[Walter_Middie]

What is loss of cabin pressure? The plane flys at 40,000 ft or more, and the cabin is pressurized to about 6,000 ft. If you start losing cabin pressure, the pilots will begin to descend. Not a crazy dive, just a fast descent down to a better altitude. The idea is that as the cabin altitude goes up, the plane is coming down. At some point the cabin altitude becomes the outside airplane altitude - they should be down to about 25,000 ft by this point - as long as there isn't a really big hole somewhere. The descent continues until you reach about 10,000 ft. Takes about 6 minutes from cruise altitude to a safe to breath altitude. Oxygen masks are needed for the descent.

This airplane supposedly went through severe turbulence - Something sudden had to happen for them not to be able to call out a may day.