Ceiling (aircraft)

With respect to aircraft performance, a ceiling is the maximum density altitude an aircraft can reach under a set of conditions, as determined by its flight envelope.

Service ceiling

The service ceiling is the maximum usable altitude of an aircraft. Specifically, it is the density altitude at which flying in a clean configuration, at the best rate of climb airspeed for that altitude and with all engines operating and producing maximum continuous power, will produce a given rate of climb (usually 100 feet per minute climb or 30 metres per minute,^[1] and 500 feet per minute climb for jet aircraft). Margin to stall at service ceiling is 1.5g.^{[citation needed]}.

The one engine inoperative (OEI) service ceiling of a twin-engine, fixed-wing aircraft is the density altitude at which flying in a clean configuration, at the best rate of climb airspeed for that altitude with one engine producing maximum continuous power and the other engine shut down and feathered, will produce a given rate of climb (usually 50 feet per minute).^{[citation needed]}

However some performance charts will define the service ceiling as the pressure altitude at which the aircraft will have the capability of climbing at 50 ft/min with one propeller feathered.

Absolute ceiling

The absolute ceiling, also known as coffin corner, is the highest altitude at which an aircraft can sustain level flight, which means the altitude at which the thrust of the engines at full power is equal to the total drag at minimum drag speed. In other words, it is the altitude where maximum thrust available equals minimum thrust required, so the altitude where the maximum sustained (with no decreasing airspeed) rate of climb is zero. Most commercial jetliners have a service (or certificated) ceiling of about 42,000 feet (12,802 m)^{[citation needed]} and some business jets about 51,000 feet (15,545 m).^[2] While these aircraft's absolute ceiling is much higher than standard operational purposes, it is impossible to reach (because of the vertical speed asymptotically approaching zero) without afterburners or other devices temporarily increasing thrust. Flight at the absolute ceiling is also not economically advantageous due to the low indicated airspeed which can be sustained: although the true airspeed (TAS) at an altitude is typically greater than indicated airspeed (IAS), the difference is not enough to compensate for the fact that IAS at which minimum drag is achieved is usually very low, so a flight at an absolute ceiling altitude results in a low TAS as well, and hence in a high fuel burn rate per distance traveled. The absolute ceiling varies with the air temperature and, overall, the aircraft weight (usually calculated at MTOW).^[1]

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