What happened
On 12 November 2020, a solo student pilot was conducting a general handling flight in a Rotorsport UK Cavalon, registration G-CKYT, over the Black Isle. After completing a successful morning dual flight with an instructor, the pilot commenced a solo training sortie from Inverness Airport.
During the flight, the aircraft performed several maneuvers, including an orbit north of Avo and a subsequent turn towards the Moray Firth coastline. Witnesses in the area reported hearing unusual engine noises, described as spluttering or a loud mechanical cracking sound. Shortly thereafter, observers noted the aircraft in a steep, erratic descent. Witnesses reported seeing the rotor blades detach from the aircraft in mid-air. The fuselage struck the ground and caught fire, resulting in one fatality.
The investigation
The AAIB investigation utilized radar data and a recovered Garmin ADS-B receiver to reconstruct the flight path. Analysis of the GNSS data revealed that the aircraft's altitude and airspeed fluctuated significantly during a specific sequence of turns.
Investigators performed a forensic analysis of the failed gimbal block and mast. They found that the separation was caused by a structural overload failure. Testing and analysis of a new production mast and gimbal block closely replicated the damage seen on the accident aircraft, demonstrating that the roll stop bar had made contact with the side of the channel on the gimbal block.
Findings
- The rotor head separated from the fuselage due to a structural overload failure caused by continuous exposure to dynamic flight loads.
- The pilot likely inadvertently entered a low-g flight regime, potentially exceeding the limits defined in the Pilot’s Operating Handbook, while attempting to fly a level left turn followed by a reversal to the right.
- This specific sequence of maneuvers caused the vertical acceleration to drop below 1g, unloading the rotor and allowing the engine torque to cause an unopposed rotation of the fuselage.
- The aircraft was correctly maintained, and no maintenance-related issues were identified as a cause of the accident.
Safety action
- The CAA is recommended to introduce mitigations to reduce the risk of catastrophic failure from contact between the gimbal block and the roll stop bar on similar gyroplane designs.
- It is recommended that the CAA reassess certification and compliance requirements for gyroplanes used in commercial operations to ensure manufacturers demonstrate adequate mitigation of dynamic load risks.
- The CAA should publish guidance regarding rotor load factor awareness for gyroplane pilot training and testing.