BRITAIN'S aviation regulator has banned the use over British seas of helicopters similar to the one that ditched off Shetland this week.
The Civil Aviation Authority (CCA) order applies to all Super Puma EC225s and its older variant AS332, except those involved in life-saving operations.
It has emerged the EC225 that came down in the sea while flying workers from Aberdeen to the West Phoenix drilling rig had gearbox and emergency lubrication problems similar to that which caused another Super Puma to make an emergency landing four months ago.
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Eurocopter has admitted extra maintenance checks ordered after an EC225 ditched in the North Sea in May failed to prevent a similar mechanical failing on the same model of aircraft on Monday.
The helicopter manufacturer has now ordered more frequent and thorough checks.
But it faced criticism from pilots union Balpa, which said the enhanced safety regime put in place in collaboration with the European Aviation Safety Agency (EASA) earlier this year had not been sufficient.
Now the CAA has stepped in to impose restrictions over flying saying: "This decision reflects the challenging conditions that UK offshore flying presents."
The CAA said it had prohibited flying of affected helicopters on all "open sea areas" around Britain.
The EASA also issued a directive increasing the frequency of checks of affected helicopters.
CHC Helicopter, the operator of the aircraft involved in the latest incident, has suspended all flights of the model, and two further operators, Bond and Bristow, have followed suit.
Balpa said there was growing concern among helicopter pilots about the Super Puma and for the oil and gas workers who regularly fly in it.
"We question what EASA and Eurocopter have done to ensure the fault identified in May's incident has been rectified on all helicopters of this type.
"Whatever action has been put in place has clearly not been sufficient if the AAIB's initial investigation proves to be correct," Balpa said.
The EC225 and the AS332, have been involved in a series of incidents that have led to fatalities and grounding of fleets.
In April 2009, 16 people died when an AS332 L2 Super Puma returning to Aberdeen from the BP Miller platform plunged into the sea after gearbox failure.
That happened six weeks after BP suspended flights involving EC225s after a Super Puma with 18 on board came down 550 yards from a company rig off the coast of Aberdeen, and Bond moved to ground other EC225s.
An Emergency Airworthiness Directive was later issued on the EC225 and the AS332 over concerns about the fairing gutter, which protects the intermediate gearbox.
An AAIB report into the latest incident – involving a CHS -operated EC225 with 19 people on board – revealed a crack on a gear shaft.
The AAIB said the problems were with a "bevel gear vertical shaft", the same type that failed when another EC225 was forced to carry out an emergency landing in the North Sea, with 12 passengers and two crew aboard, 20 miles off Aberdeen in May. All were rescued with only two receiving minor injuries.