So
AFAIK the danger of the down-wind turn does not lie in the turn itself (with airplane unaware of wind direction), but in the reaction it might provoke.
Grrr....
The danger in a downwind turn is provided by the windshear, not the pilot's perspective.
If you take off on a runway with 90o crosswind of 5 knots, your tail wind component is zero (and you are not yet a part of the moving airmass).
If you now turn downwind and climb to 500ft, into a screaming 40 knot tail wind, the aircraft does not have time to react to that sudden wind shear ( by accellerating), and loses nearly 40 knots of airspeed. And in a light aircraft, that can be fatal - both in terms of height loss or airspeed loss.
So yes, a downwind turn can be fatal - but the lurking spectre of doom is the windshear close to the ground, and not the turn itself.
Try doing some gliding, to find out what windshear can do to an airframe.
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