What happened
On 11 July 2016, an Eurocopter AS350B2 G-VGMG was conducting an operational proficiency check near Bideford, Devon. The flight involved an examiner in the left seat and a trainee pilot in the right seat. During the session, the crew performed a practice hydraulic failure procedure, which involves removing hydraulic pressure to the cyclic and collective controls, thereby increasing the physical effort required to manipulate the aircraft.
As the aircraft approached a landing at Lake Farm, the trainee pilot realized they were landing downwind. In an attempt to correct the approach, the pilot rapidly pulled back on the cyclic, causing the aircraft to pitch up and the airspeed to drop below 2/0 kt. This maneuver induced a leftward yaw. The examiner immediately took control to stabilize the aircraft but reported that the cyclic, collective, and pedals experienced a brief freezing for approximately 1 to 1.5 seconds.
Following this period of-stiffness, the helicopter continued to yaw left and descended, with the skids making contact with the ground. The aircraft then lurched 90 degrees to the right and plunged into the ground in a nose-down attitude. The impact resulted in the failure of the tail boom and caused the main rotor to strike the vertical fin. There were no injuries to the two crew members on board.
The investigation
The AAIB examined the sequence of events and the mechanical state of the aircraft. The investigation noted that the tail boom had separated below the engine exhaust pipe, held only by two hydraulic pipes, and the skids had fractured at the front attachment. Because the damage to G-VGMG was beyond economic repair, no formal fault investigation of the mechanical components was conducted.
Investigators looked into the reported control freeze. While the manufacturer stated that simultaneous freezing of the independent cyclic/collective and pedal systems is extremely improbable, the examiner noted having experienced similar brief stiffness at higher altitudes previously. The investigation also considered whether conflicting inputs between the two pilots could have caused the sensation of frozen controls, noting a similar investigation by the French BEA involving a different AS350 model.