Hello, My name isXXXXX and I would be happy to help. One of the problems I encounter with Border Collies is that sometimes they are too smart for their own good. :-)
Clearly, Maggie has been frightened by these odd things int he sky, but since she is so smart, she does not forget and she is now immediately on guard for them. It seems from your history, that even in the absence of hang gliders, that this has affected her entire enjoyment of her outdoor experience. And you are correct that Border Collies need outdoor activity for their mental and physical health.
My advise to you is to consider treatment with an anti-anxiety medication. Prozac is sold generically as fluoxetine, and is very safe and effective for dogs with anxiety issues. It works by causing a net increase of the neurotransmitter serotonin in the brain, which contributes to feelings of contentment, while quelling feelings of fear and panic. You will need your veterinarian to prescribe it for her.
The goal would be to treat her for 6 months beyond the cessation of the fear of hang gliders (prozac can take 2-8 weeks for full effect). If all is well after 6 months, then I would gradually wean her off the medication...your veterinarian can guide you exactly how to do this properly when the time comes.
I see you are offline, hence no live chat, but if you have any feedback or further concerns, please feel free to ask in Q & A mode. Wishign you and Maggie the very best!