What happened
While operating in the cruise phase of flight, the crewmembers heard a loud bang originating from the rear of the cabin. The Boeing 757 experienced a momentary shudder and a bounce during the event. Following an uneventful landing at the destination airport, the crew performed an inspection and discovered that the number two right engine cowling had separated from the aircraft and struck the right inboard elevator.
The investigation
An inspection of the aircraft revealed that the upper forward hinge of the engine cowling required three bolts to be secure. At the time of the investigation, only one bolt remained attached to the hinge. There was no evidence found to suggest that the remaining two bolts had torn away during the flight itself.
Investigators observed fretting within the area of the bolt holes. This condition was consistent with an oscillation occurring over a period of time caused by the absence of two bolts. The investigation also noted that while the airliner maintenance manual required a general inspection of this area, it did not specifically mandate the inspection of cowling hinge bolts. Furthermore, the hinges were located in an area that was not easily accessible to maintenance personnel.