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.
Accident & Incidents
Events held on the SKYbrary A&I database which include reference to the air conditioning system include:
On 17 November 2021, shortly after a Boeing 737-800 commenced initial descent into Patna from FL350, a cautionary alert indicating automatic pressurisation system failure was annunciated. When the initial actions of the prescribed non-normal procedure did not resolve the problem, the system outflow valve was fully opened and a rapid depressurisation followed. After this incorrect action, the relevant crew emergency procedures were then comprehensively not properly followed and it was further concluded that the Captain had temporarily lost consciousness after a delay in donning his oxygen mask. The context for the mismanaged response was identified as outflow valve in-service failure.
On 23 February 2016, a Boeing 737-800 departing New Chitose encountered sudden-onset and unforecast heavy snowfall whilst taxiing out. When the right engine ran down and cabin crew reports of unusual smells in the cabin and flames coming from the right engine were received, it was decided that an emergency evacuation was required. During this evacuation three passengers were injured, one seriously. The engine fire was found to have been in the tailpipe and caused by an oil leak due to engine fan blade and compressor icing which had also led to vapourised engine oil contaminating the air conditioning system.
On 13 July 2018, a Boeing 737-800 cruising at FL370 at night experienced a sudden rapid depressurisation. An emergency descent to FL 090 followed but the cabin altitude was not manually controlled and after the cabin pressure had risen to that equivalent to 7000 feet below sea level, immediate equalisation of cabin and actual altitudes resulted in a second sudden depressurisation. Diversion to Frankfurt Hahn was completed without further event. The first depressurisation had resulted from a transient and rare pressure controller malfunction but passenger injuries were considered attributable to a complete absence of pressurisation control during the emergency descent.
On 1 June 2019, a Boeing 787-8 lost all cabin air conditioning after both packs failed less than an hour from its destination, Narita. When the Cabin Altitude reached 10,000 feet, the descent already commenced was completed as an emergency descent and the flight thereafter was without further event. The Investigation found that although an amended non normal procedure restricting pack resets to a maximum altitude of 35000 feet had been issued almost two months earlier, it was still “under review” at the operator which has since amended their procedures for assessing manufacturer communications which have operational safety implications.
On 3 August 2018, smoke appeared and began to intensify in the passenger cabin but not the flight deck of an Airbus A319 taxiing for departure at Helsinki. Cabin crew notified the Captain who stopped the aircraft and sanctioned an emergency evacuation. This then commenced whilst the engines were still running and inadequate instructions to passengers resulted in a completely disorderly evacuation. The Investigation attributed this to inadequate crew procedures which only envisaged an evacuation ordered by the Captain for reasons they were directly aware of and not a situation where the evacuation need was only obvious in the cabin.
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.
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.
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.
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.
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.
On 3 December 2017, an Embraer E190 en-route at FL310 was already turning back to Helsinki because of a burning smell in the flight deck when smoke in the cabin was followed by smoke in the flight deck. A MAYDAY was declared to ATC reporting “fire on board” and their suggested diversion to Turku was accepted. The situation initially improved but worsened after landing prompting a precautionary emergency evacuation. The Investigation subsequently attributed the smoke to a malfunctioning air cycle machine. Issues with inaccessible cabin crew smoke hoods and with the conduct and aftermath of the evacuation were also identified.
On 23 August 2017, a Boeing 767-400ER which had departed Zurich for a transatlantic crossing experienced a problem with cabin pressurisation as the aircraft approached FL 100 and levelled off to run the applicable checklist. However, despite being unable to confirm that the pressurisation system was functioning normally, the climb was then re-commenced resulting in a recurrence of the same problem and a MAYDAY emergency descent from FL 200. The Investigation found that an engineer had mixed up which pressurisation system valve was to be de-activated before departure and that the flight crew decision to continue the climb had been risky.
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.
On 29 April 2017, a Boeing 787-9 which had just reached cruise altitude after despatch with only one main ECS available began to lose cabin pressure. A precautionary descent and PAN was upgraded to a rapid descent and MAYDAY as cabin altitude rose above 10,000 feet. The Investigation found that aircraft release to service had not been preceded by a thorough enough validation of the likely reliability of the remaining ECS system. The inaudibility of the automated announcement accompanying the cabin oxygen mask drop and ongoing issues with the quality of CVR readout from 787 crash-protected recorders was also highlighted.
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.