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Revision as of 10:30, 7 July 2016

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 Systems|[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.

Related Articles

Accident & Incidents

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

  • 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".)
  • A320, en-route, northeast of Granada Spain, 2017 (On 21 February 2017, an Airbus A320 despatched with the APU inoperative experienced successive failures of both air conditioning and pressurisation systems, the second of which occurred at FL300 and prompted the declaration of a MAYDAY and an emergency descent followed by an uneventful diversion to Alicante. The Investigation found that the cause of the dual failure was likely to have been the undetectable and undetected degradation of the aircraft bleed air regulation system and whilst noting a possibly contributory maintenance error recommended that a new scheduled maintenance task to check components in the aircraft type bleed system be established.)
  • 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.)
  • B741, en-route, Gunma Japan 1985 (On August 12, 1985 a Boeing 747 SR-100 operated by Japan Air Lines experienced a loss of control attributed to loss of the vertical stabiliser. After the declaration of the emergency, the aircraft continued its flight for 30 minutes and subsequently impacted terrain in a mountainous area in Gunma Prefecture, Japan.)
  • A320, en-route, west southwest of Karachi Pakistan, 2018 (On 5 March 2018, the crew of an Airbus A320 in descent towards Karachi observed a slow but continuous drop in cabin pressure which eventually triggered an excessive cabin altitude warning which led them to don oxygen masks, commence an emergency descent and declare a PAN to ATC until the situation had been normalised. The Investigation found that the cause was the processing of internally corrupted data in the active cabin pressure controller which had used a landing field elevation of over 10,000 feet. It noted that Airbus is developing a modified controller that will prevent erroneous data calculations occurring.)

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