Right said Diesel8
Flangemeister's 'experiment' is quite pointless to the question at hand as it would only be a study in windshear. An aircraft, while flying, is part of the air mass. The only time the 'wind' will affect its aerodynamics is if it changes abruptly - and even then, as has been pointed out, only transiently.
The consequences of a windshear would depend on a few factors including its strength and the aircraft's proximity to the ground. Turning at 800-1000 ft AGL on a typical downwind, I think would require quite a monstrous windshear to cause a crash.
Those which have occured must surely have been due the other reasons quoted in this thread.
On the other hand I may just be misinterpreting F'Meister. Can't quite figure out which arguement he's for or against.
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'Watch thine airspeed
Lest the ground arise and smite thee!'