The purpose of the flight was to transport the passengers, who were employees of the UK offshore oil and gas industry, to Aberdeen.
On the accident flight, the helicopter had departed the Borgsten Dolphin semi-submersible drilling platform in the North Sea, to route to Sumburgh Airport for a refuelling stop. It then planned to continue to Aberdeen Airport.
An Air Accidents Investigation Branch (AAIB) report into a helicopter crash off Shetland in which four people died records that flight instruments were "not monitored effectively" by pilots in the moments leading up to the incident. Four people died when the CHC Super Puma Helicopter dropped into the sea on its approach to Sumburgh in August 2013. A tendency to over rely on automatic processes and a lack of practical instrument flying ( which is according to an aviation expert quite common these days ) is cited as probably one of the primal causes of the 2013 Shetland helicopter Crash.
"The helicopter's flight instruments were not monitored effectively during the latter stages of the non-precision instrument approach.
"This allowed the helicopter to enter a critically low energy state, from which recovery was not possible.
"Visual references had not been acquired by the Minimum Descent Altitude (MDA), no effective action was taken to level the helicopter, as required by the operator's procedure for an instrument approach."
The report continued: "The decreasing airspeed went unnoticed by the pilots until a very late stage, when the helicopter was in a critically low energy state. The commander's attempt to recover the situation was unsuccessful and the helicopter struck the surface of the sea.
"It rapidly filled with water and rolled inverted, but was kept afloat by the flotation bags which had deployed."
No evidence of any engineering issues causing the crash was found..
Passengers Sarah Darnley from Elgin, Gary McCrossan, from Inverness, Duncan Munro, from Bishop Auckland, and George Allison, from Winchester, lost their lives in the crash on 23 August 2013. Fourteen people were rescued. The AAIB report said the impact with the water was "survivable".
Of the four dead, the report found that one was unable to escape, one was incapacitated by head injury, one drowned before reaching the surface, and the another died in the life raft, from a chronic heart condition.
Martin Tosh, one of the fourteen survivors of the crash, said he was concerned the safety recommendations made by the AAIB do not go far enough. He said the experience of "falling out the sky" was "absolutely petrifying".
He said: "I need closure but I am not going to get closure any time soon."
A total of 28 safety recommendations have been made but the report said many of them have already been implemented.
An initial AAIB report already found the crew failed to notice the helicopter's air speed dropping until it was too late.
In February 2014, the Civil Aviation Authority announced a series of measures aimed at improving offshore helicopter safety following an inquiry set up in the wake of the Shetland crash.
They included helicopters not being allowed to fly offshore in severe sea conditions, passengers having to be seated next to emergency exits, and a size limit for those on board.
Sources: BBC, AAIB, NOTAMS, CNS
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