The machine was returning to its Clydeside base from a routine call-out when it plunged on to the roof of the busy Clutha Vaults pub. Eyewitnesses have described hearing a loud bang, similar to a car engine backfiring, before it came down.
The pilot, two police constables and a further seven people in the bar were killed.
An inquiry by the Air Accidents Investigation Branch has so far been inconclusive.
An initial report in December said there was no evidence of a catastrophic engine failure .
However, a second interim report in February said both engines had flamed out as a result of fuel starvation - yet there was still 76kg of fuel in the main tank.
The key mystery is why vital pumps which transfer fuel from the main tank into the engines' supply tanks were switched off, cutting off fuel flow to the engines.
The switches to control the pumps are located in the cockpit and can only be turned off manually.