Lord Ballyedmond, a member of his staff and two personal pilot were killed shortly after take off in Norfolk.
Pilots Carl Dickerson and Lee Hoyle, expressed concerns before take off in March 2014, the jury in Norwich heard.
Coroner Jacqueline Lake said she would be raising concerns over regulations controlling flights from private land.
The inquest was told Lord Ballyedmond, born Edward Haughey, made regular visits from Northern Ireland to check progress on the refurbishment of his stately home Gillingham Hall, in his Agusta Westland AW139 helicopter.
On 13 March 2014, he was delayed in taking off because he had been overseeing the hanging of pictures, the hearing was told.
Mr Dickerson had warned they needed to take off "no later than 7pm" because of bad weather, but the four - including Lord Ballyedmond's foreman Declan Small, 42 - did not take off until 19:22, by which time fog had closed in.
Air Accidents Investigation Branch (AAIB) expert Tim Atkinson told the hearing that in such conditions the helicopter would not have been allowed to take off from a licensed aerodrome.
The jury saw video mobile phone pictures of the helicopter - which was found to have no mechanical defects - preparing for take off from the helipad, during which the pilots decided to disengage the autopilot.
In the footage, one of the pilots said: " I don't mind telling you I'm not very happy about lifting out of here. "
The other replies: "It should be ok because you can still see the moon."
Moments after the words "nose down" were spoken from the cockpit, the helicopter ploughed into a line of hay bales in a field near the take off site.
The inquest heard the two pilots were experienced but there was no evidence either had been trained in taking off vertically in low visibility.
Following the verdict, a statement was read out on behalf of the families of Lord Balleyedmond and Mr Small.
It read "It is still a mystery why the pilots did not comply with the operations manual and adopt safe take off procedures or why they chose to de-select the autopilot while attempting to take off, at night, in dense fog.
"The families continue to believe after this week of evidence that this was a preventable accident."
The deaths of one of Northern Ireland's richest men and three others in a helicopter crash were accidental, a jury decided.