BOEING MD-11F near Miami, FL — 2008-05-21

Probable cause

The insufficient predicted fatigue life of the P/N KBE6-59 adjustable-length 3/8-inch rod end, and the inadequate fatigue margin at the transcowl CDU clevis assembly bracket attachment rivets for the one actuator out condition, which resulted in the failure of a second transcowl attachment during thrust reverser activation, leading to the partial liberation of the thrust reverser’s inboard transcowl.

NTSB narrative

The airplane’s No. 3 engine thrust reverser lost a large section of its inboard translating cowling (transcowl) after thrust reverser deployment at landing. A postincident inspection found that two of the three transcowl attachments (the lower and the center) had failed. The lower ballscrew actuator attachment had failed in fatigue at its rod end. The center (CDU) attachment had failed in overload at the clevis assembly bracket attachment rivets. A fatigue analysis of the various rod ends qualified for use on CF6-80C2 series thrust reverser adjustable-length ballscrew actuators found that four of the 3/8-inch rod end part numbers (P/Ns), including P/N KBE6-59, which was on this thrust reverser, lacked sufficient predicted fatigue life. A design validation of the CF6-80C2 thrust reverser actuating system (TRAS) found that TRAS end-of-stroke impact loads can induce torque loading at the rod ends and clevis assembly bracket attachment rivets that result in bending loads not accounted for in the original design. The design validation also found that the fatigue tolerance capability of the CDU clevis bracket attachment fasteners was marginal for the “one actuator out” (OAO) condition under normal deploy loads.

NTSB coding

• Powerplant sys/comp malf/fail · Landing-landing roll

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