John Eacott said:
Rotor flicker/speed on a smartphone video is totally unreliable. Search for videos of stationary blades in flight, what you see on the video is unlikely to have any similarity to what actually happened.
John, don't know if you were responding to my post about counting video frames or just responding to some of the people eyeballing it at full speed. You're certainly right that there are plenty of videos out there where the camera rate was close enough to the rotor RPM to cause illusions like stopped rotor systems.
In fact, that actually happens with the TR in this video: you can see that it appears stopped at first, the rotates slowly backward, and rotates slowly forward, etc. However you can tell that even when it appears to be moving backward that it is actually turning forward because the bottom blade appears bent forward due to the scanout of the video sensor.
Also, unlike the main rotor, the tail rotor is turning at a rate that is a multiple of the video capture speed. (2550 * 65% = 27 revolutions per second, very close to the 30 frames per second the capture unit was likely using. A 206 main rotor blade is turning at 6.5 revolutions per second at 100% Nr so this is unlikely to be aliased with any modern cell phone or video camera IMHO (with two blades you would be seeing 13 blades per second so if you were capturing at 15 frames per second you might get aliasing but I don't think any modern cell phone is capturing at that rate unless it's someone trying to capture at 4K, and you can easily tell 15 fps capture from the jerkiness of the video - note that lots of the "stopped MR" videos are 4 bladed where you're getting a 4x multiplier in the # of blades passing per second)
In any case, although the TR is showing these sorts of things in this video, the main rotor is turning slowly enough you can clearly watch the blade go round and round without it showing these sorts of video artifacts.
Then Arnie Madsen said:
I am just making guesses like everyone else and I keep leaning toward VRS because how else could it drop so fast .
VRS could also explain the rpm increase because of unloaded blades and unloaded engine during the VRS
As far as I can tell MR pitch was never reduced ... so if it was a shaft or sprague failure a 206 should have mushed down as MRRPM decayed , not the sudden drop we saw.
That's what I was saying at first, it seemed weird to see the helicopter accelerate towards the surface when we would all expect a slow increase in vertical speed, like watching someone do a hovering auto. But when you realize he had only 65% Nr at the beginning of the video, it actually fits quite well. For whatever reason, he ran out of RPM fairly high up and the acceleration towards the surface was just gravity doing it's thing. We all expect to see a mush towards the ground because we're expecting flight RPM, but in this case he was already almost unpowered at the beginning of the video. Therefore I don't think it was anything like VRS - I think it was a classic case of running out of RPM because of loss of power.
Normally this is the part where I would lecture everybody about the importance of maintaining Nr right until impact "if you're gonna crash, crash with lots of RPM to pull right before impact". We had a bad fatal accident here in Boston a number of years back where a low time State Police pilot apparently stalled the rotor after an engine failure, the result being that they fell at the end of the sequence and were all killed. I immediately thought about that accident when I saw this video, but in reading people's comments I'm thinking there is a chance this was a deliberate decision on the part of the pilot:
A couple of people have mentioned that it was probably an emergency landing where he realized at the last second there were too many bystanders to make the landing where he planned and he ran out of options. That sounds pretty likely. Putting it right at the edge of the water certainly maximizes the chances of the water being shallow enabling everybody to get out of the aircraft (rather than mid channel, say). Maybe he used his RPM to get right next to the shore, rather than preserve it for a soft touchdown further from shore. A tough decision and one that I hope I don't have to ever make.
I'm hoping the pilot will be interviewed soon and and we'll get a first person account of exactly what was going on.