Aircraft Pressurisation Systems

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

Contents 1 Definition 2 Discussion 3 Related Articles 4 Accident & Incidents

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 managing the flow of air through the outflow valve, using a cabin pressure regulator, 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. When the maximum differential pressure is reached, then if the aircraft continues to climb 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

• Explosive Depressurisation
• Emergency Depressurisation
• Hypoxia

Accident & Incidents

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

• B738, en-route, southern Austria, 2010 (AW) (On 9 May 2010, Boeing 737-800 being operated by Swedish operator Viking Airlines on a public transport charter flight from Sharm el Sheikh, Egypt to Manchester UK and which had earlier suffered a malfunction which affected the level of redundancy in the aircraft pressurisation system, experienced a failure of the single air conditioning pack in use when over southern Austria and an emergency descent and en route diversion to Vienna were made. There were no injuries to any of the 196 occupants.)
• B744, en-route, South China Sea, 2008 (AW LOC) (On 25 July 2008, a Boeing 747 suffered a rapid depressurisation of the cabin following the sudden failure of an oxygen cylinder, which had ruptured the aircraft's pressure hull. The incident occurred 475 km north-west of Manila, Philippines.)
• B737 en-route, Glen Innes Australia, 2007 (AW HF) (On 17 November 2007 a Boeing 737-700 being operated by Virgin Blue on a scheduled passenger service from Coolangatta to Melbourne at night experienced a right hand engine bleed trip off during the take off, which was continued. A subsequent attempt at reset was not successful and once above FL170, above which APU air use was not permitted, only the left engine bleed air would be available for air conditioning and cabin pressurisation. The flight crew initially decided to cruise at a lower level than planned, FL250, but once there, icing conditions were encountered and the crew decided to continue the climb to FL350 to cruise clear of cloud.)
• RJ1H, en-route, South West of Stockholm Sweden, 2007 (HF LOC AW) (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.)
• B741, en-route, Gunma Japan 1985 (AW LOC HF) (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.)
• … further results