Left Main Landing Gear Fatigue Failure on an MD-10 (N391FE)

Accident Overview and Landing Conditions The investigation determined that the approach and landing were stabilized and within specified limits. Recorded data showed that the loads experienced by the landing gear at touchdown were within the certification limits for an intact landing gear without pre-existing cracks or flaws. Weather and runway conditions did not affect the landing. The application of braking by the accident crew, and the overall effect of the carbon brake modification, did not initiate or contribute to the landing gear fracture. ## Maintenance and Material Findings The left main landing gear (LMLG) outer cylinder on the accident airplane had been operated about 8 1/2 years since its last overhaul. During that overhaul, stray nickel plating likely was introduced in the air filler valve hole. Nickel plating is a permissible procedure for maintaining the tolerances of the inner diameter of the outer gear cylinder, but it is not allowed in the air filler valve bore hole. Literature and test research revealed that a nickel plating thickness of 0.008 inches results in a stress factor increase of 35%. ## Fatigue Crack Initiation and Growth At some point in the life of the LMLG, a load event compressively yielded the material in the vicinity of the air filler valve hole, causing a residual tension stress. During normal operations, the stress levels in the air filler valve hole were likely within the design envelope. However, the addition of the residual stress and the stress intensity factor due to the nickel plating increased these to a level high enough to initiate and grow a fatigue crack on each side of the air filler valve hole. ## Engineering Analysis The stresses at the air filler valve hole were examined via development of a Finite Element Model (FEM), which was validated with data gathered from an instrumented in-service FedEx MD-10 airplane. The in-service data and FEM showed that, for all conditions, the stress in the air filler valve hole was much higher than anticipated in the design of the outer cylinder. Fatigue analysis of the in-service findings, using the nickel plating factor, resulted in a significantly reduced fatigue life of the gear cylinder compared with the certification limits. ## Accident Sequence During the accident landing, the spring back loads on the LMLG were sufficient to produce a stress level in the air filler valve hole that exceeded the residual strength of the material with the fatigue cracks present. ## Post-Accident Actions Post-accident modifications to the MD-10 carbon brake system were implemented due to investigative findings for the purposes of braking effectiveness and reliability. Post-accident emergency response by the flight crew and ARFF was timely and correct.