Legbreaker, vortex ring is nothing new; there is nothing mysterious about it- it is a known risk of descending in the hover, made worse when descending in the hover into a confined space, and when descending quickly. Ever watch the helicopter descent in Jurassic Park? Vortex ring is the reason the helo descends so slowly into that ravine...
All it takes is a slight miscalculation of the horizontal stop point, leaving you with a longer than planned hovering descent, the need to get on the ground fast... it doesn't mean anyone made a mistake, or anything failed- it just means that things can go wrong when you have to do something dangerous (as I suspect you know better than I do...)
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