The cabin pressure expressed as an equivalent altitude above sea level.
The Cabin Altitude of a pressurised 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.
There are times when the crew may wish to maintain the cabin pressure at or near sea level. This may be for a number of reasons including:
The nature of the cargo. Some aircraft, military transport aircraft for example, may be transporting volatile chemicals such as specialist fuels or explosives which are sensitive to changes in pressure.
The medical condition of a passenger whose condition may be worsened by changes in pressure or a drop in pressure, for example an aeromedical flight with a patient with internal injuries.
In such cases, the aircraft may have to fly at a lower cruising altitude than normal so the lower cabin altitude can be maintained without exceeding the structural pressure differential limitation. This may have implications for the routing (terrain clearance) and range (higher fuel burn at lower cruising altitude).
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 pressurisation.