Discovering means of flight strikes me as being one of mankind's greatest achievements. Using this idea in conjunction with the idea of "ghost", as is done with "Ghost of Flight", appears to me as a kind of subversion- the greatness of being able to fly, offset by the empty nothingness of ghosts.
This got me thinking about the true worth of the goals we hold so dearly. We put ourselves through so much to achieve them, and then we do just that. Then what? We look back on our lives and question if it was all worthwhile.
That's what lies at the heart of "Empty Achievements".