A HELICOPTER which crashed into a North Sea oil platform during a routine flight with eight passengers aboard was flying with a faulty tail rotor which could have failed at any time, investigators have found.

The Sikorsky S92 carved gouge marks across a helipad on the West Franklin Platform when it landed heavily after a short flight from Total's Elgin rig.

Both of the crew members and the passengers onboard escaped unscathed, but the crash led to the Sikorskys92 fleet being grounded across the world for safety checks.

Now an Air Accident Investigation Branch (AAIB) probe has disclosed that engineers noticed problems with the aircraft before the incident in December 2016, but did not act on them.

An AAIB report incident said that vibrations had been detected coming from the tail rotor the day before the crash, which were investigated but not disclosed to the crew.

On the day of the flight the helicopter took off and spun slightly out of control, before stabilising. The pilots assumed the malfunction had been caused by the wind, and proceeded to fly to the rig.

But the aircraft spun haphazardly for a second time on arrival before landing on the helipad and continuing to turn, coming to a stop near the edge of the platform.

The AAIB report said: "The five-minute flight to the West Franklin wellhead platform was uneventful but, in the latter stages of landing, yaw control was lost completely and the helicopter yawed [spun] to the right.

"The crew landed the helicopter expeditiously, but heavily, on the helideck. The helicopter continued to rotate to the right and the crew closed the throttles before it came to rest near the edge of the helideck having turned through approximately 180°. There were no injuries."

And it added: "If the loss of yaw control had occurred at an earlier stage of the flight, the helicopter would most likely have made an uncontrolled descent into the North Sea."

The AAIB concluded that the fault in the rotor hadn’t been spotted due in part to "human limitations", and recommended new safety and maintenance procedures.

A spokesman for CHC said: The report includes two safety recommendations to the European Aviation Safety Authority (EASA).

“Immediately after the accident, we introduced a number of measures to strengthen the detection of impending degradation.

"We are confident that the measures we have taken and the actions directed by Sikorsky since the accident to all operators worldwide will strengthen both our procedures and our compliance monitoring continue to underpin the safety and airworthiness of the Sikorsky S-92.

“CHC’s top priority is the safety of everyone we carry in our aircraft. We have well established procedures and have been at the forefront of developing industry guidelines for HUMS best practice. We never take safety for granted and have already taken steps to further strengthen procedures and our compliance monitoring, as well as sharing this information with other operators.

“We have continued to share our learning with other operators through HeliOffshore. The offshore helicopter industry is committed to sharing best practice and driving offshore transportation safety ever higher.”

A spokeswoman for Sikorsky said: "Safety is our top priority, and Sikorsky assisted in the investigation through the NTSB to the UK AAIB.

"Following this incident, we worked closely with our customers to ensure airworthiness of the fleet, to include improved manufacturing processes and the continuous utilization of HUMS to ensure early detection of any potential anomalies.

"We are reviewing this final report and if additional safety recommendations are offered, Sikorsky will review those recommendations and take appropriate action.

She added: "Sikorsky is committed to communicating to customers promptly as soon as actionable information is available."