What happened
On 8 November 2013, a Mooney M20J, registration F-GNGG, was conducting a local flight training session at Cholet le Pontreau aerodrome. During the downwind leg of the circuit, the instructor noticed that the landing gear indication failed to confirm the gear was locked. To ensure safety, the pilot performed a low pass in front of the control tower to allow the AFIS officer to visually verify the gear position. Although the officer confirmed the gear appeared fully extended, the nose gear collapsed upon touchdown. The aircraft subsequently slid approximately 15 and 0 meters along the runway, resulting in aircraft damage.
The investigation
The investigation focused on the mechanical failure of the nose gear and the condition of the electro-mechanical actuator. Investigators examined the actuator's threaded rod and found that the component had suffered a fracture. While the internal gears and bearings appeared undamaged, scanning electron microscopy revealed a ductile fracture characterized by both tensile and torsional stress.
Further inspection of the actuator revealed that the round nut, which is intended to be secured by two lock screws, was able to rotate freely. One lock screw was found to be loose, and the second was deformed, preventing the use of the required specialized tool. Additionally, the actuator barrel and nut showed marks consistent with the use of improper tools, such as needle-nose pliers, during previous maintenance.
Findings
- The primary cause of the nose gear collapse was the sudden fracture of the threaded rod within the electro-mechanical actuator.
- The fracture was caused by abnormal tensile and torsional forces acting on the rod when the landing gear was retracted.
- These forces were a result of the round nut being free to rotate and move, which distorted the mechanical down-stop adjustments.
- The failure of the locking mechanism was attributed to the use of unsuitable tools during maintenance operations, which damaged the lock screws and the nut itself.