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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:

  • H25B / AS29, en-route / manoeuvring, near Smith NV USA, 2006 (On 28 August 2006, a Hawker 800 collided with a glider at 16,000 feet in Class 'E' airspace. The glider became uncontrollable and its pilot evacuated by parachute. The Hawker was structurally damaged and one engine stopped but it was recovered to a nearby airport. The Investigation noted that the collision had occurred in an area well known for glider activity in which transport aircraft frequently avoided glider collisions using ATC traffic information or by following TCAS RAs. The glider was being flown by a visitor to the area with its transponder intentionally switched off to conserve battery power.)
  • E195, en-route, Irish Sea UK, 2008 (On 1 August 2008, an en-route Embraer 195 despatched with one air conditioning pack inoperative lost all air conditioning and pressurisation when the other pack’s Air Cycle Machine (ACM) failed, releasing smoke and fumes into the aircraft. A MAYDAY diversion was made to the Isle of Man without further event. The Investigation found that the ACM failed due to rotor seizure caused by turbine blade root fatigue, the same failure which had led the other air conditioning system to fail failure four days earlier. It was understood that a modified ACM turbine housing was being developed to address the problem.)
  • B744, en-route, South China Sea, 2008 (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.)
  • B735, en-route, SE of Kushimoto Wakayama Japan, 2006 (On 5 July 2006, during daytime, a Boeing 737-500, operated by Air Nippon Co., Ltd. took off from Fukuoka Airport as All Nippon Airways scheduled flight 2142. At about 08:10, while flying at 37,000 ft approximately 60 nm southeast of Kushimoto VORTAC, a cabin depressurization warning was displayed and the oxygen masks in the cabin were automatically deployed. The aircraft made an emergency descent and, at 09:09, landed on Chubu International Airport.)
  • B738, Glasgow UK, 2012 (On 19 October 2012, a Jet2-operated Boeing 737-800 departing Glasgow made a high speed rejected take off when a strange smell became apparent in the flight deck and the senior cabin crew reported what appeared to be smoke in the cabin. The subsequent emergency evacuation resulted in one serious passenger injury. The Investigation was unable to conclusively identify a cause of the smoke and the also- detected burning smells but excess moisture in the air conditioning system was considered likely to have been a factor and the Operator subsequently made changes to its maintenance procedures.)

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