What happened
On February 16, 2013, an MCR 4S aircraft, registration F-PDPE, departed from Dieppe Saint-Aubin aerodrome with a pilot and two passengers. During the initial climb, the pilot attempted to retract the flaps, at which point a loud, violent noise was heard. This was immediately followed by a significant left roll of approximately 60 degrees. Subsequent attempts to operate the flap controls yielded no response. To stabilize the aircraft, the pilot reduced engine power, which managed to maintain a level flight path despite a persistent left bank of about 50 degrees. The pilot performed an emergency landing on a runway opposite the landing direction, resulting in slight damage to the aircraft.
Upon inspection on the ground, it was discovered that the left flap remained in the extended position while the right flap had retracted, confirming a failure in the symmetric deployment mechanism.
The investigation
The investigation focused on the electromechanical flap control system, which utilizes a screw-and-nut mechanism (a bronze nut moving along a threaded rod) to synchronize flap movement. Investigators found that while the materials used met manufacturer specifications, the internal threads of the bronze nut on the left flap were heavily worn, allowing the nut to slide freely along the threaded rod. This prevented the mechanism from properly guiding or maintaining the flap's position. The right side also showed signs of excessive wear, though the connection remained intact.
Further analysis revealed that the manufacturer had used an ISO-profile threaded rod, which is better suited for standard fastening than for the high-stress guiding functions required by this mechanism. Additionally, the investigation looked into maintenance records, noting that while 50-hour inspections were required to check for axial play, the aircraft's 100-hour annual inspection was overdue by two and a half months. It was also noted that recent 50-hour checks were performed by club volunteers who did not specifically check the axial play of the screw-and-nut assembly.
Findings
- The primary cause of the incident was the failure of the left flap control mechanism due to excessive wear.
- Inadequate maintenance practices meant that the increasing axial play in the bronze nut was not detected.
- The use of an ISO-profile thread for a guiding function contributed to premature wear of the assembly.
- The aircraft had completed 6/96 flight hours and 1,250 landings without the replacement of the screw-and-nut components, a high-use cycle for this specific mechanism.