Both engines failed on the helicopter which crashed into a Glasgow pub resulting in 10 deaths, an interim air accident report said today.
In the latter stages of the flight on the night of November 29, the right engine "flamed out and shortly after the left engine flamed out", said the report from the Air Accidents Investigation Branch (AAIB).
But the report also said that 76kg (168lb) of fuel was left in the main fuel tank and the continuing investigation into the crash "will seek to determine why a situation arose that led to both the helicopter's engines flaming out when 76kg remained in the fuel tank group".
The AAIB said the continuing investigation would also want to find out "why no emergency radio transmission was received from the pilot, and why, following the double engine failure, an autorotative descent and flare recovery was not achieved".
This was the second interim report into the crash into the Clutha bar in Glasgow of the Eurocopter EC135 helicopter, which was assisting police at the time.
Civilian pilot David Traill and his two passengers, Pc Kirsty Nelis and Pc Tony Collins, were killed, as well as seven people inside the pub.
Today's report said that the helicopter had 400kg (882lb) of fuel on board - sufficient for about one hour and 35 minutes of flight - when it left Glasgow City Heliport at 8.45pm to support police operations.
At 10.18pm the pilot requested clearance from air traffic control to re-enter the Glasgow control zone to return to the heliport. This was approved and no further radio transmissions were received from the pilot.
The report went on: "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 stuck the roof of the Clutha Vaults bar at a high rate of descent, in an upright attitude. Evidence indicates that the rotor blades and Fenestron tail rotor were not rotating at the moment of impact. The last recoded radar position for the helicopter was at 10.22pm showing it at an altitude of approximately 400ft (122m) amsl (above mean sea level)."
The AAIB said it had been closdely examining the helicopter's fuel system. There was no evidence that fuel had leaked from the aircraft before or during the impact with the pub.
The report went on: "Examination of all internal pipework and transfer passages has not revealed any pre- or post-impact failure and all paths still permit (correct) uninterrupted fuel flow.
"It has been established that unrestricted flow was also available from each supply tank to the corresponding engine fuel control unit, through the relevant fuel shut-off valves which were found set to the 'open' position."
More than 100 people were in the popular city centre bar, near the River Clyde, at the time of the crash.
Those killed in the pub were John McGarrigle, 57, Mark O'Prey, 44, Gary Arthur, 48, Colin Gibson, 33, Robert Jenkins, 61, and Samuel McGhee, 56.
Customer Joe Cusker, 59, was pulled from the wreckage alive but died in hospital from his injuries almost two weeks later. In total, more than 30 people were taken to hospital after the crash.
There have been calls for a public inquiry into the safety of all commercial flights in the UK in the wake of the incident.
Last month, it emerged that the helicopter operator had begun making interim payments to the victims.
Thompsons Solicitors, who are representing more than 50 victims of the Clutha tragedy, said the move would help their clients cover loss of salary while compensation is agreed.
John Fyall, spokesman for helicopter operator Bond Air Services, said: "This is another step in the AAIB's systematic investigative process. All concerned understand this process will be a long and complex investigation.
"While the investigation continues, we would urge against speculation. We owe it to the memories of those who died, those who were injured, and the families affected by this tragedy to help the investigation team answer as many questions as possible and discover exactly what happened.
"We continue to support the investigations and our thoughts remain with all those who have been touched by this tragedy. We also continue to work with Glasgow City Council and other parties to support bereaved families and the local community."
Examination of the engines showed "no evidence of foreign object damage or intake or exhaust blockage in either engine".
The left engine fuel filter was found to contain a small amount of fuel while the right engine was empty. Both engine fuel control units were "serviceable in all respects".
The report also said no faults were found with the transmission or rotor system, there was no evidence of structural failure or in-flight fire and no evidence of damage caused by bird strike or a foreign object hitting the aircraft in flight.
The helicopter was not required to have, and was not fitted with, flight recorders but the AAIB recovered and analysed some data recorded from installed systems.
This data included warnings triggered during the flight, but not when they occurred. There were intermittent low fuel warnings for the left fuel supply tank, then a permanent low fuel warning for the right tank.
This was followed by a further temporary low fuel warning for the left tank before it became permanent for the rest of the flight.
The report said these warnings are triggered by thermal sensors in the supply tanks and, for this helicopter, they indicated when there was approximately 32kg of fuel remaining in the left supply tank and 28kg in the right one.
On receipt of these warnings, the manufacturer's flight manual for the helicopter instructs the pilot to land within 10 minutes.
The report said measurements taken after the helicopter was taken from the pub showed 0.4kg of fuel in the left supply tank and none in the right tank.
The report said the data had also indicated the engine flame-outs but not the exact time they occurred.
The AAIB also said that the helicopter was at about 1,000ft (305m) amsl as it approached the accident area and its average ground speed was approximately 105 knots (121mph).
No CCTV recordings had been obtained which captured the end of the flight and the recorded radio transmissions did not contain any reference by the crew to difficulties with the aircraft.
The AAIB stressed today that it had not attempted an analysis of the facts in this interim report - known as a special bulletin - which it had published "to provide more factual information and an update on the progress of the investigation".
Airline pilots' group Balpa said: "Today's report is the next step in establishing what happened in Glasgow. Pilots want every flight to be safe through full and proper investigation of incidents and accidents like this one.
"We now know that the engines were not running at the time of the crash and it appears this was to do with the fuel system. However, pilots, like the AAIB, will not be satisfied until this is explored further and the exact reason for that failure is identified. The AAIB have our continued support in their work."