Initial analysis shows that neither the main nor tail rotor of a police helicopter was rotating at the time the aircraft crashed through the roof of a public bar in Glasgow.
The Eurocopter EC135 hit the building’s flat roof with a high rate of descent but negligible forward speed, killing all three on board as well as six people in the bar.
About 4min before the accident the helicopter’s pilot had requested to return to Glasgow City Heliport following a 93min sortie to the south and east.
The Air Accidents Investigation Branch says that air traffic control granted clearance, but that nothing more was heard from the aircraft and radar contact was lost shortly afterwards.
Investigators state that a witness heard a noise resembling a loud “misfiring car”, followed by silence, before he saw the helicopter descending rapidly.
The inquiry has not indicated the height of the helicopter at the onset of the accident sequence, but says it had been operating at around 1,000ft altitude earlier in the flight.
It had departed the heliport with 400kg of fuel and about 95 litres were drained from the tanks after the crash.
Assessment of the powerplant showed no evidence of “major mechanical disruption” to either Turbomeca Arrius 2B2 engine, says the inquiry into the 29 November accident.
It adds that, based on an initial assessment, the main rotor gearbox was “capable of providing drive” from the number two engine power turbine to the main and tail rotors. A similar continuity check could not be carried out for the other engine, owing to impact damage.
Radar data has been retrieved. But the helicopter was not fitted with flight recorders and its on-board systems did not provide a continuous recording of parameters.
But the investigators will try to obtain information from fault codes recorded by some systems, and examine camera and audio equipment for possible clues.