The missing tail section was eventually recovered and brought ashore
A height warning may have failed to activate on a helicopter which ditched in the North Sea after it hit a bank of fog, a report has revealed.
All 18 people on the Super Puma survived last month when it came down as it approached a platform about 125 miles east of Aberdeen.
The search was "hampered by limited personal beacon homing transmissions", the report also said.
The Air Accidents Investigation Branch (AAIB) inquiry is continuing.
Its interim report revealed witnesses on the platform at the time of the accident said it was in a "fog bank", the top of which was estimated to be 10ft above the platform's helideck.
The report said: "The co-pilot was visual with what he believed was the platform's helideck lighting as well as the lights of the platform and the commander was visual with what he believed were the lights of the platform but could not identify the helideck.
The helicopter overturned after the ditching in the North Sea
"Shortly after that the aircraft landed heavily on the surface of the sea".
The report said the search for the passengers and crew was "hampered by limited homing transmissions from personal locator beacons and poor visibility".
Black box recorded data recovered from the sea indicated that an audio voice warning telling the crew they were at 100ft had not come on.
Aviation writer Jim Ferguson said the Terrain Avoidance Warning System (TAWS) was fairly new and may still require some debugging.
He added: "There was no height warning and I suspect that this was a major strand in this accident."
A preliminary examination revealed "no evidence of any pre-impact malfunction of any major mechanical components of the helicopter".
The AAIB said the aircraft's operators Bond had amended its offshore night operation manual to improve approaches to platforms.
This included making it compulsory for aircraft to gain height and re-attempt a landing if visual contact with an installation is lost or becomes uncertain.
The AAIB said analysis of data and the wreckage, and a review of emergency and survival equipment performance, was continuing.
Operators and the Health and Safety Executive (HSE) had already decided not to issue the personal beacons to workers flying to platforms until a technical solution can be found, after the accident.
The decision was made after the beacons were found to interfere with emergency equipment on helicopters.
Those on board the ditched Super Puma praised the crew and their rescuers.
Jake Molloy, from the offshore arm of the RMT union, told BBC Scotland: "Obviously there's concern they were trying to land in these weather conditions."
Mr Ferguson said he hoped the inquiry, which he said could take up to a year, would also look at the rescue operation.
"The helicopter arrived fairly quickly," he added.
"But the fast response craft took just a little under two hours to get there and I think we're entitled for an explanation for this."