I have seen wingtip vortices (condensation trails) hang in the air permanently at least one after a plane producing them has crashed - one possible occurance (though it may be a trail from something else) can be seen in "the black death" track, the other I saw while playing online.
Edited this post to revise.
Last edited by fireship4; 09-10-2011 at 12:08 AM.
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