Along with service ceiling and combat ceiling, absolute ceiling is a term used to describe the maximum density altitude that a particular aircraft can operate under Standard Air conditions. Specifically, absolute ceiling is the highest altitude that an aircraft can maintain level flight without being affected by the pressure difference of the outside pressure and the in-cabin pressure. At absolute ceiling level, a cabin pressurization system on an aircraft will not be able to provide an adequate oxygen level for the passengers and crew inside, or the atmospheric pressure on the aircraft will be too great on the structural design of the aircraft.
It can also be said that an aircraft has reached the absolute ceiling when it can no longer climb in altitude. In order to gain altitude during takeoff and flight, the pilot can either utilize the aircraft’s best angle-of-climb (Vx) or best rate-of-climb (Vy). The best angle-of-climb gives the greatest altitude gain in the shortest horizontal distance, while the best rate-of-climb gives the greatest altitude gain in the shortest time. As the pilot continues to climb and altitude increases, the speed for the best angle-of-climb increases while the speed for the best rate-of-climb decreases. With that said, it can be concluded that the point at which these two speeds meet is the absolute ceiling of the particular aircraft.
A related term is service ceiling, as mentioned above. This term refers to the altitude at which an airplane is only able to maintain a maximum climb of only 100 feet per minute. Therefore, the service ceiling is lower than the absolute ceiling, and more commonly used due to its practicality. As a reference, most commercial jets have an absolute ceiling of about 42,000 feet, while some business jets can fly at around 52,000 feet.