Puzzling Details Emerge in Glasgow Helicopter Crash

 - December 10, 2013, 3:07 PM

The Eurocopter EC135 light twin that crashed into a busy pub on November 29 in Glasgow, Scotland, was intact when it hit the roof, but neither the main rotor nor the fenestron tail rotor were rotating at that time, the UK Air Accidents Investigation Branch preliminary report reveals. The helicopter had been flying for one hour and 33 minutes when the pilot requested clearance to return to Glasgow City Heliport. No further radio transmission was received.

Four minutes later, radar contact was lost and the helicopter was seen and heard by a witness who described hearing a noise like a loud “misfiring car,” followed by silence. The helicopter struck the roof with a high rate of descent and negligible forward speed.

Investigators have found no evidence of major mechanical disruption of either Turbomeca Arrius 2B2 engine. Moreover, the main rotor gearbox retained its ability to provide drive from the number-two engine power turbine to the main rotor and to the fenestron drive shaft, they said. Twenty-five gallons of fuel remained in the tank.

The crash killed nine and injured 32.

Comments

Hessbergerj's picture

I read with interest the report on the tragic crash of the helicopter in Scotland. As an experienced pilot and training pilot (retired Okanagan Helicopters), I would like to offer a possible scenario that might have lead to the accident.

If I understand it correctly, the helicopter was technically sound during the moments prior to the crash. If this is the case, then there would have been no way that the rotors would not have been turning. So, either the report/witness is incorrect or there is perhaps one other explanation.

If the helicopter was operating with little or no forward speed, as is possible during police surveillance type work, the helicopter might have entered a condition called Ring State Vortex (RSV). RSV is the condition whereby the helicopter enters a possibly small sink rate, in a hover, or near hover, causing the vortices of the main rotor to overlap the rotor tips. The helicopter's rotor would then descend in its own, already descending column of air.

It is entirely possible that the pilot may not have had enough time to recognize the condition. At an operating altitude of about a thousand feet, he would have had mere seconds to respond. The only action possible would be to dump the collective and jam the cyclic forward to get out of the ring state and then attempt to fly away or at least land somewhere under control. Even that would probably not work anymore as the descent rate would be very high.

If he did not attempt to recover, but pulled hard on the collective, in order to arrest the descent rate another condition called over pitching would be entered. This would exacerbate the problem plus cause perhaps another one. This is now guess work on my part.

I believe that with the aircraft basically in free fall, all the normal airflows around the engines would be disturbed. It seems to me that a possible explanation for the bang would be that the airflow through the turbines may have reversed, causing compressor stall. This compressor stall might have snuffed out the fire in the engine with the accompanying bangs. in addition, if the pilot has then, through his terror of the situation, pulled the collective hard up. The resultant blade angle, combined with the lack of engine power could have stopped the blades.

I hope that this explanation might be helpful in the investigation.

BTW. I have experienced vortex ring state twice in my career, once in a Bell 212, in the Arctic and another time on a Sikorsky S-76 in Thailand. If anyone is interested in these occurrences, I would be happy to elaborate.

Joe Hessberger

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