Synopsis
On 8 October 2016, during taxi across a grass parking area at Hunsdon Airfield, Hertfordshire, the left main landing gear leg of a Flight Design CTSW (registration G-CGVG) collapsed as a result of a fatigue crack that had propagated from the main attachment bolt hole. The aircraft settled onto its left wingtip, causing damage to the wingtip, the landing gear leg, and surrounding structure. Both occupants were uninjured.
History of the Flight
The pilot had completed a normal landing on grass Runway 03. While taxiing across a grass parking area, the left main landing gear leg failed. The aircraft rotated downwards on its left side, damaging the left wingtip and left fuselage near the landing gear leg. The pilot shut down the engine normally, and he and his passenger exited without further incident.
Aircraft Information
The CTSW is a two-seat microlight with a fixed tricycle landing gear. The main landing gear legs are tapered beams made of 7075-T6 aluminium alloy. The leg is supported at its upper end by a bolted joint in a welded tubular steel socket and restrained by an M8 steel main attachment bolt that passes through a hole in the leg and a glassfibre rear reinforcement plate.
Aircraft Examination
Examination revealed that the landing gear leg had failed due to a fatigue crack propagating from the aft lower edge of the main attachment bolt hole. The crack extended through approximately 60% of the lower section thickness before the leg failed in ductile overload. The left fuselage was locally damaged from excessive upward rotation of the leg during collapse, and the rear reinforcement plate was delaminated. The left wingtip was damaged by ground contact.
Previous Events and Service Bulletins
Following a similar failure on a CT2K (a similar model with identical landing gear) in July 2006, the UK agent issued Service Bulletin CT123 in February 2007. That bulletin required removal of both main landing gear legs for dye penetrant crack inspection at the top and at the main attachment bolt hole positions, applicable to certain aircraft at 300 flying hours. G-CGVG, built in 2011, was not subject to SB CT123 because the aircraft was manufactured after the bulletin's applicability date, and no heavy landings had been reported.
Scheduled maintenance in the CTSW Operator's Manual requires dye penetrant crack testing of the main landing gear legs while installed every 300 flying hours. It could not be determined whether this inspection was performed on G-CGVG, as the relevant aircraft logbook had been lost in a burglary. The in-situ inspection does not require removal of the legs, so it would not detect a crack on the forward edge of the bolt hole, which is not accessible without removal.
Safety Actions
In response to this accident, the UK agent intends to issue Service Bulletin CT145. This new bulletin will require removal and dye penetrant crack inspection of the main landing gear legs on all CT2K and CTSW aircraft at intervals of 300 flying hours, superseding SB CT123. Additionally, it will introduce modification M309, which peens the edges of the main attachment hole to create residual compressive stress intended to suppress fatigue crack initiation. The requirement for inspection following heavy landings remains.