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Aircraft Pressurisation Systems

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Article Information
Category: Flight Technical Flight Technical
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary

Definition

A system which ensures the comfort and safety of crew and passengers by controlling the cabin pressure and the exchange of air from the inside of the aircraft to the outside.

Discussion

Aircraft engines become more efficient with increase in altitude, burning less fuel for a given airspeed. In addition, by flying above weather and associated turbulence, the flight is smoother and the aircraft less fatigued. Crews will therefore normally fly as close to the aircraft’s Cruise Ceiling as they can depending on flight rules and any other constraints such as the aircraft oxygen system. In order to be able to fly at high attitudes, the aircraft needs to be pressurised so that the crew and passengers can breathe without the need for supplemental oxygen.

The cabin and cargo holds (or baggage compartments) on most aircraft are contained within a sealed unit which is capable of containing air under pressure higher than the Ambient Pressure outside of the aircraft. Bleed Air from the turbine engines is used to pressurise the cabin and air is released from the cabin by an Outflow Valve. By using a cabin pressure regulator, to manage the flow of air through the outflow valve, the pressure within the aircraft can be increased or decreased as required, either to maintain a set Differential Pressure or a set Cabin Altitude.

In practice, as an aircraft climbs, for the comfort of the passengers, the pressurisation system will gradually increase the cabin altitude and the differential pressure at the same time. If the aircraft continues to climb once the maximum differential pressure is reached, the differential pressure will be maintained while the cabin altitude climbs. The maximum cruise altitude will be limited by the need to keep the cabin altitude at or below 8,000 ft.

A safety valve:

  • acts as a relief valve, releasing air from the cabin to prevent the cabin pressure from exceeding the maximum differential pressure,
  • acts a vacuum relief valve, allowing air into the cabin when the ambient pressure exceeds the cabin pressure, and
  • acts as a dump valve, allowing the crew to dump cabin air manually.

A Cabin Altimeter, Differential Pressure Gauge, and Cabin Rate of Climb gauge help the crew to monitor the aircraft pressurisation.

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Accident & Incidents

Events held on the SKYbrary A&I database which include reference to the air conditioning system include:

  • B733, en-route, northwest of Athens Greece, 2005 (On 14 August 2005, a Boeing 737-300 was released to service with the cabin pressurisation set to manual. This abnormal setting was not detected by the flight crew involved during standard checks. They took no corrective action after take-off when a cabin high altitude warning occurred. The crew lost consciousness as the aircraft climbed on autopilot and after eventual fuel exhaustion, the aircraft departed controlled flight and impacted terrain. The Investigation found that inadequate crew performance had occurred within a context of systemic organisational safety deficiencies at the Operator compounded by inadequate regulatory oversight.)
  • LJ35, Aberdeen SD USA, 1999 (On 25 October 1999, a Learjet 35, being operated on a passenger charter flight by Sunjet Aviation, crashed in South Dakota following loss of control attributed to crew incapacitation.)
  • A319, Helsinki Finland, 2018 (On 3 August 2018, smoke appeared and began to intensify in the passenger cabin but not the flight deck of an Airbus A319 taxiing for departure at Helsinki. Cabin crew notified the Captain who stopped the aircraft and sanctioned an emergency evacuation. This then commenced whilst the engines were still running and inadequate instructions to passengers resulted in a completely disorderly evacuation. The Investigation attributed this to inadequate crew procedures which only envisaged an evacuation ordered by the Captain for reasons they were directly aware of and not a situation where the evacuation need was only obvious in the cabin.)
  • RJ1H, en-route, South West of Stockholm Sweden, 2007 (On 22 March 2007, climbing out of Stockholm Sweden, the crew of a Malmö Aviation Avro RJ100 failed to notice that the aircraft was not pressurised until cabin crew advised them of automatic cabin oxygen mask deployment.)
  • B737 en-route, Glen Innes NSW Australia, 2007 (On 17 November 2007 a Boeing 737-700 made an emergency descent after the air conditioning and pressurisation system failed in the climb out of Coolangatta at FL318 due to loss of all bleed air. A diversion to Brisbane followed. The Investigation found that the first bleed supply had failed at low speed on take off but that continued take off had been continued contrary to SOP. It was also found that the actions taken by the crew in response to the fault after completing the take off had also been also contrary to those prescribed.)

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