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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:

  • B773, en-route, east northeast of Anchorage AK USA, 2015 (On 30 December 2015, a Boeing 777-300 making an eastbound Pacific crossing en-route to Toronto encountered forecast moderate to severe clear air turbulence associated with a jet stream over mountainous terrain. Some passengers remained unsecured and were injured, one seriously and the flight diverted to Calgary. The Investigation found that crew action had mitigated the injury risk but that more could have been achieved. It was also found that the pilots had not been in possession of all relevant information and that failure of part of the air conditioning system during the turbulence was due to an improperly installed clamp.)
  • A320, en-route, north of Öland Sweden, 2011 (On 5 March 2011, a Finnair Airbus A320 was westbound in the cruise in southern Swedish airspace after despatch with Engine 1 bleed air system inoperative when the Engine 2 bleed air system failed and an emergency descent was necessary. The Investigation found that the Engine 2 system had shut down due to overheating and that access to proactive and reactive procedures related to operations with only a single bleed air system available were deficient. The crew failure to make use of APU air to help sustain cabin pressurisation during flight completion was noted.)
  • A320, en route, north of Marseilles France, 2013 (On 12 September 2013, pressurisation control failed in an A320 after a bleed air fault occurred following dispatch with one of the two pneumatic systems deactivated under MEL provisions. The Investigation found that the cause of the in-flight failure was addressed by an optional SB not yet incorporated. Also, relevant crew response SOPs lacking clarity and a delay in provision of a revised MEL procedure meant that use of the single system had not been optimal and after a necessary progressive descent to FL100 was delayed by inadequate ATC response, and ATC failure to respond to a PAN call required it to be upgraded to MAYDAY.)
  • B738, en-route, near Lugano Switzerland, 2012 (On 4 April 2012, the cabin pressurisation controller (CPC) on a Boeing 737-800 failed during the climb passing FL305 and automatic transfer to the alternate CPC was followed by a loss of cabin pressure control and rapid depressurisation because it had been inadvertently installed with the shipping plug fitted. An emergency descent and diversion followed. The subsequent Investigation attributed the failure to remove the shipping plug to procedural human error and the poor visibility of the installed plug. It was also found that "the pressurisation system ground test after CPC installation was not suitable to detect the error".)
  • 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.)

... further results