A 12 inch crack on the crown skin forced a Southwest Boeing 737-700, to make a rapid descent from 39,000 feet to 22,000 feet on 13 March Friday.
A Southwest flight WN-1685 with 123 passengers from Las Vegas, USA (LAS) to Boise, USA (BOI) was enroute at FL390 about 130nm southwest of Salt Lake City,UT, when the crew initiated a rapid descent to FL220 due to unstable cabin pressure.
The aircraft levelled off at FL220 about 7 minutes and the cabin pressure stabilized at FL220. The passenger oxygen masks had not deployed, so that the crew continued the flight to Boise maintaining FL220.
The aircraft landed safely in Boise about 38 minutes after leaving FL390.
An inspection by the FAA revealed a 12inch/30cm long crack in the crown skin area behind the cockpit. This area was required to be inspected every 1500 flight cycles and mandated by the FAA to detect cracks and missing or loose fasteners.
The last inspection of that area had been done 500 flight hours before the occurrence.
The FAA is investigating the occurrence, it is too early to tell whether more frequent examinations of these areas will be required.
Southwest currently operates a fleet of 707 Boeing 737s.