller hub is lost, or when the pilot mismanages power and pitch settings -- for example, applying high manifold pressure with the propeller control set to a low pitch (high RPM) condition during a steep descent. Sustained overspeed can damage the engine, propeller, and accessories.
Plain English
The propeller is spinning faster than the engine is designed to allow, usually because the system that controls how the blades cut through the air has failed or been mismanaged.
Context Anchor
Seen in propeller-system discussions, emergency procedures, and in flight when the RPM indicator shows the propeller going above the red-line limit.
Derivation
“Overspeed” combines “over,” meaning beyond a limit, and “speed,” meaning rate of motion. In this aviation use, the limit being exceeded is the propeller’s rotational speed, measured in RPM.
Why Pilots Care
Uncorrected overspeed can quickly destroy the engine through excessive centrifugal forces on the propeller and connecting rods.
Analogy
It is like running a car engine past the red line on the tachometer. The engine may still be running, but it is being pushed beyond the range it was designed to handle safely.
Intuition Check
Do not read “overspeed” as airplane speed here. In this term, it means the propeller is rotating faster than its allowed RPM.
Example Sentence 1
During the descent, the pilot noticed the tachometer climbing past red-line and reduced throttle to prevent a propeller overspeed.
Example Sentence 2
After a propeller overspeed event the pilot landed at the nearest suitable airport and had the engine inspected before further flight.