What happened
On 9 August 2018, a Rotorsport UK Cavalon, registration G-CKYV, was being operated for a private delivery flight at Wolverhampton Halfpenny Green Airport. During the pre-departure phase, the pilot engaged the pre-rotator system to accelerate the rotor. While the rotor initially reached 200 rpm, the pilot then observed the rotor RPM indication drop to zero.
In an attempt to increase the rotor speed using airflow, the pilot commenced a takeoff roll using reduced power. As the aircraft reached approximately 30 kt, the cyclic control was violently wrenched from the pilot's hands. The aircraft subsequently pitched up, yawed, and rolled to the left before falling upright and sliding along the runway. The pilot sustained minor injuries and was assisted from the wreckage by the Fire Service. The aircraft was damaged beyond economic repair.
The investigation
The investigation examined the mechanics of the pre-rotator system and the aerodynamic phenomena occurring at low rotor speeds. It was established that the pilot's decision to proceed with the takeoff roll was an attempt to use the aircraft's forward motion to generate sufficient airflow to raise the rotor RPM.
Investigators analyzed the phenomenon of 'blade sailing,' which can occur when rotor RPM is low. In this state, the advancing blade experiences increased lift and flaps upward, while the retreating blade experiences a sudden loss of lift and flaps downward. This creates significant instability in the rotor disc. The investigation determined that as the airspeed increased, the aircraft entered this unstable state.
Findings
- The pilot's primary cause of the accident was the decision to attempt a takeoff roll despite the rotor RPM indicator showing zero.
- The aircraft likely encountered a blade sailing event as airspeed increased, causing the rotor disc to move violently and displace the flight controls.
- The pilot's focus on successfully initiating the takeoff distracted him from the necessity of aborting the flight when the initial RPM drop was first noted.