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Cabin Pressurisation – If Lost?

On 10 Jun 1990, on BA flight 5390, Captain Tim Lancaster was sucked out halfway out of the aircraft, when an improperly installed windscreen pane failed. While the first officer made an emergency landing in Southampton, the cabin crew firmly held on to Tim, bringing him down safely [*].

In another accident in 1994, navigator of an AN-32 aircraft was sucked out after his glass bubble broke. This accidental cabin depressurisation was attributed to metal fatigue, where the metal sheets surrounding the bubble had worn out and the wires holding the bubble had loosened. The pressure difference at that high altitude was beyond the affected airframe’s capability [**].

Loss of cabin pressure can vary from a slow leak, due to some minor mechanical fault such as a failure of the canopy seal, to a rapid or even ‘explosive’ decompression due to rupture of the cabin wall or loss of the canopy or window. During decompression, the potential ill effects are associated with the rapid expansion of gas within the body. As per the duration of decompression, there are three types of decompression:

  • Explosive Decompression – < 1 second
  • Rapid Decompression – > 1 second and < 20 seconds
  • Fast Decompression – > 20-30 seconds

 Effect of decompression on occupants depends on three major factors viz.:

  • Rate of decompression 
  • Pressure change during decompression 
  • Pressure in the cabin after decompression

The occurrence of a rapid decompression is indicated by a loud noise due to the sudden release of pressure and surrounding mist. The compressed air within the cabin rushes out of the defect at a velocity near the speed of sound until the cabin pressure reaches equivalence with the ambient. As air rapidly escapes from the cabin, the remaining gas expands. This causes the temperature of the air within the cabin to drop to its dew-point, and the water condenses as a mist. This mist can be so dense that it interferes with the occupant’s vision. The loud noise and the mist due to rapid decompression often leads the aircrew to believe that their aircraft is on fire or severely damaged.

In case of a slow leak there is no such dramatic happening. The first sign is either the audio or visual warning of the cabin pressurisation failure, or a higher cabin altimeter reading than indicated.

Effects of Cabin Decompression on occupants. The physiological effect of cabin decompression on the occupant’s is as follows:

  • Expansion of body gases
    • Lungs may get damaged if free venting is obstructed
    • Expansion of intestinal gases
    • Ear/Sinuses problem during descent
  • Hypoxia – This is the most important hazard of the decompression of the pressure cabin
  • Decompression Sickness
  • Hypothermia – This is due to the ill-effects of severe cold at higher altitudes.

There are three main factors, which govern the rate of decompression, viz.

  • Volume of the pressure cabin, 
  • Size of the cabin defect, and 
  • Pressure ratio between the cabin and the outside atmosphere.

The larger the pressure cabin, the longer it will take for the air to escape during the decompression and conversely small cabins will experience much more rapid decompression (time taken for decompression and equalisation of cabin and ambient pressure is less) for a given size of defect. It is for this reason that the combat aircraft, with smaller cockpits, have been designed with low differential cabins. The larger the cabin defect, the faster is the decompression, since a greater volume of air can escape within a shortened period of decompression.

Aerodynamic Suction Effect on Rapid Decompression. In case, rapid decompression occurs due to a defect in the cabin wall, rather than due to failure of the aircraft pressurisation system, the final cabin altitude may exceed the actual pressure altitude at which aircraft is flying. This happens due to the flow of ambient air over the defect which tends to suck the residual air out of the cabin, a classical aerodynamic suction effect. The magnitude of this effect varies with the aircraft type, the position of the defect in relation to the atmospheric air-stream and the aircraft’s altitude and speed.

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Reference

  • *British Airways Flight 5390
  • ** Old Workhorse, New Crisis
  • 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.

In case you wish to share your experiences, offer comments/feedback on this Blog, you may use the form below or send an e-mail to avmed@avmed.in

Acknowledgement  Image courtesy Wikimedia Commons

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