I think it is worth aggregating some witness statements from those who had a chance to see or hear the events unfolding and are doing their best to describe the last moments:
Eyewitnesses give accounts of Glasgow pub police helicopter crash | Glasgow & West | News | STV
Eyewitness John Bremner said he was in Trongate when he saw the helicopter drop like a 10p piece. He said: Literally in seconds it just stopped in mid-air and just fell. It dropped in the most eerie silence followed by a bang. Not even a bang, more a rumble. It wasn't that high, maybe 100ft above the Holiday Inn. It was staying there. It wasn't hovering, just there, not purposely. It dropped instantly. Like something out of a disaster movie. The lights were still on. That made it stand out. They were still on as it dropped. It just dropped. Like dropping a 10p. Followed by a rumble.
Mark Weatherall, who was in the area at the time of the crash, said: Me and my girlfriend saw it happening from about 30 seconds to a minute before it happened. We were walking through the car park at the back end of St Enoch's at the other side of Slaters. We could see a bright light shining down and saw it was a helicopter, it didn't take long to realise it was rapidly decreasing in altitude before my girlfriend said it was flying really low. Next thing we heard something pop in the helicopter which I assume was the engine or something mechanical before the helicopter just dropped out of the sky disappearing behind the building looking over Clutha. We heard a bang and saw people running towards the scene and when we got round there was a large group outside with casualties being brought out of the pub and people helping them to get out.
@crushtina 10:29 PM - 29 Nov 13
Horrible plane descending sound, big crash, people screaming and shouting on each other, sirens and megaphones at St Enoch's square. Anyone?
Rutherglen worker speaks of Clutha horror - Daily Record
Derek had his window down and no music in the car. I came up for the Holiday Inn and I remember thinking I hope the lights don’t change or I’ll be stuck here for ages, but the lights did. I was sitting looking straight at the Clutha at the point of impact. “There was no sound, it was so quiet. When the helicopter hit there were no lights either. On impact...it was like a bellow of dust that flew out the door. There was a slight echo-y sound and the dust came out the front door. I thought a chimney had collapsed.
Glasgow helicopter crash: 'They formed a human chain to pass out survivors' - Telegraph
Gordon Smart, a journalist, who was on the sixth floor of a multi-storey car park nearby. I looked above me – couldn’t work out where it was coming from. It got louder and louder. I spotted a helicopter falling out of the sky. The rotor blades were not spinning as the helicopter fell, said Mr Smart. The helicopter was turning in a strange position and dropping at a great speed. It seemed to be coming straight for the car park but changed direction and disappeared out of sight behind a block of flats
Meanwhile, Eddie Waltham, a retired fireman, was heading for the Clutha Vaults to meet a friend. I was only 50 metres away when I heard the explosion which stopped me in my tracks. When I looked up, there was a very obvious cloud of smoke and I could see it was coming from the pub. He ran towards the building. I had absolutely no idea it was a helicopter.
From their flat overlooking the Clutha, Iain and Victoria Simpson, looked on, first in shock, then in horror. We heard the big bang first of all and looked out, said Mrs Simpson.
Police killed in Glasgow helicopter crash had won bravery commendations | UK news | The Guardian
A witness told the Guardian he saw a shower of sparks fly out of the helicopter as if a firework had gone off. Craig Russell, 52, said he had been returning home from the Barras, a famous covered market near the crash site in the city's east end, soon before 10.25pm on Friday when he saw the Eurocopter EC135 T2 flying far lower than expected. It was making an odd noise as if the engine was misfiring, with the rotor blades sounding fine for several beats before an odd metallic grating noise could be heard, and that sequence of sounds continued, he said, as it flew. As Russell walked through Glasgow Cross, he said, he saw the helicopter flying westwards over the Saltmarket main road and railway bridges that run parallel to it, only a few hundred metres east of the Clutha Vaults. He added that it then flew low over a railway bridge: It was really low, as you looked up you could make out the silhouette as it passed and I said: 'Dear God, that's low, we could almost touch it'. It went over the arch at Osborne Street and then it was as if someone had fired a firework. There was a big flash of sparks. Alarmed by what he had witnessed, Russell, a volunteer with the Barras Trust charity, which runs the market, said he ran towards the King Street car parks just north of the Clutha Vaults because he suspected the helicopter might have been trying to land there. Instead, he saw people outside a nearby pub deep in conversation as if nothing had happened. By then, he was unable to see or hear the aircraft, so, reassured, he assumed it had continued flying and went home to his flat nearby. About 30 minutes later, his sister phoned him to tell him a helicopter had crashed on to the Clutha's roof.