A helicopter carrying 19 people was forced to ditch in the North Sea due to a gearbox failure, according to an Air Accidents Investigation Branch (AAIB) report.
The aircraft was carrying an oil crew from Aberdeen to a rig 86 miles (138km) north-west of Shetland when it ditched at around 3.30pm on Monday.
The 17 passengers and two crew were taken off their life raft by a rescue craft launched from the Nord Nightingale vessel, which was close to the scene.
They were taken back to the tanker and flown by RAF and Bond rescue helicopters to Kirkwall in Orkney.
No-one was injured and they returned to Aberdeen on Tuesday.
The AAIB report said a problem with the main gearbox lubrication system caused the pilots to take action.
The report said: "The crew of the helicopter carried out a controlled ditching following indications of a failure of the main gearbox (MGB) lubrication system and, subsequently, a warning indicating failure of the emergency lubrication system.
"All passengers and crew evacuated the helicopter and were subsequently rescued without injury."
The crew told the AAIB that lights on the helicopter's central warning panel illuminated, indicating that the oil pump system had failed and the emergency lubrication system, which is supposed to keep the gearbox going for 30 minutes, also failed.
An inspection of the aircraft, which was brought ashore at Peterhead harbour today, found a crack on a gear shaft.
The report said: "An initial visual examination has identified a 360-degree circumferential crack on the bevel gear vertical shaft, in the vicinity of the weld that joins two sections of the shaft. Therefore, the main and stand-by oil pump gears were no longer being driven."
A study of vibration equipment which monitors the engines and main rotor gearbox found that vibrations on the bevel gear, which eventually cracked, exceeded an alert threshold before the ditched flight.
The vibration monitoring information is recorded on a memory card which was last downloaded by the operator on Sunday. Data from earlier trips on the day of the ditching was not downloaded but there was no requirement to do so, the AAIB report said.
The ditching is the fourth serious helicopter incident in three years.
In May, all 14 passengers and crew members on a Super Puma helicopter were rescued after it ditched about 30 miles (48km) off the coast of Aberdeen. It was on a scheduled flight from Aberdeen Airport to a platform in the North Sea.
On April 1 2009, 16 people died when a Super Puma plunged into the sea off the Aberdeenshire coast. The gearbox of the Bond-operated helicopter failed while returning from the BP Miller platform.
The tragedy happened about six weeks after another Bond Super Puma with 18 people on board ditched in the North Sea as it approached a production platform owned by BP. Everyone survived that incident.
CHC Helicopter, the operator of the aircraft involved in the latest incident, suspended all flights with the EC225 Super Puma model after the ditching.
Two further operators, Bond and Bristow, also suspended scheduled flights with the model.
A spokesman for CHC said: "We are aware that the AAIB has issued a special bulletin. Representatives from across the industry will reconvene in meetings in Aberdeen tomorrow to agree the terms and timing against which to return 'on hold' aircraft to service."
The AAIB said an engineering investigation of the helicopter is continuing along with the manufacturer, Eurocopter, and the operator. Audio recordings from the flight will also be studied.
The European Aviation Safety Agency and Eurocopter are reviewing directives they issued after the incident in May in which operators of the EC225 model fitted with bevel gear shafts of a certain part number and serial were told to monitor the parts at set intervals after flights.
Before the AAIB report was published RMT offshore organiser Jake Molloy said gearbox problems were becoming concerning.
"It's been rumoured it was another gearbox issue and that is a major concern because that would be the third issue with this aircraft-type gearbox in the last few years," he said.
Mr Molloy added that workers had been frustrated over the delay to deal with safety issues.
"Many of the workforce will be extremely frustrated as some will be stuck offshore or alternatively stuck onshore, some of them perhaps not being paid.
"But I would hope that the frustration the guys are feeling will be tempered by the fact that the decision to suspend flights has been done in their best interests, and we are looking to prevent a recurrence of Monday's event."