The helicopter crash which killed Tory peer and self-made multimillionaire Lord Ballyedmond may have been triggered by an error in perception along with a lack of training and procedures to handle the flight which took off in thick fog.
An error in perception by the crew who had lacked visual clues before the flight, known as the somatogravic illusion, may also have played a part in the crash on March 13 2014, according to the Air Accidents Investigation Branch (AAIB).
The two passengers - Lord Ballyedmond, 70, and Declan Small, 42, who worked for him - along with captain Carl Dickerson, 36, and co-pilot Lee Hoyle, 45, were killed.
The Agusta AW139 G-LBAL helicopter crashed just 460 yards (420m) after taking off from the peer's home at Gillingham Hall in Norfolk at around 7.20pm.
Take-off had originally been set for about 6.30pm but by the time the passengers were ready to leave, a dense fog which cut visibility down to "the order of tens of metres" had set in, according to the AAIB report.
The investigators pointed out that because the flight was from a private landing site, there was no requirement for a particular minimum visibility.
The AAIB report notes: "The helicopter departed the private site in fog and at night. Operation from the site in such conditions was permissible under existing regulation. Departure from a licensed aerodrome in such conditions would not have been permitted."
The investigators concluded: "Evidence suggests that the flight crew may have been subject to somatogravic illusion caused by the helicopter's flight path and the lack of external visual cues.
"The absence of procedures for two pilot operation, the pilot's lack of formal training in such procedures, and the limited use of the automatic flight control system, may have contributed to the accident."
Lord Ballyedmond was considered to be one of Ireland's richest men, with estimated wealth in excess of £800 million.
He was best known as chairman and founder of Co Down-headquartered Norbrook Laboratories, the largest privately-owned pharmaceutical company in the world. The father of three had a range of other business interests.
He was a life peer with a seat in the House of Lords, first on behalf of the Ulster Unionist Party before switching to the Conservative Party. He had also previously sat in the upper house of the Republic of Ireland's parliament, the Seanad.
Somatogravic illusion could have led to the progressively abnormal attitude of the helicopter "feeling" normal to those on board, the investigators suggested. They explained that in the absence of visual clues, the "down" direction is sensed from accelerations that can be felt. This sensation can be compromised in an accelerating aircraft when gravity is not the only force being sensed.
The helicopter had reached an altitude of 82ft (25m) above ground level and a ground speed of 90 knots (just over 103mph) when it crashed nose-down in a field.
It hit a line of large hay bales lying in the ploughed field and the "cabin structure was destroyed", according to the report.
In the final few seconds of the flight the co-pilot had made two verbal prompts to the captain regarding the aircraft's pitch attitude. Recorded data showed that steps had been taken to rectify this.
The helicopter manufacturer said that, based on the recorded data, "the helicopter had responded appropriately to the crew inputs". There is no evidence the helicopter struck a tree or any other object during the flight.
A second impact mark was made 45m beyond the first ground mark and three of the five main rotor blades had been stripped away.
The layout of the wreckage indicated the helicopter had become airborne after the second ground impact.
It was noted that an accident in 1996 involving a G-HAUG helicopter had been caused by a "loss of situational awareness" by that pilot and that the G-LBAL pilots had experienced a similar condition. Another similarity was the use of a procedure not recognised for a civil helicopter pilot which gave little margin for error.
The report concludes that "opportunities to reduce the likelihood of such an event, presented by the report into the operator's previous fatal accident, appeared not to have been taken".
The UK is to adopt new regulation involving non-commercial complex aircraft operation in 2016.
In light of the G-LBAL accident and other helicopter incidents investigated by the AAIB, the Civil Aviation Authority is also review into Instrument flight rules (IFR) outside controlled airspace.