Definition
The maximum altitude at which a supercharged or turbocharged reciprocating aircraft engine can maintain its rated takeoff (or rated full-throttle) manifold pressure at full throttle in standard atmospheric conditions. Above this altitude, the supercharger or turbocharger can no longer compress the thinner air enough to keep manifold pressure at the rated value, and engine power begins to fall off.
Plain English
The highest altitude at which a boosted piston engine can still produce its full rated power. Climb above it and power starts to drop because the air is too thin for the booster to fully compensate.
Context Anchor
Seen in piston-engine performance discussions, especially for aircraft with supercharged or turbocharged engines.
Derivation
‘Critical’ here comes from the Latin ‘criticus’, meaning a turning point or decisive moment. In this context it marks the altitude where engine behaviour changes — below it the engine holds rated power, above it power begins to fall.
Why Pilots Care
Knowing critical height lets a pilot anticipate power loss, adjust climb planning, and avoid attempting to maintain altitude or speed beyond the engine's capability.
Grounding Statement
As the aircraft climbs, outside air gets thinner; critical height is the point where the engine’s air-boosting system can no longer fully make up for that thinner air.
Intuition Check
Critical does not mean the airplane is automatically in danger at that height. It means the engine has reached a performance limit, and power will start dropping if the aircraft climbs higher.
Example Sentence 1
Above the engine's critical height, the pilot noticed manifold pressure beginning to drop even with the throttle fully open.
Example Sentence 2
Above the published critical height the manifold pressure began to drop even with the throttle fully forward.