A REPORT investigating problems with a Wiltshire Air Ambulance helicopter that caused it to spin out of control has been published.

The Bell 426 was undergoing a power assurance check in June 2018 when it span out of control at the charity’s air base in 2018, spinning around two-and-a-half times in 10 seconds.

It was temporarily grounded. The same pilot experienced another incident, again while preparing for a power assuance check, in January 2019.

This check is part of the daily safety routine and verifies that the engines are working properly.

The 49-year-old pilot had almost 4,000 flying hours under his belt.

On June 15 2018, the pilot heard a loud clunk and crunch from the right side of the helicopter just behind the seat while carrying out this check then felt a lurch downwards to the right and thought that the landing gear was collapsing on the right side.

The report by the Air Accident Investigations Branch notes that the helicopter yawed rapidly to the left, then the pilot felt very disorientated and realised that action was needed to try and contain the situation.

The pilot lowered the collective lever and then tried to move the engine switches to cut them off.  After some brief difficulties, both engines were shut down. The pilot applied the rotor brake and, after another 180-degree rotation, the helicopter came to rest. 

The report added: "The pilot could have stopped the rotation using the pedals but, for several reasons, did not do so. The loud “clunk” and sudden unexpected motion would likely have been startling. The angle of the pilot’s head while looking down may have led to disorientation caused by the Coriolis illusion. A fast, un-demanded yaw on the ground had never been trained or discussed and the pilot concluded that the right landing gear must have collapsed. Despite being confused and disorientated, the pilot lowered the collective lever quickly and then shut down the engines and applied the rotor brake to stop the rotation."

On January 2 2019, the pilot was preparing to conduct another Power Assurance Check in the same helicopter while keeping their feet on the pedals this time.

The pilot noticed unusual feedback forces in the pedals which were trying to move the helicopter sharply to the left but the pilot managed to stop this by pressing down on the pedals and pressing a button to clear the pressure.

The report found that the pilot had accidentally caused a "yaw trim runaway" which made the helicopter lean to one side while taking off but had managed to stop it with his quick-thinking response.

As a result of this investigation, the heli's manufacturers Bell Textron Canada Limited published a revision to the Rotorcraft Flight Manual to reduce the risk of a yaw trim runaway which included procedures for responding to a runaway so that control of the helicopter would be maintained.

The manufacturer also recorded the problems with their flight control system software  that the report highlighted and will fix them with future enhancements to the automatic pedal trim function of the software.

A Wiltshire Air Ambulance spokesman said: "Wiltshire Air Ambulance has now received a full report from the Air Accidents Investigation Branch relating to incidents in June 2018 and January 2019.

"The charity takes matters of this nature very seriously, with the safety of its staff and patients absolutely paramount.

"Wiltshire Air Ambulance is pleased with the findings and conclusions of the report, which have resulted in the manufacturer being required to make a revision to the Rotorcraft Flight Manual and Integrated Avionics Manual."

Read the full report here.