UNANSWERED questions over how crucial fuel switches came to be turned off in the Clutha helicopter means there is no way to prevent a similar crash in future, an aviation safety expert has warned.

David Learmont, a former RAF pilot and aviation safety writer, said "something that is not ordinary" had occurred on board the stricken aircraft, but the lack of a black box meant the mystery would probably never be solved.

He added that the circumstances of the air crash were unprecedented.

He said: "This was always going to be complicated, because something that is not ordinary happened that night. But because there was no flight recorder we may never know what that was.

"It normally only takes about a year to get a helicopter accident report. The fact that it has taken much longer was an indication that [the AAIB] were struggling with it, and everybody in the business said they would struggle, because we've never seen anything like this before."

The final report from the Air Accidents Investigation Branch (AAIB) is set to be published later today [Friday], but the families of those killed in the disaster have already expressed their anger and frustration after a briefing by senior investigators on Wednesday night raised "more questions than answers".

The helicopter's engines flamed out within seconds of one another after being starved of fuel, despite 76kg remaining in the main tank.

Aviation specialists have been left baffled as to why vital switches, which control the pumping of fuel from the main tank into the engines' supply tanks, were found in the off position when investigators examined the wreckage.

The switches, which are located in the cockpit and can only be turned off manually, should have been on throughout the flight.

It is understood that the AAIB has been unable to rule out "human factors", but have also pointed towards an unexplained technical malfunction.

A series of recommendations have been made by investigators, thought to include advice to fit black boxes to all passenger-carrying helicopters, but Mr Learmont added that the lack of clarity around what happened left the industry largely powerless to prevent a similar tragedy in future.

"The only purpose for an air accident investigation is to find out what happened and to make recommendations to prevent it happening again," he said. "If they really don't know what happened, they can't make any recommendations to stop it happening again."

Pilot Captain David Traill died along with PC Kirsty Nelis and PC Tony Collins after the Police Scotland helicopter, a Eurocopter EC135, plunged into the roof of the Clutha Vaults pub on November 29, 2013, as it was returning from routine operations.

John McGarrigle, Mark O'Prey, Gary Arthur, Colin Gibson, Robert Jenkins, and Samuel McGhee, who were inside the pub, were killed when the roof caved in. Joe Cusker, who was also at the venue, died from his injuries in hospital 13 days later.

The Crown Office can now consider mounting criminal prosecutions or a Fatal Accident Inquiry (FAI) based on the conclusions of the AAIB.

David Bell, a solicitor at Irwin Mitchell who is representing 17 people bereaved or injured in the crash, said he "would be surprised" if criminal proceedings were launched, and also cast doubt on the likelihood of an FAI.

He said: "The purpose of an FAI is not to attribute blame but to make recommendations on how to avoid similar incidents in future. As the AAIB report already makes those recommendations it is difficult to see what an FAI could add."

An FAI is normally automatic in circumstances where people have died at work, but in previous tragedies - such as the sinking of the Flying Phantom tugboat in 2007, when three crewmen died - the Lord Advocate concluded that the reasons for the tragedy had already been established during criminal trials.