What happened
On 18 November 2008, a Glasair IIS RG, registration G-KSIR, was performing a training flight when it returned to Upfield Airfield, Whitsund, following a navigation exercise. The pilot executed a standard approach to Runway 05. Upon touchdown, the nose landing gear collapsed toward the rear of the aircraft. This movement caused the nose of the plane to descend until the propeller made contact with the runway surface. The engine sustained a shock load during the impact. The aircraft slid along the concrete runway before coming to a halt, and both occupants were able to exit the aircraft without injury.
The investigation
Investigators examined the nosewheel assembly and discovered that the nosewheel had partially separated from the fork. The investigation established that the nut securing the axle bolt to the wheel hub had become detached. This loss of the nut allowed the wheel to move rearwards and become jammed against a guide strap, which effectively created a locked-wheel condition upon touchdown. The resulting friction and force generated by this lock-up caused the nose leg to collapse.
Further examination of the axle bolt revealed thread debris, which suggested that the internal threads of the nut may have failed due to an overload in tension. The investigation also reviewed manufacturer guidance, noting that a 2007 Service Bulletin had previously addressed similar incidents of nose wheel axle detachment in Glasair aircraft. While the bulletin suggested replacing the existing shear nut with a high-tensile tension nut, the investigation noted that width constraints in the nose gear well might make such a change difficult for certain aircraft models.
Findings
- The nose landing gear collapsed because the axle nut had detached, allowing the wheel to migrate and jam.
- The loss of the nut caused the wheel to act as a locked wheel during touchdown, creating the forces necessary to collapse the gear leg.
- Evidence of thread debris on the spindle bolt indicated a possible overload failure of the nut threads in tension.