What happened
On 10 April 2013, a private flight involving a Rotorsport UK Calidus, registration G-CGMD, was attempting takeoff from Wickenby Aerodrome in Lincolnshire. Following the standard pre-rotation procedure, the pilot aligned the aircraft on the runway and applied full throttle. As the aircraft reached approximately 30 mph, the pilot applied aft pressure to the control stick to lift the nosewheel.
During this maneuver, a hammering vibration was felt, and the aircraft veered toward the left. The pilot initially believed this was caused by a rough patch on the runway surface. However, the aircraft subsequently left the paved surface and veered sharply to the left, causing the main rotor blades to strike the ground. The impact threw the aircraft onto its right side. The pilot successfully exited the aircraft through the canopy without assistance, though he sustained one minor injury to his right leg caused by the impact of a yaw pedal.
The investigation
The investigation examined the aircraft's flight manual procedures and the pilot's control inputs. The aircraft is equipped with a pre-rotator system designed to reduce takeoff distance. The manual specifies that once the nose rises, the pilot should allow the nosewheel to float and then balance the aircraft by reducing back pressure.
Investigators analyzed the possibility that the rotor RPM had decreased during the takeoff roll. They noted that the pilot's decision to apply aft pressure specifically to lift the nosewheel, rather than waiting for the nose to lift naturally and then adjusting the stick, may have disrupted the airflow through the rotor disc. This disruption could have led to a loss of lift on the advancing blade and potentially triggered a retreating blade stall.
Findings
- The pilot applied aft control pressure prematurely to lift the nosewheel, rather than following the manual's guidance to allow the nosewheel to float.
- A reduction in rotor RPM likely occurred because the pilot did not maintain the stick in the correct position during the takeoff roll.
- The sudden change in the rotor disc angle at low RPM may have caused a retreating blade stall and a subsequent loss of lift on the left side.
- The impact of the rotor head reaching its travel limit likely caused the observed hammering vibration.