History
On 8 November 2013, a Mooney M20J (registration F-GNGG) operated by Aéroclub du Pontreau was conducting a local training flight with an instructor and a student pilot. While on the downwind leg, the instructor reported to the AFIS officer that he had no indication the landing gear was locked. He performed a low pass in front of the control tower, and the officer visually confirmed the landing gear appeared fully extended. The instructor then proceeded to land. During touchdown, the nose gear collapsed, and the aircraft slid on the runway for approximately 150 m before coming to a stop.
Damage
The aircraft sustained damage to the propeller: both blade tips were bent from contact with the runway during the collapse. The left and right nose gear doors showed marks from rubbing on the runway. The threaded part of the nose gear actuator rod was found fractured inside the gearbox. The position of the limit switch indicated that the nose gear had not locked during the accident flight.
Examination
Examination of the electromechanical actuator revealed marks on the barrel and round nut that appeared to be tool marks, possibly from nose pliers. One of the two lock screws on the round nut was loose and no longer performing its locking function; the other was marked and deformed, preventing use of the specific loosening tool. The round nut could be rotated freely. Circular marks were observed on the mechanical down stop shims at the gearbox input. Scanning electron microscopy showed dimples over the entire fracture face, characteristic of a ductile fracture. Ridge lines and small dimples on the outer edges indicated torsional stress, while the core showed non-oriented dimples consistent with tensile stress. No significant damage was found in other actuator components.
Maintenance Issues
According to the investigation, maintenance disassembly operations to uncouple the round nut and rod barrel tube may have damaged the lock screws. With the nut no longer held in place, it could rotate freely, distorting the adjustment of the mechanical down stops. When the landing gear was retracted, abnormal tensile and torsional forces were transferred to the actuator rod, causing it to fracture due to overload. On 12 December 2013, the manufacturer issued Service Instruction M20-122, backed by FAA Bulletin CE-14 12, reminding owners of the importance of full landing gear system inspections and checks of landing gear settings as described in the maintenance manual.
Conclusions
The nose gear collapsed during landing due to the sudden fracture of the threaded rod of the landing gear’s electromechanical actuator. Because the rod was fractured, the nose gear could not be extended and locked by any means available to the pilot (control on the instrument panel or emergency control). The two main landing gear, unlike the nose gear, were able to lock down under gravity. A contributing factor was the use of unsuitable tools during landing gear system maintenance operations.