A number of people have asked me about details of losing pressurisation in flight at altitude. Flight crews are trained to react quickly to situations were there is a loss of presurisation. The human body relies on pressure so that the lungs can work getting oxygen out of the air and into the bloodstream. No pressure, no oxygen transfer. If the aircraft cabin is punctured or the aircaft pressurisation systems fails such that the pressure inside the cabin becomes the same as that outside, this can cause major issues when cruising at altitude. For instance at 35,000 ft a typical person has 45 seconds of useful consciousness. In other words you have just 45 seconds to be able to think and react before you start suffering hypoxia. Hypoxia is another name for altitude sickness. If a person is hypoxic they most likely will feel drunk and believe that everything is fine. If the situation continues it is likely that person will lapse into unconsciousness and, in fact may possibly die. If you are cruising at 40,000 ft you have about 15 secs of useful consciousness. Not much time really. In fact when cruising above 41,000ft it is mandatory for flight crew to be always wearing oxygen masks.
Now you may be saying, well I can just hold my breath at 35,000ft. This will not work as I mentioned before you need pressure on the lungs to be able to be able to oxygenate the blood.
OK. So what happens if you are cruising along at 35,000 ft and a window blows out. This would create what is termed an explosive decompression. There would be a bang and air in the cabin would rush out the hole. It would be very noisy, lots of dust would be kicked up, even foggy. Various gases in your body would be also trying to escape (burp/fart) and it is likely you would have stomach cramps and ear aches. The 747-400 has 3 airconditioning packs that can actually handle a hole the size of a blown out window so it is not THAT big an issue. Never-the-less a lot of things may get sucked out that hole. Hopefully no people. The oxygen maskes would drop automatically. The crew, after putting on their oxygen masks, would immediately initiate a rapid descent. That is idle the engines back, point the nose downward and descent at maximum speed with airbrakes extended. Aim is to get down ASAP to 10,000 ft. Typically it would take 3-4mins to get from 35,000 ft to 10,000 ft. Once at 10,000 ft the oxygen masks can be removed and the crew would fly the aircraft to the nearest airport.