This issue has come from pilots reports: using certain trolleys for take off during aero tow with a tail may lead to potential danger. The matter in question - trolleys with V-shaped keel support.
When the keel tube goes too deep in the V-shaped support, the support upper points may (most probably will) catch the tail during take off.
You can see on the video:
that the keel tube rises from the support at the same time with the glider lifting off the trolley. Large deepening in the support will lead to the tail catching the upper points of the support and the glider pitching nose down and crash right after departing from the trolley. We have never discovered this problem because we are using different trolleys where the keel tube of the glider doesn’t go deep inside, the deepening for the keel tube is no deeper than the keel tube diameter. Having discovered this potential danger we recommend to pilots that aero towing with tails to use only trolleys where the deepening for the keel tube in the keel support is no bigger than the keel diameter.