THE exact cause of a helicopter crash in North Weald remains a mystery despite an extensive investigation.

David Richards, 53, chief executive of Prodrive which runs the Subaru World Rally Team, and his wife, Karen, 53, escaped uninjured when the Eurocopter EC135 came down in a field off Church Lane last September.

The crash happened less than 24 hours after the couple’s friend, rally driver Colin McRae, died in a helicopter crash in Scotland.

The couple were returning from the Belgian Grand Prix where Mr Richards had led the tributes to McRae.

Their miraculous escape happened when Mr Richards believed there had been a double engine failure.

The tail of the five-year-old helicopter hit the ground first and then rolled onto its side leaving the machine extensively damaged.

But experts have been unable to come to a firm conclusion over what exactly happened.

An air accident investigation report states that Mr Richards had heard a dull thud from above and behind him, similar to a bird strike. He also felt the helicopter change altitude.

Mrs Richards, who was sitting in the front left passenger seat, had been reading a magazine when the helicopter pitched nose-down. She later said the helicopter felt ‘wobbly’. The report adds: “The manufacturer [of the equipment] has unsuccessfully attempted to reproduce the thud during test flights and has stated that they are unaware of any other cases where this has occurred.

“The dull thud could have been caused by an external influence such as a bird strike, turbulence, wake vortex or the helicopter manoeuvring, but none of these factors applied to either of the reported occurrences.

“It is therefore most likely that the thud was a consequence of a slight change in the pitch of the rotor blades as a result of a disturbance in either the hydraulic system of the flying controls.

“Unfortunately the extensive damage to the helicopter meant that it was not possible to test the hydraulic system, or to test the AFS (auto flight system) dynamically.”

In conclusion, the report states: “From the available evidence it would appear that the accident sequence started with the disengagement of the autotrim function when the pilot was flying ‘hands off’ at a fast cruise speed.

The pilot misread his triple tachometer gauge and, aware of the thud from the engine area, believed that he had suffered a total engine failure and therefore entered an autorotation.

“He successfully positioned the helicopter for a power-off landing in a suitable field but misjudged the landing flare and the tail pylon broke off when it struck the ground first.

“As the helicopter travelled over the field, a skid dug into the soft earth causing the helicopter to roll onto its side.

“The investigation could not identify the reason why the autotrim disengaged or the cause of the dull thud which the pilot heard at the start of the accident sequence.”