Loss of Cabin Pressurisation
Loss of Cabin Pressurisation
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Definition
Depressurisation of the aircraft cabin as a result of structural failure, pressurisation system malfunction, an inadvertent crew action or a deliberate crew intervention.
Description
Loss of pressurisation is a potentially serious emergency in an aircraft flying at the normal cruising altitude for most jet passenger aircraft. Loss of cabin pressure, or depressurisation, is normally classified as explosive, rapid, or gradual based on the time interval over which cabin pressure is lost.
The cabins of modern passenger aircraft are pressurised in order to create an environment which is physiologically suitable for humans (Aircraft Pressurisation Systems). Maintaining a pressure difference between the outside and the inside of the aircraft places stress on the structure of the aircraft. The higher the aircraft flies, the higher the pressure differential that needs to be maintained and the higher the stress on the aircraft structure. A compromise between structural design and physiological need is achieved on most aircraft by maintaining a maximum cabin altitude of 8,000 ft.
The composition of atmospheric air remains constant as air pressure reduces with increasing in altitude and since the partial pressure of oxygen also reduces, the absolute amount of oxygen available also reduces. The reduction in air pressure reduces the flow of oxygen across lung tissue and into the human bloodstream. A significant reduction in the normal concentration of oxygen in the bloodstream is called Hypoxia.
The degree to which an individual’s performance is affected by lack of oxygen varies depending on the altitude of the aircraft, and on personal factors such as the general health of the person and whether he/she is a smoker. Below 10,000 ft, the reduced levels of oxygen are considered to have little effect on aircrew and healthy passengers but above that, the effect becomes progressively more pronounced. Above 20,000 ft, lack of oxygen leads to loss of intellectual ability followed by unconsciousness and eventually respiratory and heart failure. When suddenly deprived of normal levels of oxygen, estimates of the Time of Useful Consciousness are a pertinent guide - at 35,000 ft it is less than one minute. See the separate article on Hypoxia for more detailed information.
Note that some military flights may involve deliberate depressurisation at high altitude for the purpose of dropping troops or equipment by parachute. Such flights are normally conducted in accordance with specific special procedures.
Causes
- Structural Failure: Failure of a window, door, or pressure bulkhead for example, or in-flight explosion. An in-flight explosion may be due to a system failure, dangerous cargo, or a malicious act consequential on such as an explosive device on board.
- Pressurisation system failure: Malfunction of some part of the pressurisation system such as an outflow valve.
- Inadvertent system control input(s): Accidental or incorrect activation of a critical pressurisation control.
- Deliberate Act: A drastic measure but one which an aircraft captain might consider, for example, as a way of clearing the cabin of smoke.
Effects
- Crew Incapacitation. Depending on the altitude of the aircraft when depressurisation takes place, loss of pressurisation can very quickly lead to the incapacitation of the crew and passengers unless they receive supplementary oxygen.
Solutions
- Oxygen. In the event of loss of pressurisation, it is essential that the flight crew don oxygen equipment as soon as possible. In the case of a deliberate depressurisation, the crew should be on oxygen before the depressurisation commences.
- Emergency Descent. In the case of an uncontrolled depressurisation, the crew will want to descend immediately to an altitude at which they and the passengers can breathe without supplementary oxygen - usually given as 10,000 feet amsl subject to adequate terrain clearance.
For further information see the articles Pressurisation Problems: Guidance for Flight Crews and Emergency Depressurisation: Guidance for Controllers.
Accidents and Incidents
On 6 June 2023, a Boeing 717-200 was on base leg about 10 nm from Hobart, Australia, when chlorine fumes became evident on the flight deck. As the aircraft became fully established on final approach, the captain recognised signs of cognitive impairment and handed control to the initially unaffected first officer. Just before touchdown, the first officer was similarly affected but was able to safely complete the landing and taxi in. The same aircraft had experienced a similar event two days earlier with no fault found. The Investigation determined that the operator’s procedures for responding to crew incapacitation in flight had been inadequate.
On 8 June 2016, a Boeing 737-800 en-route to Seville, Spain, had already reverted to alternate automatic pressurisation control when this also failed. Manual system control was attempted but was unsuccessful, so an emergency descent followed by diversion to Toulouse, France, was completed without further event. A similar pressurisation control fault had occurred earlier that day but had not been properly dealt with by an appropriately qualified engineer. Both system controllers were showing faults and were replaced, as were a ruptured flexible hose and a series of malfunctioning drain valves. More reliable controllers and routine checking of system performance were recommended.
On 8 February 2022, a Boeing 767-300ER inbound to Madrid at FL340 experienced a failure of automatic pressurisation control, followed almost three hours later by a failure of manual control and rapidly rising cabin altitude. An emergency was declared and descent made to FL120 where manual control was regained. The flight was completed without recurrence. The failure cause was found to have been water leaking from a tube with a broken clamp which, when it froze, had blocked the air conditioning outflow valve doors. Elements of the system design, scheduled maintenance requirements, and fault detection were identified as contributing factors.
On 17 November 2021, 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 not properly followed. 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.
Further Reading
Airbus
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