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Cabin Pressurisation – Hazards of Rapid Decompression

Incidence of failure of cabin pressurisation in military aircraft, even in peace time, is higher than commercial aviation. The commonest cause of loss of pressurisation in military aviation, especially in fighter flying, is engine flameout.

Hereafter, the discussion focusses on decompression in military aviation, particularly with the perspective of its occurrence in a combat aircraft.

The physiological effect of decompression depends upon the pressure differential, the duration of decompression and the final cabin altitude. The most severe hazards associated with a rapid decompression to high altitude is hypoxia. This hazard is particularly significant for the following three reasons.

  • a.  In the case of high differential cabins with a cruising cabin altitude of 1,500 m – 2000 m (6000 to 8,000 ft), the crew is most unlikely to use oxygen equipment at the time of the decompression.
  • b.  If the final cabin altitude is very high i.e. above 10,000 m (33,000 ft), the time of useful consciousness (TUC) for various crew members breathing air may be reduced by as much as one third from the figures which would normally be expected for that ambient altitude. This is due to the fact that during the escape of gas from the lungs, the partial pressure of oxygen in the alveoli is reduced to below 40 mm Hg which is the approximate value for the oxygen tension in the venous blood. There is an actual reversal of the oxygen diffusion gradient across the alveolar membranes and oxygen passes back into the lungs from the venous blood. Immediately following a rapid decompression to these very high altitudes, therefore, the arterial blood leaving the heart would be carrying a little or no oxygen and the onset of hypoxia would be very rapid. This shows the advantage of having the pilot or one of the pilots on oxygen the whole time when ambient aircraft altitude exceeds 8.5 Km (30,000 feet).
  • c.  The final altitude after decompression may be above 12,000 m (40,000 ft) and positive pressure breathing would be required to prevent hypoxia.

When the immediate problem of hypoxia has been overcome, the crew may still be faced with the possibility of developing symptoms of Decompression Sickness (DCS), especially so if there is a need to continue the flight at a cabin altitude greater than 7,500 m (25,000 ft). Susceptible individuals might develop DCS even at an altitude as low as 5,500 m (18,000 ft).

Depending upon the size and position of the defect in the cabin structure, cold may also limit sustained flight at altitude after a rapid decompression. If for example, the canopy has been lost or the windscreen shattered, there would be extremely severe wind chill effect, which could be debilitating and life threatening.

Barotrauma, due to rapid expansion of gases in semi-closed cavities, is not so often reported under normal conditions of flight and pressurisation.

There is another aspect of severe or explosive decompression, which is a distinct possibility in a military transport or bomber aircraft. The violent rushing out of cabin air through a hatch or window opening could endanger aircraft occupants who are not strapped in. There is a possible danger to the passengers or crew being displaced from their seats if they are not restrained by a harness at the time of decompression. They can be pulled out of their seats and even out of the aircraft, depending upon the severity of the decompression and their position in relation to the defect in the aircraft structure.

In-Flight Measures after Loss of Cabin Pressure. Emergency actions must be initiated to prevent hypoxia. The actions required are to commence an immediate descent to safe altitude, while donning/tightening the Oxygen masks. In fighter aircraft, if the resultant cabin altitude is greater than 12,000 m (40,000 ft) the subsequent action will depend upon the type of positive pressure breathing assembly being used. A full pressure suit, which provides the pilot with the capability of staying at altitude and with it functioning correctly, minimises the threat of hypoxia. On the other hand, if a partial pressure assembly, which is a ‘get-you-down’ system, is used, an immediate descent below at least 12,000 m (40,000 ft) at maximum rate will be necessary. In either case, the remaining mission should be undertaken as per the operational requirement, available fuel, the threat of low cabin temperatures on the occupants as well as aircraft equipment, besides the possibility of DCS.

The pressure cabin is an essential part of the high performance combat aircraft, allowing it the capability to fly at higher altitudes. Without pressurisation, aircrew would be able to carry out high altitude missions using their personal oxygen equipment continuously. At higher altitudes, this would be not only cumbersome but shall have an adverse effect on their efficiency and performance during flight. On the other hand, however attractive be the concept of a so-called ‘shirt-sleeve’ environment, it is not achievable in a military aircraft – the defined roles of the modern combat aircraft exposes it to a higher risk of rapid decompression due to the loss of cabin pressure. This also necessitates provisioning of personal oxygen assembly to prevent hypoxia, irrespective of the phase of flight.

The modern commercial aircraft on the other hand, have designed aircraft with lower cabin altitudes. The Boeing 776 and Airbus 380 maintain a maximum cabin altitude of about 2100 m (6900 feet) and 1500 m (5000 feet) while cruising at altitudes of 12,000 m (39,000 feet) and 13,000 m (43,000 feet), respectively, making it as near physiologically compatible for human occupant as possible [*].

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Reference

  • Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.
  • Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.
  • Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.
  • * Cabin Pressurization

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Acknowledgement Image courtesy Wikimedia Commons

2 comments

  1. Rajat Baijal

    Engine flame out is a common occurrence in military combat flying. It is generally a short term event leading to a successful relight or inevitable ejection in case of a failure to relight. Loss of cabin pressurization is possible due to use of engine bleed air, but has the rate of loss / level of loss of cabin pressure and its significance from physiological point of view in case of Engine Flame out ever been studied / reported.

    1. Av Med

      Thanks Rajat for your query. Regrettably, such incident/accident reports being in the military domain, even if studied, are not available in public domain. Though an odd anecdote may be heard occasionally!

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