History of the Flight
The pilot of the Piper PA-28-180, registration G-AXSG, conducted a landing at St Mary's Airport, Isles of Scilly on 7 April 2023. After touching down and applying the brakes, the aircraft veered left onto grass, where the left main landing gear leg collapsed. The remaining gear structure dug into the ground, turning the aircraft 180 degrees before it came to a stop. The pilot and two passengers were uninjured.
Aircraft Examination
Examination revealed that the left main landing gear wheel, torque link, and oleo strut assembly had detached from the landing gear cylinder but remained attached by the brake hose. Both upper torque link attachment lugs had fractured from the main landing gear cylinder. The base of the cylinder had broken into several pieces near the lug attachments. Additional damage included scrapes on the left flap and wing surface. The left wheel brake disc showed corrosion but no witness marks from pad contact.
Metallurgical Analysis
Fracture surfaces of the torque link attachment lugs were examined using stereomicroscopy and scanning electron microscopy. Both lugs exhibited areas of stable, progressive fatigue crack growth with fine striations, initiating at the inner lower radii. The inboard lug had a fatigue crack depth of 2 mm, the outboard 3 mm, before instantaneous fracture occurred. No pre-existing material or mechanical defects were found at the crack origins. The main cylinder fractures were overload failures that occurred after the lug failures.
Maintenance History and Service Bulletins
The aircraft had a valid Airworthiness Review Certificate issued on 18 July 2022 at 8,044 hours. The last annual inspection was concurrent. A previous brake binding issue in February 2023 was addressed with new piston seals. Piper Service Bulletin SB1131A, which calls for visual inspection with dye penetrant of the cast landing gear cylinder for fatigue cracks every 100 flight hours, had been last complied with on 24 October 2019 at 7,948 hours. At the time of the accident, the aircraft had flown 141 hours and 299 landings since that inspection. No cracks were recorded then. The EASA Airworthiness Directive that mandated SB1131A was cancelled in 2020 due to the perceived low number of cast cylinders and low risk. There is currently no mandate for SB1131A.
Conclusion
The left main landing gear torque link attachment lugs failed due to fatigue cracking, causing the wheel assembly and oleo leg to detach from the cylinder during landing. It is possible that the cracks were present but not detected during the last dye penetrant inspection, which was conducted with the leg still fitted to the aircraft.