I don't want to spoil it too much partly because it is a short story so I am sticking with:
Scootaloo has Fibrosarcoma and would rather die with her wings than live without them.

Every rising star must eventually fall. Rainbow Dash is locked in a struggle against her own past, and with the help of Twilight is about to make a discovery that will change the way she flies forever. But what will she lose in the process?

Massive thanks to Furor1 and Nyuuchandiannepie for the ridiculously awesome artwork, you should really check them out on DeviantArt if you've not already.

Dedicated to Donald Campbell and his Bluebird, for chasing his horizons for no other reason than the fact that they were there. Also to Steve Hogarth and Marillion for writing Out Of This World, the song that inspired this story.