Mystery still surrounds a helicopter crash that left two people seriously injured and one pilot dead at Wycombe Air Park last year.

Three people were left fighting for their lives after a AS350 B3e Ecureuil, G-MATH helicopter “rolled” to its left before hitting the ground at the Booker airfield on the morning of May 5, 2017.

When emergency services arrived at the scene at around 9.30am, the passenger in the rear seat was standing, but he had suffered serious injuries to his face and had a fractured pelvis.

The instructor had suffered multiple cuts, broken ribs and collarbone and was semi-conscious, however, the training pilot was unconscious and not breathing.

He was extracted from the wreckage and revived, before being taken to hospital via air ambulance but he passed away weeks later.

An investigation into the cause of the crash, carried out by the Air Accidents Investigation Branch (AAIB), could find no technical issues with the aircraft, or a reason why the instructor, who was controlling the helicopter at the time, was unable to roll the helicopter back upright before hitting the ground.

The report, published this week, explains that the crash happened during “hydraulic failure training”, a technique which would allow a pilot to maintain control of the aircraft if it lost power during flight.

There were three people on board during the training – the instructor and a pilot in the passenger seat and a pilot under training.

Investigators found that on approach to landing on two different occasions, the instructor became “dissatisfied” with the way the helicopter was being controlled by the training pilot and took control of the aircraft in the “latter stages” because he felt the speed was too slow for a successful landing.

On the third landing attempt, the instructor found he was unable to control the helicopter and it rolled beyond 90 degrees before it “descended rapidly and struck the ground, coming to a rest on its left side”.

After the crash, the instructor said he had told the training pilot not to keep his hand on the cyclic – a control stick for the helicopter – but he continued to do so.

When interviewed, the instructor stated that he had been unable to move the cyclic control to the right to correct the roll to the left. He also told how the training pilot had allowed the speed of the aircraft to “drop too far and allowed the approach to get crooked”.

Describing the final moments of the flight, the instructor said: “Shortly after initiating a climb and then left turn, the aircraft rapidly and involuntarily banked to the left and I was unable to correct the attitude of the aircraft through any amount of physical force.

“The controls seemed to be completely jammed. As far as I recall the aircraft maintained this extreme angle of bank to the left until it impacted the ground as I was unable to influence any control upon the aircraft’s flight trajectory.”

The helicopter had been fitted with a video and audio system and crash investigators were able to access footage from the moments before the crash.

None of the occupants were wearing helmets at the time, but there was no requirement for them to wear one.

Investigators found that while the left front harness for the instructor had been cut to release him from the wreckage, there were “conflicting reports” about whether the training pilot had his harness fastened at the time of the crash.

AAIB investigators concluded that clearer instructions for “hydraulics-off” flight in the AS350 flight manual would help prevent similar crashes in future.

In response to the investigation, the helicopter manufacturer has amended the flight manual and added warnings not to conduct low speed manoeuvres with hydraulics off due to the danger of loss of control.

It has also prepared a safety video describing how to perform hydraulics-off training.