Definition
A specific range of engine or propeller revolutions per minute (rpm) at which destructive vibration or harmonic stress can occur in the engine, propeller, or airframe. Operation within this range is restricted, and the rpm indicator is typically marked with a red arc to warn the pilot to pass through it quickly rather than operate within it.
Plain English
A band of engine speeds where the aircraft shakes itself harder than is safe. The pilot must move the throttle through this range quickly and not stay there.
Context Anchor
Seen in aircraft operating limitations, maintenance information, and tachometer markings for engines, propellers, or rotor systems.
Derivation
"Critical" comes from the Greek krisis, meaning a turning point or decisive moment — here, a range where something important changes for the worse if ignored. "Rpm" stands for revolutions per minute, the standard measure of how fast the engine crankshaft is turning.
Why Pilots Care
Prolonged operation can cause propeller imbalance, fatigue cracks, or blade failure.
Intuition Check
Critical does not just mean important here. It means a specific unsafe speed band where vibration can build up and damage parts.
Example Sentence 1
During climb-out, the pilot advanced the throttle smoothly through the critical rpm range rather than letting the engine settle there.
Example Sentence 2
The POH directs the crew to avoid continuous operation in the critical RPM range of 1950 to 2250.