Cabin pressure

Mechanics

All exhaust air is dumped to atmosphere via an outflow valve, usually at the rear of the fuselage. This valve controls the cabin pressure and also acts as a safety relief valve, in addition to other safety relief valves. If the automatic pressure controllers fail, the pilot can manually control the cabin pressure valve, according to the backup emergency procedure checklist. The automatic controller normally maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position so that the cabin altitude is as low as practical without exceeding the maximum pressure differential limit on the fuselage. The pressure differential varies between aircraft types, typical values are between 540 hPa (7.8 psi) and 650 hPa (9.4 psi). At 39,000 ft (11,887 m), the cabin pressure would be automatically maintained at about 6,900 ft (2,100 m), (450 ft (140 m) lower than Mexico City), which is about 790 hPa (11.5 psi) of atmosphere pressure.