Event Description
While climbing through 35,000 feet for a cruise altitude of 37,000 feet, the crew heard a loud thump and the aircraft depressurized. As the crew descended to 11,000 feet, the cabin altitude climbed to 30,000 feet. The flight diverted to a nearby airport and safely landed.
Examination Findings
Examination of the aircraft disclosed that repetitive pressurization cycles caused the initiation of a fatigue crack and the resultant rupture of a forward pressure bulkhead.
Historical Context
Over 14 years earlier, Douglas issued service bulletins advising operators that leaks could occur in the forward pressure bulkhead area because of metal fatigue. Douglas recommended that specific corrective action be taken which involved inspecting the area at 1,500 hour intervals or making permanent structural airframe modifications.
Operator's Maintenance Approach
Contrary to the majority of aircraft operators, United chose not to structurally modify this aircraft. Rather, it chose to perform recurring visual inspections for leaks in the suspect area. The aircraft's pressure bulkhead ruptured 1,367 hours after its last inspection.