Microstructure Modifications due to Abusive Grinding Resulted in Unexpected Catastrophic Failure of Main Landing Gear in Civilian Aircraft

The chain events which lead to this failure, started with the fact that prior to takeoff, flight and ground crew of a Boeing airplane heard a loud noise that sounded like a strong impact, accompanied by a certain lateral and vertical movement of the aircraft. After a normal takeoff, a "GEAR DISAGREE" message was displayed during main landing gear (MLG) retraction and the left MLG was "DOWN" but not locked by two braces. The crew decided to return and landed safely. The MLG was inspected, and the back side arm was found broken. This MLG failure occurred after 160 landings from the overhaul process. The MLG structural material was a high-strength low-alloy steel 4340 with hardness of 50–55 RC. The failure analysis has been performed under the supervision of chief investigator from the Ministry of transport, included visual inspection, micro-hardness, macro- and micro-etching, fluorescent particle inspection (FPI), fracture modes classification, Barkhausen mapping, hydrogen content and chemical analysis. Based on the findings, it was drawn that abusive grinding during the overhaul process was responsible for this early fracture. The results pointed out on the possible embrittlement oriented mechanism which causes for both the cracking initiation and propagations stages. Stress corrosion cracking and/or, hydrogen embrittlement and fatigue, were considered to be the failure mechanisms. The current paper highlights the failure mechanism based on the findings reported in metallurgical reports. The dominance of intergranular fracture near the fracture surface pointed out that some microstructure changes which have been occurring during the grinding process that in retrospect was found to be very abusive based on the macro-etching and on the FPI inspection.