The AAIB study does not blame any individual for the crash but states that without a cockpit voice recorder it is not possible to know what was said on board and why extra tasks were undertaken when the low fuel warnings were active.

It says the causes of the crash included the fuel in the main tank being unusable because the switches were turned off, and the aircraft not landing within 10 minutes of the warnings. It also questions why no Mayday call was received from the pilot.

It recommends that the Civil Authority Authority should make black box flight data recorders that capture data, audio and images compulsory in police helicopters.

It also suggests the CAA should consider making black box recorders mandatory for all state aircraft, including military aircraft, and suggests the European Aviation Safety Agency should make the technology compulsory for all air ambulances.

More than 100 people were enjoying a night out at the pub on November 29, 2013, when the police helicopter, which was returning to its base on the banks of the River Clyde, crashed through the roof.

The lengthy report describes in detail the helicopter’s operations that night after it took off in Glasgow at 8.44pm. It spent half an hour above the city helping to find someone who was thought to have been hit by a train, before flying east to Dalkeith, outside Edinburgh, for routine surveillance.

It turned back towards Glasgow at 9.45pm and during that leg of the journey the transfer of fuel from the main tank to the supply tank stopped, leaving only the fuel in the supply tank available to the engines.

It added: “It was concluded that this was due to the fuel transfer pumps in the main fuel tank being switched off, either simultaneously or in sequence.”

The report said one or other of the switches were sometimes switched off but there was no explanation for switching off both, which left around 76kg of fuel in the main tank inaccessible to the engines.

The reports states that lights and alarms warning of low fuel for both engines came on after 9.59pm as the aircraft approach Bothwell, Lanarkshire, which would have been heard by all three people on board.

At that point Mr Traill should have switched the pumps back on and landed within 10 minutes if the alerts remained in place. The helicopter fell on to the pub at 10.22pm after both engines failed.

John McGarrigle, whose father died, said that fact that helicopter remained in the air so long after the warnings was “baffling”, adding: “Once the fuel level reaches a certain point he has 10 minutes to find space to land. As far as I know these are the rules.

“The pilot is the only person in control of the fuel switch so far it to be on and then turned off shows he has acknowledged it. You would not expect a highly experienced pilot to miss that.”

Mr Traill, 51, died with Pc Tony Collins, 43, and Pc Kirsty Nelis, 36. The accident also claimed the lives of the pub customers John McGarrigle, 57, Mark O’Prey, 44, Gary Arthur, 48, Colin Gibson, 33, Robert Jenkins, 61, Samuel McGhee, 56 and Joe Cusker, 59. More than 30 people were injured