Old Jul 22, 08, 6:00 pm
Join Date: Jun 2008
Posts: 943
Originally Posted by doobierw View Post
Takes about an hour to restow the masks. Did that once when the masks were all dropped by an overzealous maintenance guy..... IF however the oxygen generators were activated, then it's a big operation to replace all of them.

There are two scenarios here: One is that the crew noticed the plane was not pressurizing properly as they were climbing....and were able to get down soon enough to 'beat' the masks deploying. The cabin is normally pressurized to around 6 or 8k feet equivalent. We get an alarm around 10k pressure, and the masks deploy around 14k pressure equivalent. In that scenario, the masks will generally not deploy......

The second scenario is the explosive decompression, which is an instantaneous loss of pressure at altitude. The cabin will go from 6k to 'flight level k' in seconds. That is a serious event, and the masks WILL deploy automatically.

The more common event is scenario one. Had it once out of BNA, and we just leveled off at 9,000 feet while we worked the scenario. Regained cabin pressure control manually and proceeded uneventfully to destination.

Aircraft have a cabin outflow valve (located on the aft, left part of the fuselage for you techno-geek enthusiasts) that is full open on the ground, and will modulate closed as we climb. The closure of that valve maintains a certain psi differential, which in turn maintains cabin pressure at a pre-determined altitude. The outflow valve has two controllers which are electric driven motors, and they can also be controlled manually in the event both motors fail.

Thanks for the info! Good to know/understand.
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