The absolute ceiling is the highest altitude at which an aircraft can sustain level flight. Due to the thin air at higher altitudes, a much higher true airspeed is required to generate sufficient lift on the wings. The absolute ceiling is therefore the altitude at which the engines are operating at maximum thrust, yet only generate enough lift to match the weight of the aircraft. Hence, the aircraft will not have any excess capacity to climb further. At absolute ceiling, the aircraft can no longer accelerate, since any acceleration will lead to higher airspeed and therefore excess lift. Stated technically, it is the altitude where the maximum sustained (with no decreasing airspeed) rate of climb is zero.
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