Shetland helicopter crash: Pilots 'failed to spot reduced airspeed'
Air accident investigators have said a reduction in airspeed that caused the fatal crash of a helicopter off Shetland was not noticed by the pilots.
Four people died when the Super Puma crashed on approach to Sumburgh Airport on 23 August.
The ongoing Air Accidents Investigation Branch (AAIB) investigation has so far found no evidence of a technical fault.
The investigation will now focus on the effectiveness of pilot monitoring of instruments and the training of crews.
An interim report published by the AAIB said that, as the helicopter approached to within 2.3 nautical miles of the airport, the commander noted that its airspeed had reduced to 80 knots.
The commander increased the collective pitch, intending to maintain the speed.
But the helicopter's airspeed reduced to below 80 knots, unobserved by the crew.
A short time later, there was an automated audio call of "check height", which was acknowledged by the commander, and then a comment by the co-pilot to draw the commander's attention to the airspeed.
At this time, the helicopter's airspeed was only 35 knots and reducing.
There was then a second automated call of "check height" followed by a "100 feet" automated call.
The report stated: "At some point the commander saw the sea, but he was unable to arrest the helicopter's descent and it struck the water shortly thereafter.
"The co-pilot, realising that the helicopter was about to enter the water, armed the helicopter's flotation system.
"After striking the surface the helicopter rapidly inverted, but remained afloat, the flotation equipment having successfully deployed".
There were 18 people on board the helicopter at the time.
Sarah Darnley, 45, from Elgin, Gary McCrossan, 59, from Inverness, Duncan Munro, 46, from Bishop Auckland, and George Allison, 57, from Winchester, lost their lives in the incident.
The AAIB investigators also called for a review of rescue operations in the water surrounding Sumburgh Airport.
A number of lifeboats and helicopters were sent to the scene, but the report said one rescue boat supposed to leave from a slipway to the west of the airport was delayed due to tidal conditions and instead had to leave from the south of the airport.
It took almost an hour to reach the crash scene.
The report said: "It is recommended that the Civil Aviation Authority reviews the risks associated with the current water rescue provision for the area of sea to the west of Sumburgh Airport and take appropriate action."
Oonagh Werngren, operations director of Oil & Gas UK, said: "We now have a greater appreciation of what happened that day but we still don't fully understand why.
"It is important, both for those directly involved in this tragedy and for all the men and women who work offshore, that the AAIB continues its very important task and reaches conclusion in a timely fashion.
"It is also good to see that it is already making recommendations."
She added: "In the meantime there are a number of different investigations under way and Oil & Gas UK will continue to work with the industry, the regulators and relevant bodies to ensure that the lessons are shared and the recommendations appropriately addressed."
Les Linklater, team leader of Step Change in Safety, said: "This bulletin helps us to have a better understanding of what happened but not yet why it happened, and that will form part of the conclusion of the investigation.
"It's very important that we all understand why this tragic accident happened and this will be a significant part of the on-going investigation."
He added: "We will continue to monitor the investigation and act accordingly where safety improvements are identified. We are also continuing to work with the helicopter operators Bristow, Bond and CHC who have already instigated a review, which is focusing on safety-related processes."