<P> The part of the bleed air that is directed to the ECS is then expanded and cooled to a suitable temperature by passing it through a heat exchanger and air cycle machine known as the packs system . In some larger airliners, hot trim air can be added downstream of air conditioned air coming from the packs if it is needed to warm a section of the cabin that is colder than others . </P> <P> At least two engines provide compressed bleed air for all the plane's pneumatic systems, to provide full redundancy . Compressed air is also obtained from the auxiliary power unit (APU), if fitted, in the event of an emergency and for cabin air supply on the ground before the main engines are started . Most modern commercial aircraft today have fully redundant, duplicated electronic controllers for maintaining pressurization along with a manual back - up control system . </P> <P> 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 7.8 psi (54 kPa) and 9.4 psi (65 kPa). At 39,000 feet (12,000 m), the cabin pressure would be automatically maintained at about 6,900 feet (2,100 m) (450 feet (140 m) lower than Mexico City), which is about 11.5 psi (79 kPa) of atmosphere pressure . </P> <P> Some aircraft, such as the Boeing 787 Dreamliner, have re-introduced electric compressors previously used on piston - engined airliners to provide pressurization . The use of electric compressors increases the electrical generation load on the engines and introduces a number of stages of energy transfer; therefore, it is unclear whether this increases the overall efficiency of the aircraft air handling system . It does, however, remove the danger of chemical contamination of the cabin, simplify engine design, avert the need to run high pressure pipework around the aircraft, and provide greater design flexibility . </P>

The primary function of the cabin pressurization system outflow valve is to
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