THE mystery of how two vital fuel switches came to be turned off is now the focus for investigators trying to unravel why a police helicopter crashed in Glasgow, according to experts.

The error is said to be the "smoking gun" behind the tragedy, but aviation analysts are baffled over why an experienced pilot would knowingly turn them off. The switches, which control the flow of fuel to the engines, are both supposed to be on throughout flight.

The latest bulletin from the Air Accident Investigation Branch (AAIB) revealed that the Eurocopter EC135 suffered double engine failure as a result of fuel starvation, but the report downplayed the cause by simply stating that "the switches were set to the off position" without highlighting their significance.

The aircraft had 76kg of fuel in its main tank when it plummeted into the Clutha Vaults pub on November 29 last year, killing all three on board and seven more on the ground.

However, it is understood that none of the fuel was reaching the engines because the key transfer pumps - known as aft and fore - had been switched off.

As a result fuel could not ­transfer from the main tank into the two supply tanks, which in turn pump fuel to the left and right engines.

The switches which control the pumps are located in the cockpit ceiling to the rear of the pilot and can only be turned off manually.

The AAIB report on February 14 stated that there was zero fuel in the right supply tank and just 400g in the left supply tank at the time of impact. It noted that "in the latter stages of the flight, the right engine flamed out, and shortly after the left engine flamed out". A series of low-fuel warnings were also recorded during the flight.

The report added that investigators will now "seek to determine why a situation arose that led to both the helicopter's engines flaming out when 76kg of fuel remained in the fuel tank group".

David Learmont, an aviation safety expert, said the situation pointed to human error.

He said: "The AAIB have probably worked out, or are trying to work out, whether they could have been knocked into the off position on impact, but they're not saying anything. It seems highly unlikely though as that doesn't explain why there was virtually nothing left in the supply tanks.

"The other option, that they were turned off during maintenance, doesn't add up either. The helicopter could not have operated as long as it did that night - well over an hour - if those switches were set to off from the beginning. So it only leads to one conclusion.

"The thing is, the engines did not 'fail'. They stopped working because they were starved of fuel.

"There are lots of human factors in this crash, but the bottom line is: why were the switches off? It's up to the pilot to set them. So if it didn't happen on impact, why were they off? That's the big mystery."

Captain David Traill, 51, had accumulated 646 flying hours on the EC135 since 2010. As a former RAF pilot and instructor who had served in both Gulf wars he was highly experienced, making the possibility that he could have made such a fatal mistake all the more baffling.

Captain Traill was killed along with both passengers, police constables Kirsty Nelis and Tony Collins. Six people inside the Clutha Vaults also died when the roof caved in, with a seventh victim dying from his injuries later in hospital. Lawyers are now pursing compensation for those affected by the tragedy.

The development with the switch has been seen as hugely important by manufacturer Eurocopter, now known as Airbus helicopters, as it appears to rule out technical fault.

However, aviation analyst Chris Yates warned against ­jumping to conclusions. While he agreed that the fuel switches were key to the crash, he added that they might also reveal Captain Traill's desperate final attempts to save the aircraft.

"A lot depends on when they were reset," said Mr Yates. "It could be that the pilot was cycling the system to try to get it to function - that he was reacting to the 'low fuel' warnings by turning them off and on. But we might never know because there was no voice recorder on board."

A third specialist, who did not want to be named, suggested that fuel quantity sensors in the supply tanks may have been malfunctioning. As a result, Captain Traill would not have received the low fuel warnings "until it was too late" and the "sudden loss of both engines meant he did not have time to switch on the transfer pumps".

There was no indication that the aircraft was in trouble when Captain Traill requested permission to return to base at 10.18pm, four minutes before the chopper went down, and no mayday was issued.

The AAIB are also probing why autorotation - a form of emergency landing which pilots are routinely trained to perform in the event of double engine failure - was not achieved. The manoeuvre forces the rotors to continue turning without power, but investigators have confirmed that all rotors were at a complete standstill at the time of impact.

A spokeswoman for the AAIB said: "The investigation is ongoing."