Rented movie disappeared from iTunes
My first suggestion would be to launch iTunes and from the menu bar select Store -> Check Available Downloads

If that does not work, just contact Apple Support. Reference your order number, and they should push the download back down to you. 

They do have a 24 hour turn around on this, so do it earlier rather than later so you'll have it for your next flight!

http://www.apple.com/support/contact/