Definition
An operating state of a constant-speed propeller system in which the engine RPM is below the value selected by the propeller control. The governor responds by decreasing propeller blade pitch (a finer blade angle), which reduces aerodynamic load on the engine and allows RPM to rise back to the selected setting.
Plain English
The engine is turning slower than the pilot asked for. The propeller governor reacts by flattening the blade angle so the engine can speed up to the chosen RPM.
Context Anchor
Seen in constant-speed propeller, engine governor, and aircraft systems discussions where a selected rotating speed is being maintained automatically.
Derivation
Under' meaning below, plus 'speed' meaning rotational speed (RPM in this case). It simply names the situation: the engine is running below the speed the pilot has set.
Why Pilots Care
An underspeed condition reduces available thrust and may signal low oil pressure to the propeller hub or a governor malfunction that requires corrective action.
Grounding Statement
Underspeed means actual rotating speed is below the selected or required rotating speed.
Intuition Check
Underspeed does not mean the airplane is necessarily flying too slowly. Here it means a rotating part, such as the engine or propeller, is turning below its target speed.
Example Sentence 1
As the aircraft entered a climb and engine RPM began to drop, the governor sensed an underspeed condition and decreased blade pitch to restore the selected RPM.
Example Sentence 2
During cruise the propeller governor failed and the engine settled into an underspeed condition until the pilot increased RPM with the control.