Crash investigators yesterday ordered "urgent" inspections on helicopter gearboxes following the North Sea disaster that claimed 16 lives earlier this month.
The European Aviation Safety Agency said it would require all European Super Puma helicopter operators to conduct the new checks "by next Friday", or they would be grounded.
In an initial report into the 1 April crash, the Air Accidents Investigation Branch (AAIB) said the gearbox of the Eurocopter AS33L2 Super Puma had suffered a catastrophic failure before the craft plunged into the sea off the coast of Aberdeenshire.
Today, in a second report, investigators said the gearbox of the Super Puma was already being checked after metallic debris was found in it just 34 flying hours before the helicopter crashed.
The small pieces of debris were found on the gearbox metallic detector, which is used to attract any bits which can get into the system.
The 14 passengers and two crew had been returning from BP's Miller platform when the helicopter crashed, 11 miles north-east of Peterhead, Aberdeenshire.
The appearance of such debris is seen as an early warning of a possible gearbox failure.
The AAIB report said: "However, during the period between the discovery of the chip and the accident, no signs of an incipient gearbox failure were detected."
Investigators added that they were recommending the new checks "as a matter of urgency to ensure the continued airworthiness of the main rotor gearbox".
The checks will be conducted on AS33L2 Super Pumas as well as the EC225LP Super Puma, which was the type of helicopter that crashed in the North Sea on 18 February in a separate accident in which all 18 passengers survived.
The investigators issued a short statement saying further work was needed to find out just what caused the failure, as metal debris had been carried through the gearbox, bursting it apart.
It added this had resulted in a large amount of secondary damage to all gearbox components, which was "potentially masking the initial failure".
Kieran Daly, executive editor of Flight International magazine, said: "For the AAIB to use language such as 'a matter of urgency' is quite unusual.
"Investigators are finding it difficult to understand why the gearbox failed. Unless a way can be found to anticipate failures then there has to be a serious question mark over whether these helicopters should be allowed to fly.
"The manufacturers have got to come up with a new way of inspecting these aircraft."