Aircraft Pressurisation Systems
From SKYbrary Wiki
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.
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.
- Explosive Depressurisation
- Rapid Depressurisation
- Gradual Depressurisation
- Loss of Cabin Pressurisation
- Aircraft Oxygen Systems
Accident & Incidents
Events held on the SKYbrary A&I database which include reference to the air conditioning system include:
- B734, en-route, east northeast of Tanegashima Japan, 2015 (On 30 June 2015, both bleed air supplies on a Boeing 737-400 at FL370 failed in quick succession resulting in the loss of all pressurisation and, after making an emergency descent to 10,000 feet QNH, the flight was continued to the planned destination, Kansai. The Investigation found that both systems failed due to malfunctioning pre-cooler control valves and that these malfunctions were due to a previously identified risk of premature deterioration in service which had been addressed by an optional but “recommended” Service Bulletin which had not been taken up by the operator of the aircraft involved.)
- 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.)
- B738, en-route, near Sydney Australia 2018 (On 12 July 2018, a Boeing 737-800 was climbing through FL135 soon after takeoff from Sydney with First Officer line training in progress when the cabin altitude warning horn sounded because both air conditioning packs had not been switched on. The Captain took control and descended the aircraft to FL100 until the situation had been normalised and the intended flight was completed. The Investigation noted that although both pilots were experienced in command on other aircraft types, both had limited time on the 737 and concluded that incorrect system configuration was consequent on procedures and checklists not being managed appropriately.)
- A320, vicinity Dublin Ireland, 2015 (On 3 October 2015, an Airbus A320 which had just taken off from Dublin experienced fumes from the air conditioning system in both flight deck and cabin. A 'PAN' was declared and the aircraft returned with both pilots making precautionary use of their oxygen masks. The Investigation found that routine engine pressure washes carried out prior to departure have been incorrectly performed and a contaminant was introduced into the bleed air supply to the air conditioning system as a result. The context for the error was found to be the absence of any engine wash procedure training for the Operator's engineers.)
- 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.)