IT was the deadliest helicopter crash in Scotland for 20 years, and almost three months on from the Clutha tragedy the question marks over the cause appear to be multiplying.

The latest bulletin from air accident investigators, published on Friday, provides the bereaved with little in the way of answers over what led the EC135 police helicopter to "drop like a stone" from the night sky over Glasgow on November 29 last year.

In December, investigators said there was no evidence of engine or gearbox failure, yet neither the roof nor tail rotors was turning when the aircraft came down.

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It was described by some as a "head-scratcher".

Aviation experts to whom I spoke suggested that if there was plenty of fuel (95 litres) to complete the journey back to base, and apparently no problem with the engines, then the likeliest explanation might be a problem with the fuel supply from the tanks. Then last week, the Air Accident Investigaton Branch (AAIB) confirmed that first the right then the left engines had indeed "flamed out", or failed, on the night of the crash.

But why?

Under laboratory testing the engines and all their parts were found to be fully functional, with only minor external damage caused on impact.

There was also an "unrestricted flow" of fuel between the engines and the tanks with an examination of all internal pipe work and passageways showing no evidence of failure, either before or after the crash.

Likewise, no fault has been detected in the transmission and rotor systems that could explain a sudden loss of power.

There was no electrical outage, fire, structural failure or even bird strike.

And yet, in those final four minutes of flight - between Captain David Traill seeking permission to return to base at 10.18pm and the crash at 10.22pm - something went abruptly and horribly wrong.

No mayday was issued and there is nothing to indicate that Captain Traill attempted an emergency landing, known as autorotation.

This would be normal procedure in the event of double-engine failure.

In 1994, a RAF Chinook crashed on the Mull of Kintyre killing all 29 people on board.

The pilots were initially blamed, before eventually being cleared of responsibility in 2011. The actual cause remains a mystery.

Oddly, both crashes are linked by Fadec, an on-board engine software system once described by the Ministry of Defence's own engineers as positively dangerous.

Fadec (full authority digital engine control) allows aircraft engines to operate at maximum efficiency by continually monitoring everything from temperature to fuel flow, and automatically correcting them.

When it works, it is a pilot's best friend. But, if it malfunctions, there is no form of manual override available for the pilot to take back control.

Quite simply,: "If a total Fadec failure occurs, the engine fails".

Several experts involved in the Mull of Kintyre investigation believed Fadec was to blame.

With simple answers vanishing fast, Clutha investigators are likely to be placing it under fresh scrutiny.