Page:CAB Accident Report, Pan Am Flight 4.pdf/1

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ADOPTED: September 28, 1966

A Pan American World Airways, Inc., B-707-21B, N761PA, experienced an explosive disintegration of the third stage turbine disk of the No. 4 engine at approximately 1410 P.d.t., June 28, 1965. The accident occurred shortly after takeoff from San Francisco International Airport, San Francisco, California, at an altitude of about 800 feet above the ground. Disintegration of the turbine disk was followed by a fire in the No. 4 engine area and an explosion in the outboard reserve fuel tank. The No. 4 engine and approximately 25 feet of the right outer wing separated from the aircraft.

The fire was extinguished and a successful emergency landing was accomplished at Travis Air Force Base, California, with no injuries to the 143 passengers or 10 crewmembers aboard the flight.

The Board determines that the probable cause of this accident was the failure of the third stage turbine disk. This failure was caused by a transient loss of operating clearance between the third stage turbine disk and the third stage inner sealing ring. This loss of clearance resulted from a combination of improper turbine rotor positioning during engine assembly, the use of serviceable worn parts, and an operating clearance which was less than predicted in design analysis.

1.1 History of Flight

Flight 843, a Pan American World Airways B-707-321B, N761PA, was a regularly scheduled international passenger operation. The flight departed San Francisco International Airport, San Francisco, California, at approximately 1409 P.d.t. June 28, 1965, for Honolulu, Hawaii.

Witnesses stated that the takeoff and initial climb were normal. Shortly after the climb was initiated at an altitude of approximately 800 feet, ground witnesses observed a fire around the No. 4 engine. At this time the crew noted heavy vibration, and yawing of the aircraft. The crew initiated shut down procedures of the No. 4 engine until they received an intermittent fire warning signal. They then initiated engine firefighting procedures for the No. 4 engine. During this