what is cabin altitude

Cabin altitude is the cabin pressure expressed as an equivalent altitude above sea level. In other words, it is the equivalent altitude above mean sea level having the same atmospheric pressure according to a standard atmospheric model such as the International Standard Atmosphere. The cabin altitude of a pressurized aircraft is normally maintained at and altitude of 8,000 ft or less as a compromise between the physiological needs of the crew and passengers and the structural limitations of the aircraft. At 8,000 ft, the use of supplemental oxygen is not required. The cabin altitude is displayed on a Cabin Altimeter which is often combined with a Cabin Differential Pressure Gauge. Warning systems are also incorporated to alert the crew of an excessive cabin altitude or loss of pressurization.