Definition
The highest altitude at which an engine can maintain its rated power output under standard atmospheric conditions. Above this altitude, available power decreases as air density continues to drop, regardless of throttle position.
Plain English
The highest altitude where the engine can still produce its full rated power. Climb above it, and the engine starts losing power because the air is too thin.
Context Anchor
You encounter this term when thinking about stalls, steep turns, low-altitude maneuvering, approaches, go-arounds, and any situation where the airplane is close to the ground with little energy to spare.
Derivation
From Latin 'criticus,' meaning 'decisive' or 'turning point.' Critical altitude is the turning point above which the engine can no longer hold its rated power.
Why Pilots Care
Descending below this height with low energy leaves no margin for recovery, turning a stall or steep descent into an unrecoverable impact.
Analogy
It is like trying to stop a bicycle before a wall. Far enough away, a normal brake input works. Too close, even the right action may come too late.
Grounding Statement
If the airplane is low, slow, and descending, critical altitude is the height where delay can turn a recoverable situation into one that cannot be fixed in time.
Intuition Check
Critical altitude does not mean one exact number that is the same for every flight. Here it means the lowest height that still leaves enough room to recover, and that height changes with the situation.
Example Sentence 1
Above the engine's critical altitude, the pilot noticed manifold pressure begin to drop even with the throttle fully forward.
Example Sentence 2
If airspeed and altitude continue to decay together, the aircraft may reach critical altitude with no remaining option for a safe recovery.