What happened
On 24 February 2018, a Dyn’Aero MCR01 ULC microlight, identified as 21ABU, was performing a local flight at Saint-Nazaire Montoir aerodrome in France. The pilot, who had approximately 10 hours of experience on this specific aircraft type, was conducting circuit patterns. After completing the first circuit without incident, the pilot began the final approach for a full-stop landing on runway 07.
During the final approach, the pilot issued a distress call, declaring a "MAYDAY" and stating the aircraft was in danger. An eyewitness observed the aircraft entering a very steep bank angle at an altitude of approximately 50 meters. The aircraft subsequently struck the ground and was destroyed, resulting in one fatality.
The investigation
The BEA examined the wreckage and the flap control mechanism to determine the cause of the loss of control. Investigators found that the aircraft struck the ground in an inverted position with a shallow nose-down attitude. Crucially, the examination of the flap control mechanism revealed that the flaps were in an asymmetrical position: the left flap was fully retracted, while the right flap was partially extended.
Technical analysis of the electromechanical flap control system focused on the universal joints that connect the worm screws to the pulley system. Investigators discovered that both worm screws were bent. On the left side, the universal joint pin had failed due to fatigue. This failure was attributed to abnormal friction and jamming within the joint, likely caused by a lack of lubrication or contamination. On the right side, cracks were identified in the heat-affected zone of the weld on the pin.
Regarding maintenance, while the aircraft had undergone recent 100-hour inspections, there was a lack of documentation for the period between 200-7 and 2013. The investigators noted that the lack of lubrication could have been a result of improper storage or maintenance shortcomings.
Findings
- The primary cause of the accident was asymmetric flap extension during the final approach, which rendered the aircraft nearly impossible to control at a low altitude.
- The asymmetry was caused by the fatigue failure of a component within the left-side flap control universal joint.
- The fatigue was driven by alternating torsional stresses resulting from internal jamming of the joint's pin.
- This jamming was likely exacerbated by a lack of lubrication or the presence of wear debris within the mechanism.
Safety action
Following the investigation, SE Aviation issued a service bulletin and updated the maintenance manual, requesting that the universal joints on the flap control systems for all relevant MCR aircraft be inspected every 50 hours. Additionally, SE Aviation has proposed replacing the current flap control system with a more robust design.