Best guess, pilot disorientation from the lighting conditions.

A wing Ailron went bad in Flight?
Lost signal?

Radio conexion lost..?? or aileron servo failure

FrSky ACCST D16 v1 Uncommanded Servo Movement bug?

Unexpected barrel roll would be a classic example of it.

I would suggest that the glider caught some lift and pulled the tail of the tow plane slowing it down to stall speeds and then it snap spinned into the tree.

On second thought it was the sailplane that went wide while the tow plane was trying to circle back. This caused the glider to pull hard on the tow slowing the tow plane down to stall speeds.

I've done that water skiing behind a junky old boat. It slowed the boat down when I went wide and I could feel my weight on the line increase as centripetal forces were pulling hard on the boat.

It did seem like the glider drifted a bit to the right during the initial climb and was a fair ways out of place when the tow plane started the left turn. And the distance back to the glider seems oddly short in the view after being zoomed in on the pair. Plus the glider seems smaller than it should be given the initial relative sizes during the takeoff rollby.

So I'm also seeing the same idea as ducatirdr. I'm guessing that the line is attached to the tow model at the upper rear of the wing. A strong pull to the side from the glider being off track a fair amount to the right would slow it down and pull the model into a right roll...... or it was just coincidence and the aileron servo failed.

I’m never done aero tow. It seems to me the glider did not stay high enough. Assume the line is attached to the top of the wing, the line caught the vertical when the glider went low and to the side. That caused the tow plane to spin in. The tow plane is low and to the side. Even if it did not get caught it would pull the tow plane over as said by others.

Art

To me it looks like the classic stall/spin scenario. Tow plane should have increased airspeed when entering the downwind leg.

Originally Posted by masterscross7

A wing Ailron went bad in Flight?
Lost signal?

Originally Posted by FLYINGBOXER

Radio conexion lost..?? or aileron servo failure

Originally Posted by mawz

FrSky ACCST D16 v1 Uncommanded Servo Movement bug?

Unexpected barrel roll would be a classic example of it.

I suspect it was one of the above. The roll to the right was far too rapid and positive for it to be a stall spin. Besides, the tug was still flying at a good speed.
The glider was "in station", or certainly appeared to be. There is no apparent problem there.
Besides, the tug rolled to the right. If the glider was turning too wide, the effect of that (if any), would be to roll it to the left.

Originally Posted by mawz

FrSky ACCST D16 v1 Uncommanded Servo Movement bug?

Unexpected barrel roll would be a classic example of it.

Good one

.....hmmmmmm, tought call. The glider is slightly larger than the tug and at one point, due to size perspective, the glider appears to be far right and above the tug. At about 22 seconds the tug begins this right wing down attitude. A guess would be if the tow line cross over the vertical stab and was pulling from the left side of the vertical stab the tug pilot was fighting the rightward pull and high amp draw crashed the receiver or caused aileron servo to fault out.

All the time this is going on the glider maintains its altitude, direction, closes on the tug and appears to cut the tow at about that 22 second mark.

However, a stall appears to be more likely. The tug motor sounded anemic and was on a long roll, or it seems to me, before it was airborne. The glider being to the right and high stalled out the tug and it spun in.

Upon takeoff the glider stayed fairly high in relation to the tow plane. Right before the tow plane rolled the glider drops down then pulls back up. As the glider is rising you see the tow plane start the roll. I think the tow line got under the left side of H Stab when it dropped and then when it rose the line under the stab rolled the tow plane. The glider rise happened in concert with the roll. It doesn't look like a stall to me..

Very possible...I noticed the same as well on take off. What I did not see, or missed in the scenario you mention is the tail rising with the glider rising. But, what appears to be a right roll and dive may be as you describe.

Originally Posted by Lufo

Very possible...I noticed the same as well on take off. What I did not see, or missed in the scenario you mention is the tail rising with the glider rising. But, what appears to be a right roll and dive may be as you describe.

You see the tow plane turning left and it looks like the glider is quite a ways inside the turn circle. It's kind of hard to say for sure that the line got under the stab but it sure looks like it got caught on something on the left side. I have watched this several times. At first I was focusing on the tow plane then started watching the glider more closely.