What happened
During the landing roll of an MD-11, the center landing gear (CLG) experienced a failure in a rearward direction. The flight data recorder (DFDR) captured a momentary peak in both vertical and longitudinal accelerations. This peak occurred approximately 5.75 seconds after the main landing gear touched down and 2.5 seconds after the nose wheel touchdown. During this event, the vertical acceleration momentarily decreased while the longitudinal acceleration momentarily increased.
The investigation
Mechanical examination revealed that the lugs on the upper attach point of the lower drag brace on the CLG had failed. This failure caused the landing gear retraction cylinder to be pulled apart, leading to a loss of hydraulic fluid within the #3 hydraulic system. A metallurgical examination of the failed lug provided evidence of overload.
Investigation into the aircraft's history noted five prior documented events that were identical to this occurrence. Following the first three such incidents, McDonnell-Douglas performed two additional duplications using instrumented aircraft. These efforts led to a service bulletin for modifications to the CLG brakes and anti-skid system; the subject aircraft had already received these modifications. While the operator conducted additional testing to identify high loads on the CLG during landing, no high loads were detected, and the specific cause of the oscillation remained undetermined.