Reely340
With the prime pump switches on, the effect of any air sucked with fuel from the supply tanks would have effectively been reduced to zero until the fuel actually ran out. So when the engines failed due to fuel exhaustion, the effect would have been like flicking a switch to turn them off.
The rotor rpm would drop more quickly than in a simulated engine failure where the throttle is rolled off in a more gradual manner.
Once the rotor stalled the air resistance would slow rotor rotation to effectively zero in less than ten seconds.
The aerodynamic design would tend to keep the aircraft upright while falling, all the heavy bits are mounted below the plane of the rotor disk. The stalled rotors would still exhibit aerodynamic resistance.
The free fall time from 1000 feet is less than 10 seconds, however the aerodynamic resistance of the falling helicopter would probably slow the descent rate to around 20 seconds to impact the ground from 1000 feet. Hence the single return from the aircraft on ground radar showing a height of 400 feet after consistently being tracked at 1000 feet, i.e. it was pinged by radar mid fall.
The roof of the Clutha pub was in no way of flimsy construction. It had been originally constructed to take the weight of several floors of warehousing built above, but since removed. The strength of the roof probably prevented many more casualties in this tragic incident.
The primary mechanical causes of the accident are that the fuel transfer switches to the supply tanks were switched off (for reasons unknown). The prime switches were switched on (possibly mistaken fro the fuel transfer switches). When the engines failed due to fuel starvation, they ran down immediately and without warning due to the prime pumps operating to remove any entrapped air in the fuel being supplied to the engines. The rotor speed decayed significantly faster than in simulated or practise autorotations.
Additionally there may have been problems with the information the pilot was seeing on his flight display and when the second engine stopped, the battery may not have been capable of carrying the system load so effectively all the lights went out in the cockpit.
It is possible that the whole incident arose through two switches being miss selected and a display fault preventing suitable warnings being visible to the pilot. The alternative is that the pilot continued to fly the aircraft ignoring several warnings and in contravention of the directions given in the flight manual and operators procedures. That will be for a fatal accident enquiry to determine.