A helicopter accident on a North Sea oil rig that led to the grounding of all Sikorsky S92 models was caused by a damaged tail piston, an initial investigation has found.
The helicopter was landing on the West Franklin platform on December 28 when it pitched to the side and left significant gouge marks on the deck, the Air Accidents Investigation Branch (AAIB) said.
Nine passengers and two crew were on board the helicopter at the time but no-one was injured.
All Sikorsky S92 helicopters were grounded on Tuesday to undergo maintenance and inspection work following the accident.
The AAIB report said the helicopter ``yawed rapidly to the right, reaching a maximum rate of 30 degrees per second'' when it was 4ft above the landing platform, causing the landing gear to strike the deck.
It had ``yawed unexpectedly to the right through 45 degrees'' on an earlier departure but it was put down to turbulence at the time.
The AAIB said ``it was immediately apparent that the tail rotor servo piston was damaged'' when inspectors removed helicopter panels.
There were also signs of ``severe overheating'' and ``extreme wear'' on bearings.
The helicopter also spun more than 180 degrees during the landing.
It was shut down after the accident and was later lifted by a crane from the helideck on to a ship which took it back to Aberdeen for inspection, AAIB said.
The AAIB special bulletin, published to give preliminary information from inspections, said the tail bearing deteriorated quickly.
The report read: ``Initial findings indicate that the failure of this specific bearing was rapid; a period of 4.5 hours had elapsed from the first exceedance of the relevant bearing condition indicator recorded on the operator's health and usage monitoring system (HUMS) to the point of failure.''
The recall issued by Sikorsky demanded a visual inspection of tail rotors before the next flight.
A spokeswoman for the manufacturer said on Tuesday: ``Safety is our top priority and Sikorsky is working closely with our customer and investigative authorities to determine the root cause of the loss of tail rotor authority in the December 28 installation landing.
``Although the investigation into the December 28 incident has not been completed, Sikorsky released an alert service bulletin on January 10 to define additional interim inspection requirements for the S92 tail rotor pitch change shaft (PCS).
``Those procedures include an off-aircraft check of the PCS bearing and that check must be done before next flight with some leeway for getting back to base.''
Jake Molloy, regional organiser for the RMT union, welcomed the precautionary move.
He said: ``It means the aircraft has to be on the ground for some hours but I'm quite sure every worker in the North Sea would rather that that rod and that bearing were inspected to ensure that we don't have a repeat event of December 28.''
Among helicopters grounded were a fleet from the UK Coastguard but they are now back in service.