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 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 have returned to Aberdeen.
Operators CHC and two other offshore helicopter companies, Bond and Bristow, have suspended their use of the model in the wake of the incident.
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."
The crew told the AAIB that lights on the helicopter's central warning panel illuminated, indicating that all oil had drained from the main gearbox, 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 on Wednesday, 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.