UK investigators have determined that the Turbomeca Arrius 2B2 engines on an Airbus Helicopters EC135T2 that crashed through the roof of a pub in Glasgow had flamed out despite the presence of fuel in the system.

However, they remain unable to pinpoint the cause of the accident.

All three occupants of the helicopter – operating on behalf of the Police Scotland by Bond Air Services – were killed in the 29 November incident. A further seven people in the bar also died.

The UK Air Accidents Investigation Branch says in an interim factual report that although there was no flight recorder fitted, it was able to extract data from the non-volatile memory of some of the aircraft’s avionics systems, including its engine FADEC units.

“Recorded data indicates that, in the latter stages of the flight, the right engine flamed out and, shortly after, the left engine flamed out. The helicopter descended and struck the roof of the Clutha Vaults Bar at a high rate of descent in an upright attitude,” says the AAIB.

“Evidence suggests that the rotor blades and fenestron tail rotor were not rotating at the moment of impact.”

However, the AAIB has been unable to accurately determine a precise timeline of events.

It discovered that the main fuel tank contained 76kg of fuel, with 0.4kg in the left-hand engine supply tank and nothing in the second tank.

Additionally, no faults have been found with the fuel-supply system or its components. However, the AAIB notes that the switches controlling the fore and aft fuel-transfer pumps, which supply fuel from the main tank to the supply tanks, were found in the off position. But switches for the prime pumps in the supply tanks were found in the on position, as were the switches controlling the engine fuel shut-off valves.

Other causes for the crash ruled out by the AAIB Include engine mechanical failure, birdstrike, hydraulic or control system failure, and failure of either the main or tail rotor gearbox, or their oil pressure.

The pilot did not communicate any problems to air traffic control during the approximately 1.5h flight, nor was an emergency call made, says the AAIB. But the non-volatile data from the helicopter’s cockpit warning unit shows there had been low fuel alarms at some point for both fuel-supply tanks, says the report. Operating manuals mandate a landing within 10min in these circumstances.

Aside from attempting to discover what caused the engines to cut out, the AAIB will also look to discover “why, following the double engine failure, an autorotative descent and flare recovery was not achieved”.