What happened
On 13 July 2017, a HK36 TC Super Dimona, registration G-FMKA, was conducting a biennial refresher training flight near Brimslade Farm, Wiltshire. The flight was intended to help the aircraft owner revalidate his class ratings. The aircraft departed from Draycot at approximately 1810 hrs, initially appearing to be in normal flight at around 1,000 ft agl.
Shortly after, witnesses observed the aircraft performing manoeuvres at a much lower altitude. Some observers noted the aircraft was in a left turn, while others described it as appearing nose-down and potentially spinning. The aircraft struck a crop field in a near-vertical attitude at high speed. The impact caused a fire that spread to the surrounding barley crop. Both the pilot and the instructor sustained 2 fatal injuries.
The investigation
The AAIB investigation examined the wreckage and the flight characteristics of the aircraft. Aerial surveys of the accident site revealed symmetrical arcs from the wing leading edges, indicating the wings experienced significant upward bending during impact. Analysis by the manufacturer suggested the aircraft was subjected to a positive acceleration approaching 9 g, which would require a significant elevator control input.
Investigators found no evidence of pre-impact structural failure in the flying control surfaces. The examination of the propeller hub indicated the engine was likely in a powered flight setting at the time of impact. While the intense post-crash fire destroyed much of the fuselage, the investigation could not rule out a pre-existing defect in the control mechanisms located within the midships section of the airframe.
Findings
- The aircraft departed from controlled flight at a low altitude.
- The sudden loss of control was possibly caused by a power-on stall.
- Ground impact marks suggest an attempt was made to recover the aircraft using elevator input, but there was insufficient altitude to prevent the crash.
- The instructor did not hold the required TMG class rating to provide instruction on this specific aircraft type.
- Weather conditions were clear and dry, making meteorological factors an unlikely cause.