Rapid Depressurization

Cruise FL390. A sudden loud bang — structural failure. CABIN ALT HI warning fires. Cabin altitude is rising above 14,000 ft. Time of useful consciousness at FL390 without oxygen is 15–20 seconds sitting quietly — but a rapid decompression roughly halves it, so you have about 8–10 seconds to get the mask on.

Crew dons O2 masks IMMEDIATELY. Announce to passengers. Initiate emergency descent to FL100. Squawk 7700. Declare MAYDAY. Plan diversion.