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 27 May 2023, a Cessna 650 had just taken off from Aalborg when non-electrical smoke began to enter the flight deck and became so dense that it was impossible to navigate visually. An immediate MAYDAY return was made during which the smoke decreased sufficiently to make an overweight visual landing. It was concluded that failure of one of the air cycle machines had been caused by a contaminated heat exchanger due to maintenance requirements not clearly outlined in the aircraft maintenance programme.
On 12 July 2023, the crew of an Airbus A380-800 in the climb at night after departing from Johannesburg detected an acrid smell in the air circulated in both the flight deck and the passenger cabin. Concern at a possible health issue if the intended long flight was completed prompted the decision to return to Johannesburg. A MAYDAY was declared and the return accomplished without any further worsening of the situation. The cause of the contaminated airflow was later found to be a malfunctioning Air Cycle Machine.
On 17 October 2023 a Boeing 737-800 passing FL130 after departing Manchester received a cabin pressurisation warning, and the crew recognised that both engine air bleeds were off. After correcting this, climb was continued without donning oxygen masks until an air conditioning pack fault occurred, which prompted the operator to request a return which was uneventful. The bleeds-off condition had arisen after maintenance and had then not been recognised at release to service or during crew preflight checks.
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.
Further Reading
Airbus
ATSB
