OIL workers who died when their helicopter ditched off Shetland were let down by pre-flight safety briefings which gave a "false impression" about emergency breathing equipment, investigators have said.

A special bulletin by the Air Accident Investigation Branch (AAIB) said investigators were concerned the briefing material "does not include fully representative information about emergency breathing systems (EBS)".

Four people died when the Super Puma crashed and capsized in the North Sea on August 23 last year. Three of those who lost their lives - Sarah Darnley, 45, from Elgin, Duncan Munro, 46, from Bishop Auckland, and George Allison, 57, from Winchester - died as a result of drowning, according to their death certificates. The fourth fatality, Gary McCrossan, 59, from Inverness, suffered heart failure.

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They were among 18 passengers and crew on board the helicopter, which was making its way from the Borgsten Dolphin oil and gas platform to Sumbugh Airport on Shetland on a Friday night, when it ditched around 6.20pm.

Air accident investigators have blamed the crash on a reduction in air speed which was not noticed by the crew. No technical fault with the aircraft has been detected.

However, the AAIB has now also highlighted shortcomings in pre-flight training, noting that in 60% of helicopter crashes on water the aircraft overturns.

In the bulletin published yesterday it says: "The EBS can bridge the gap between the breath-hold time of an occupant and the time required to complete an underwater escape, thereby increasing the chances of survival.

"EBS were introduced in UK North Sea offshore helicopter operations as a voluntary industry standard; at present there is no regulatory requirement for such equipment."

There are three types of breathing equipment in use by the ­industry, but the AAIB said safety briefings could lead oil workers to confuse different types.

They include compressed air systems with a mouthpiece - similar to the type of equipment used by scuba divers - and rebreather systems, which allow the user to rebreathe the air contained in their lungs by breathing into a bag before they enter the water. A third piece of equipment, the hybrid system, combines the rebreather and compressed air system.

The hybrid system is commonly used in the offshore industry in the form of a life-jacket incorporating emergency breathing equipment.

Offshore helicopter passengers are required to complete initial and repeated training on the operation of EBS, and safety videos are also shown prior to every flight as a refresher.

However, the AAIB said this pre-flight material was not "fully representative" about the use of EBS.

The special bulletin stated: "It does not highlight that the EBS provided may be a hybrid rebreather containing an air supply which is discharged ­automatically into the rebreather bag, or that the system can be used even if the wearer has not taken a breath before becoming submerged.

"Incomplete information in the pre-flight safety briefing material may give passengers the false impression that hybrid rebreathers such as the widely used LAP system are only of benefit if the user has taken a breath prior to becoming submerged.

"Knowledge that hybrid rebreathers contain their own supply of air may therefore influence a passenger's decision on whether or not to use the EBS in an emergency situation."

AAIB has now approached helicopter operators working in the offshore industry to raise concerns about pre-flight training, which is now being modified.