Definition
The highest altitude at which a turbocharged or supercharged engine can maintain its rated sea-level manifold pressure at full throttle. Above this altitude, the induction system can no longer compress enough outside air to keep the manifold pressure at its rated value, and engine power begins to fall off.
Plain English
The highest altitude where a boosted engine can still produce its full rated power. Climb above it, and the engine starts losing power because the air is too thin for the boost system to fully compensate.
Context Anchor
Seen during takeoff performance planning, especially for hot days, high-elevation airports, heavy loading, short runways, or obstacle clearance planning.
Derivation
"Critical" here means the threshold or limit point — the altitude at which a key condition (full rated manifold pressure) can no longer be held. Not "critical" in the sense of dangerous.
Why Pilots Care
Above this altitude full power is unavailable, so takeoff distance increases and climb performance decreases; pilots must verify they still meet required margins before attempting departure.
Grounding Statement
On a hot day at a mountain airport, the airplane may act as if it is much higher than the airport elevation, and that can push it beyond the altitude condition where it can safely take off.
Intuition Check
Do not read “critical” as meaning the airplane is already in an emergency. Here it means the limiting point for this specific takeoff, with this airplane, load, runway, wind, temperature, and obstacle situation.
Example Sentence 1
Above the engine's critical operating altitude, the turbocharger can no longer maintain full rated manifold pressure, and climb rate begins to decrease.
Example Sentence 2
Because the planned mountain airport sat above the airplane's critical operating altitude the pilot elected to wait for cooler temperatures to improve density altitude.