What happened
While climbing through 35,000 feet toward a planned cruise altitude of 37,000 feet, the crew of the aircraft heard a loud thump, followed immediately by a loss of cabin pressure. As the flight crew initiated a descent to 11,000 feet, the cabin altitude rose to 30,000 feet. The flight was diverted to a nearby airport, where the aircraft landed safely with no injuries.
The investigation
An examination of the aircraft revealed that repetitive pressurization cycles had caused a fatigue crack to initiate, resulting in the rupture of a forward pressure bulkhead.
Records indicated that over 14 years prior, Douglas had issued service bulletins warning operators that leaks could occur in the forward pressure bulkhead area due to metal fatigue. The manufacturer's recommended corrective action involved either performing inspections at 1,500-hour intervals or implementing permanent structural airframe modifications. Unlike many other operators, United Airlines had elected not to perform the structural modifications, opting instead to rely on recurring visual inspections for leaks in the affected area. The rupture of the pressure bulkhead occurred 1,367 hours after the aircraft's last inspection.