What happened
The landing of the MD-10 was performed within established parameters, with the approach and landing being stabilized and the weather and runway conditions remaining favorable. Data from the event confirmed that the landing gear was subjected to loads during touchdown that were within the certified limits for an intact gear, and no pre-existing flaws were detected in the gear structure prior to the impact. The crew applied braking normally, and the modified carbon brake system did not contribute to the fracture. Following the incident, the emergency response from the flight crew and airport rescue and firefighting services was executed correctly and promptly.
Findings
Investigation into the left main landing gear (LMLG) outer cylinder revealed that the component had been in service for approximately 8.5 years since its previous overhaul. The primary cause of the failure was identified as improper nickel plating within the air filler valve bore hole. While nickel plating is a standard procedure for maintaining tolerances in the inner diameter of the cylinder, the application of this plating inside the valve hole was unauthorized.
Research indicated that a plating thickness of 0.008 inches increased the stress factor by 35%. The investigation determined that a previous load event had caused compressive yielding near the valve hole, creating residual tension stress. While normal operational stresses were typically within design limits, the combination of the nickel plating and the residual stress created a high enough stress intensity to initiate and propagate fatigue cracks on both sides of the valve hole.
Using a Finite Element Model (FEM) validated by data from an instrumented FedEx MD-10, investigators found that stresses in the valve hole area were significantly higher than originally anticipated during the design phase. During the landing in question, the spring back loads on the LMLG reached a level that exceeded the residual strength of the material weakened by these fatigue cracks, resulting in the gear fracture. As a result of these findings, modifications were made to the MD-10 carbon brake system to improve reliability and braking effectiveness.