Air accident investigators have called for improved safety systems on Super Puma helicopters, following the North Sea crash which killed 16 people.
The Air Accidents Investigation Branch's initial report found that the crash was caused by a "catastrophic failure" in the aircraft's gearbox.
But the report said the reason for the gearbox failure was not yet understood.
It also urgently recommended that "additional inspections" be carried out on other Super Puma helicopters.
The Rail Maritime and Transport Union (RMT) has called for all Super Pumas with the same gearboxes to be grounded for inspections and urged offshore staff concerned about safety not to fly.
The report said the aircraft's manufacturer, Eurocopter, should carry out "additional inspections and enhanced monitoring" on its AS332L2 models of Super Puma helicopters, to ensure the airworthiness of the gearboxes.
It also recommended that the European Aviation Safety Agency evaluates these checks and, when satisfied, makes them compulsory.
In addition, investigators advised that Eurocopter improves its gearbox monitoring and warning systems on its AS332L2 helicopters.
The AAIB report concluded that the gearbox's failure led to the main rotor blades breaking off and hitting the helicopter.
It continued: "Examination of the wreckage indicates that the accident occurred following a catastrophic failure of the main rotor gearbox.
Immediately after impact he saw the four main rotor blades, still connected at their hub, strike the water. Around this time, he also heard two bangs, close together
"This resulted in the detachment of the main rotor head from the helicopter and was rapidly followed by main rotor blade strikes on the pylon and tail boom, which became severed from the fuselage.
"It is apparent that there was also a rupture in the right-hand engine casing."
It added that the investigation revealed that the gearbox had suffered a "major failure", which was not yet fully understood.
A Scottish Government spokesman said: "We welcome this initial report and look forward to the findings of the full investigation, as it is vital that the cause of the crash is established so lessons can be learned and these types of tragedies can be avoided in the future.
"It's thanks to the commitment and hard work of the emergency services that the bodies, helicopter and black box were recovered, assisting the pace of the investigation.
"Our thoughts continue to be with the family and friends of the sixteen men who lost their lives through this tragic accident."
Investigators said the black box flight recorder found that, after take-off at 1303 BST, the crew had been engaged in "routine cockpit activities" and there were no problems.
At 1354 BST the co-pilot made a routine call to say the estimated time of arrival at Aberdeen Airport was 1414 BST.
Twelve seconds after this, one of the pilots issued a mayday call, which was picked up by air traffic controllers.
Air traffic controllers tried unsuccessfully to contact the crew, before asking the crew of another helicopter in the area to examine an area based on the Super Puma's last radar position.
The report said radar information showed the helicopter flying inbound towards Aberdeen at 2,000ft, climbing momentarily to 2,200ft, then turning right and "descending rapidly".
An eyewitness working on a supply vessel near the crash site heard the helicopter and saw it come down before it hit the sea.
The report states: "Immediately after impact he saw the four main rotor blades, still connected at their hub, strike the water.
"Around this time, he also heard two bangs, close together.
"He immediately raised the alarm and the ship turned towards the accident site, which by now was marked by a rising column of grey then black smoke."
The failure of the gearbox caused the main rotor blades to break off
Before setting off from Aberdeen, the report stated that the helicopter was serviceable, except for a "deferred defect", affecting part of its ice detection system.
The daily in-flight checks had been completed and were found to be satisfactory.
The helicopter landed on the Miller platform, was refuelled, with the rotors running, and 14 passengers boarded.
Weather conditions were said to be mild, with light south to south-easterly winds and good visibility with generally clear skies.
Flying conditions were reported as smooth and the sea was calm.
The AAIB was notified of the crash within minutes and a team of inspectors, including engineers, pilots and flight recorder specialists travelled to Aberdeen that evening.
The wreckage was later recovered from the seabed and transported to the AAIB headquarters at Farnborough in Hampshire.
It included the black box flight recorder, the fuselage with the engines and rotor gearbox attached, the separated rotor head with the main rotor blades still attached, and the separated tail.
The flight recorder was found to contain 24 hours of flight data and one hour of cockpit voice recording, which stopped just before the first mayday was issued.
A memorial service for those who died is to be held on Wednesday in Aberdeen.
It will be attended by Prince Charles and Camilla, Duchess of Rothesay.
The service will be held in Aberdeen's St Nicholas' Kirk and conducted by the Rev Andrew Jolly, chaplain to the North Sea oil and gas industry.
The 14 passengers and two crew were returning from BP's Miller platform when the helicopter crashed in the sea, 11 miles north-east of Peterhead on April 1.
The bodies of the victims have all been recovered and identified.
Eight of the victims came from the north east of Scotland, seven from the rest of the UK, and one from Latvia.
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