Definition
A condition in which a constant-speed propeller's rotational speed (RPM) exceeds the maximum value allowed by the engine or propeller manufacturer, typically because the governor or pitch-change mechanism has failed to increase blade pitch enough to hold the selected RPM.
Plain English
The propeller is spinning faster than it is allowed to spin. Normally the propeller's control system keeps RPM at the value the pilot has set, but something has stopped it from doing that, and the engine and propeller are now turning too fast.
Context Anchor
Seen in constant-speed propeller operation, especially when monitoring the tachometer during takeoff, climb, descent, or a propeller control problem.
Derivation
"Overspeed" simply means "speed beyond the allowed limit." The word matters here because it points to a specific limit set by the manufacturer, not just "fast." An engine running at high cruise RPM is not overspeeding; an engine exceeding the red line is.
Why Pilots Care
Continued overspeed can cause engine damage, propeller blade failure, or loss of aircraft control.
Grounding Statement
The key idea is simple: the airplane may be flying normally, but the propeller itself is turning faster than it should.
Intuition Check
Do not read “propeller overspeed” as the airplane going too fast through the air. It means the propeller’s RPM is too high.
Example Sentence 1
When the governor failed, the pilot reduced throttle and lowered the nose to prevent a propeller overspeed.
Example Sentence 2
The pilot monitored RPM closely in the descent to avoid a propeller overspeed from the increasing airspeed.