The maximum altitude at which, in standard atmosphere, it is possible to maintain, at a specified rotational speed, a specified power or a specified manifold pressure. Unless otherwise stated, the critical altitude is the maximum altitude at which it is possible to maintain, at the maximum continuous rotational speed, one of the following:
- The maximum continuous power, in the case of engines for which this power rating is the same at sea level and at the rated altitude.
- The maximum continuous rated manifold pressure, in the case of engines, the maximum continuous power of which is governed by a constant manifold pressure.
The maximum altitude under standard atmospheric conditions at which a turbocharged engine can produce its rated horsepower. (PHAK definition).