What happened
On 28 October 2003, a Piper PA-28-161 Cherokee Warrior, registration G-BTBC, was conducting a flight instructor training mission when it experienced a landing gear failure at Wellesbourne Mountford, Warwickshire. The flight, involving two crew members, proceeded normally through the approach and touchdown on Runway 18. However, during the landing roll, the occupants felt vibrations through the airframe, noted the right wing dipping toward the runway, and observed the aircraft veering to the right. The aircraft eventually came to a stop at the edge of the runway, resulting in minor damage to the right main landing gear and flap, with no injuries to the crew.
The investigation
Investigators examined the aircraft's components and discovered that the lower torque link bolt had failed. This failure caused the wheel and axle assembly to pivot at an angle nearly perpendicular to the direction of travel, creating intense drag forces that led to a bending overload of the landing gear leg.
Metallurgical analysis of the bolt revealed that the shank had fractured due to a low cycle, reverse bending fatigue mechanism. The examination showed that approximately 80% of the fracture was caused by fatigue, with multiple initiation points around the circumference. Additionally, the bolt exhibited significant pitting corrosion and wear, suggesting that the component had likely suffered from inadequate lubrication over time. The investigation also noted that a large washer under the stiff-nut had undergone plastic deformation, indicating the assembly had been over-tightened at some point, which likely reduced its resistance to fatigue.
Findings
- The primary cause of the failure was the fracture of the lower torque link bolt due to reverse bending fatigue.
- Extensive pitting corrosion and wear on the bolt shank indicated a lack of sufficient lubrication.
- Evidence of over-tightening during assembly increased the bolt's susceptibility to fatigue.
- The physical design of the installation makes visual inspection of the bolt surface difficult, especially when wheel fairings are present.
- While previous UK airworthiness directives had addressed the centre torque link bolt, no such mandatory inspection regime was in place for this specific lower bolt at the time of the accident.