What happened
On 17 November 2023, a private flight involving a Flylight Nine, registration G-CMRF, experienced a mechanical failure approximately five minutes after departing from Darley Moor Airfield. During the climb, the bolt and nut securing the rear base bar control wires to the wing keel tube became detached.
Upon realizing the wires had disconnected, the pilot applied back pressure to the base bar to maintain tension on the front wires, attempting to preserve aerodynamic balance. To avoid further instability, the pilot kept the engine running and performed a close-range return to the airfield. The aircraft touched down on a soft, boggy runway. As the aircraft lost lift and became nose-heavy, the control wires went slack, allowing the wing to pivot forward. This movement caused the right wing to strike the propeller, resulting in significant damage to the right wing and propeller. The pilot exited the aircraft without any injuries.
The investigation
The AAIB examined the aircraft and reviewed onboard camera footage, which captured the exact moment the rear wires slackened. Investigators found that the bolt and nyloc nut used to secure the wires to the keel tube were missing from the wreckage.
Analysis of photographs taken after the aircraft's construction revealed that the bolt was insufficiently secured; only a single thread was visible protruding through the nyloc nut. It was determined that the nut likely vibrated loose, allowing the bolt to fall out due to gravity and vibration. Because the wires were functional during taxi and takeoff, it was concluded that the hardware was present during the initial pre-flight inspection but failed during flight.
Findings
- The primary cause of the incident was the failure of the nyloc nut and bolt securing the rear control wires to the keel tube.
- The insufficient thread engagement (only one thread protruding) likely allowed the nut to vibrate loose.
- The loss of the rear attachment caused the base bar to pivot, leading to the wing folding and striking the propeller upon landing.