Cabin Pressurization - Mechanics of Pressurization

Mechanics of Pressurization

Pressurization is achieved by the design of an airtight fuselage engineered to be pressurized with a source of compressed air and controlled by an environmental control system (ECS). The most common source of compressed air for pressurization is bleed air extracted from the compressor stage of a gas turbine engine, from a low or intermediate stage and also from an additional high stage, the exact stage can vary, depending on engine type. By the time the cold outside air has reached the bleed air valves it is at a very high pressure and has been heated to around 200 °C (392 °F). The control and selection of high or low bleed sources is fully automatic and is governed by the needs of various pneumatic systems at various stages of flight.

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 of the 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.

At least two engines provide compressed bleed air for all of 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.

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. In the event that 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, which varies between different aircraft types but is normally around 8.6 psi. At 39,000 feet, the cabin pressure would be automatically maintained at about 6,900 ft (450 feet lower than Mexico City), which is about 11.5 psi of atmosphere pressure (79.3 kPa).

Some aircraft, such as the Boeing 787 Dreamliner, have re-introduced the use of 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, simplifies engine design, avoids the need to run high pressure pipework around the aircraft and provides greater design flexibility.