Pilot error caused a helicopter to crash into the sea of Sumburgh, Shetland seven years ago, killing four oil and gas workers, a fatal accident inquiry into the tragedy has ruled.

Sheriff Principal Derek Pyle, who heard 15 days of evidence in August and September, said in his determination published on Monday the crash was "a dreadful accident with long-term repercussions for the survivors and the families of the deceased".

The CHC Helicopter-operated Super Puma L2 aircraft, with 16 passengers and two crew on board, hit the sea on its approach to Sumburgh airport on 23 August 2013.

Three passengers — Sarah Darnley, 45, from Elgin, Duncan Munro, 46, from Bishop Auckland and George Allison, 57, from Winchester — all drowned.

Gary McCrossan, 59, from Inverness, suffered a cardiac arrest triggered by the "emotional and physical stress" of the ditching, the inquiry heard.

A fifth passenger, Samuel Bull, aged 24 at the time of the accident, suffered from post-traumatic stress disorder and died by suicide in December 2017.

The 51-year-old commander of the helicopter, the 40-year-old co-pilot and 12 passengers survived.

The helicopter was on a routine flight plan transporting oil and gas workers to and from drilling platforms in the North Sea. The crash happened on the third flight of the day, from the Borgsten Dolphin drilling platform to Sumburgh for a refueling stop.

'Critically low energy state'

The inquiry heard the helicopter’s crew — commander Captain Martin Miglans and co-pilot Alan Bell — failed to monitor flight instruments correctly, which meant they failed to maintain the correct approach speed.

The aircraft then entered a "critically low energy state", which resulted in them losing of control of the aircraft.

Sheriff Pyle said: "The flight crew of the helicopter failed to maintain the target approach airspeed — 80 knots — and the stabilised approach criteria, contained in the operator’s operations manual, during the latter stages of the non-precision approach to Sumburgh Airport."

The helicopter crashed into the sea and immediately capsized and filled with water.

'Plainly no wilful neglect'

Sheriff Pyle said Captain Miglans was a pilot of "huge experience with a first-class record of flying over many years".

"There was plainly no wilful neglect," he added. "Rather, there was, as one witness described it, a perfect storm of circumstances which resulted in all the safety barriers in place not preventing – or remedying – his one failure, to maintain the correct speed."

"While it is the case that the cause of the accident was pilot error, it is not known why. One possible reason is in the developing knowledge of the inability of the human brain continuously to monitor flight instruments in all forms of aircraft, particularly in automation modes."

Sheriff Pyle said the aviation industry and regulatory authorities are looking closely at this developing science.

"The expert inspectors from the Air Accident Investigation Branch of the Department of Transport concluded that the accident could have occurred with other pilots flying," he said.

'Millions to one against'

One expert before the inquiry described the circumstances as "millions to one against".

During the inquiry, expert witnesses praised Bell for his quick thinking in releasing the emergency flotation system, which probably saved lives, as well as his actions to release the life rafts.

One expert described his conduct as brave.

Despite suffering from a serious back injury, Captain Miglans insisted that he be the last survivor winched from the sea.

While some of the passengers experienced difficulties in escaping from the helicopter, and with their survival equipment, there was no obvious defect in survival training and instruction or the equipment, the inquiry heard.

Understanding

Sheriff Principal Pyle said he hoped the inquiry the inquiry had at "least assisted in an understanding of what occurred, the reasons for it and what has been done to ensure so far as practicable that such an accident does not occur again".

He added: "My condolences go, in particular, to the families of the deceased, including – lest it be forgotten – the family of Mr Bull.”

The inquiry heard Bull took an "active part" in the care of the other survivors, including giving CPR to McCrossan.

Call for public inquiry

Following the publication of the determination, the RMT trade union repeated its call for a full judge-led public inquiry into the impact of commercial pressures on the helicopter industry, in line with the findings of the Transport Select Committee findings in 2014, which looked at the Sumburgh accident.

The RMT also said it was disappointed that the FAI had "predictably kept the scope of the inquiry very narrow, looking at the events at the precise moment of the tragedy".

General Secretary the RMT union, Mick Cash, said: "There was no consideration of the wider aspects of what led to the tragedy and the loss of four lives, including RMT member Sarah Darnley."

Cash said the tragedy would have been avoided if safety systems in the helicopter had been enabled.

"Those systems were inhibited because the pilots hadn’t been trained how to use them," he said.

"The fact the safety systems were fitted in the aircraft tell us the risks of such an event occurring were apparent to the helicopter manufacturer. The fact the pilots weren’t trained to use the safety systems tells us that commercial pressures were a factor in a tragedy which was completely avoidable."

Deirdre Michie, chief executive of trade body Oil & Gas UK, said: "This was a tragic accident and our thoughts today are with the survivors, and the families and friends who have lost loved ones. While we are a big industry we are a close-knit community and I know that many colleagues will be reflecting and remembering with great sadness today."