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Puma Crash: Struggle To Retrieve Black Box
Salvage experts believe they know the location of the black box flight recorder from the Super Puma helicopter which crashed off Shetland, killing four people.
A locator beacon on the black box is still sending out signals from the seabed.
However, a heavy swell in the water is diffusing the signal, meaning searchers cannot pinpoint an exact location.
The rough conditions are also making it difficult for divers to search the seabed.
The black box should contain vital information as to why the helicopter suffered a catastrophic loss of power.
Without it, crash investigators have been frustrated in their efforts to pinpoint a cause.
Currently, the salvage teams are focusing on an area half the size of a football pitch and have told Sky News they rate their chances of recovering the black box at "50/50".
Sea conditions are expected to become calmer today, providing a window of increased opportunity, before becoming rough once again.
Important sections of the aircraft have, however, been found. They include both engines, the gearbox and the rotor head of the Super Puma.
Earlier, John Henderson, of Shetland marine engineering firm Ocean Kinetics, told Sky News: "Ocean Kinetics have successfully located, lifted and passed the gearbox and rotor head of the helicopter.
"We have also located both engines and parts of the cockpit, which will likely be recovered on Thursday.
"We are still searching for the flight recorder which we believe is located at the Point of Garths Ness. There is a heavy swell running, hampering diving operations."
The Air Accidents Investigation Branch said the helicopter appeared to proceed normally until three miles from the runway, "when there was a reduction in airspeed accompanied by an increased rate of descent".
"The helicopter struck the sea approximately two miles west of the Runway 09 threshold," the AAIB said in a statement.
"The evidence currently available suggests that the helicopter was intact and upright when it entered the water. It then rapidly inverted and drifted northwards towards Garths Ness.
"The helicopter was largely broken up by repeated contact with the rocky shoreline."
Representatives from across the North Sea oil and gas industry will meet on Thursday for the second time in two days to decide whether or not to lift a flying ban on Super Puma helicopters.
A meeting broke up without agreement on Wednesday after some companies argued that Super Puma models, other than the L2 which crashed off Shetland, should be allowed to resume flights.
The Unite union has made it clear that it wants flights of all Super Pumas suspended until the cause of the tragedy is known.
Contingency plans have been drawn up to maintain North Sea operations in the absence of the Super Pumas, which account for more than 50% of all flights to and from North Sea platforms.
Oil and gas companies are sharing transport and sourcing other aircraft from around the world.
Some have also commissioned boats to ferry workers to and from rigs, although sea transport does not offer an attractive alternative.
Journeys can take upwards of 12 hours and transferring staff from boat to platform typically involves a "basket transfer", in which three or four people at a time are strapped inside a capsule and lifted by crane onto the rig. It is not a pleasant experience in flat, calm conditions; in a rough sea it can be impossible.
The RMT union held a rally at their Aberdeen office calling for improved safety. General Secretary Bob Crow told Sky News that lessons must be learned.
He said: "One thing's for sure, our members shouldn't pay the ultimate price of losing their life as a result of going to work."
Four oil workers were killed when the Super Puma AS332 L2 travelling from the Borgsten Dolphin support vessel came down off the southern tip of Shetland on Friday with 16 passengers and two crew on board.
The victims have been named as Duncan Munro, 46, George Allison, 57, Sarah Darnley, 45, and 59-year-old Gary McCrossan.
The crash was the fifth incident involving Super Pumas in the North Sea since 2009.