Aircraft and Flight Details
The aircraft involved was a Piper PA-28-161 Cherokee Warrior, registration G-BTBC, manufactured in 1979 and powered by a single Lycoming O-320-D3G piston engine. The flight was a training sortie for flight instructor training, with two crew on board and no passengers. The commander held an Airline Transport Pilot's Licence, had 9,672 total flying hours (645 on type), and the flight occurred on 28 October 2003 at 1408 UTC.
Incident Sequence
The aircraft was conducting a normal approach and landing on Runway 18 at Wellesbourne Mountford, Warwickshire. The runway had an asphalt surface with 912 metres of landing distance available. Weather conditions were good with a surface wind of 240°/7 kt. During the landing roll, the pilots noticed shaking through the airframe, the right wing began to lower towards the runway, and the aircraft veered right. It came to rest on the right side of the runway, after which the crew performed shutdown checks and evacuated. The right main wheel and leg had broken away from the underside of the wing but remained attached by the hydraulic brake pipe. Damage was minor to the right main landing gear and flap.
Examination of Components
Metallurgical examination revealed that the lower torque link bolt had failed. The bolt shank had fractured across its diameter due to a low cycle, reverse bending fatigue mechanism, with approximately 80% of the fracture surface showing fatigue and the remainder overload. Multiple fatigue initiation sites were around the circumference. The reverse bending likely occurred about a vertical axis due to oscillatory loads from wheel spin-up on touchdown and braking. Additionally, the bolt shank exhibited pitting corrosion, and a band of wear was present where the shank contacted the outboard lug. The shank diameter at the wear band was 0.280 inches compared to 0.290 inches elsewhere. A large washer under the stiff-nut showed plastic deformation, indicating over-tightening at some stage, which would have reduced bending fatigue resistance. The bolt assembly included a steel sleeve and washers/shims that were not recovered after the incident.
Maintenance History and Airworthiness Context
The bolt was not subject to a finite life, but the maintenance schedule required inspection every 100 flying hours for condition and security, and lubrication every 50 hours. The torque links had grease nipples. The installation of the bolt, especially with wheel fairings, made inspection difficult. The aircraft manufacturer had issued Service Letter No 842 addressing failures of the centre torque link bolt, and the UK CAA issued Additional Airworthiness Directive (AAD) No 016-02-80, later revised, which mandated inspections. However, that directive only applied to the centre bolt, not the lower bolt that failed in this incident. The lower bolt had not been subject to such inspection requirements.
Summary of Findings
The failed bolt's poor condition, including corrosion pitting and wear, indicated inadequate lubrication over its service life. The over-tightening evidenced by the deformed washer likely increased susceptibility to fatigue. The design of the installation limited visual access for inspection. The incident highlighted that the lower torque link bolt, though similar in location to the centre bolt, had not been covered by the existing airworthiness directive.