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Old 10th Apr 2014, 14:49
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tom775257
 
Join Date: Jan 2001
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I would suggest that a structural failure casing rapid depressurisation would be a very unlikely scenario. A much more common (but still VERY rare) cause of depressurisation on the type I fly (I suspect on others too) would be a slow depressurisation due to dual air bleed failure (air taken off the engines) or dual pack failure (aircon units). Generally it starts with an innocuous single failure, but the extra stress due to becoming the single source can sometimes send the other one into failure too. Now the aircraft will drive its outflow valve shut (where air from the cabin is exhausted) but now you are relying on seals to keep the pressurised air in the cabin.

The good thing about this scenario is you can beat the cabin down, so emergency descent circa 6000' per minute, the cabin climbs about 1500' per minute depending on the seals etc. on the aircraft. Generally you can get the aircraft below 10,000' (MSA depending) before the cabin hits 14,000. Not too bad on the ears, no fog, no masks for the pax. Also with dual bleed failures, we can get the APU lit in the descent and get some bleed air off that at 20,000' and restore normal ops.

A full on proper depressurisation is a well practiced procedure, sure you might blow the odd eardrum of someone with a bad cold, but it isn't going to kill you. A pilot friend of mine burst an eardrum flying, first thing she knew about it was a bit of blood out of her ear, not particularly painful for her apparently.
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