On a capture the flag game mode, all you have to do is pick up the flag carrier with the air vehicle, aka a helicopter. You must be the pilot.
I joined a game, got in a chopper, landed and when my teammate with the flag jumped in with me, the achievement unlocked straight away.
Edit: If it doesn't unlock straight away, just pull up off the ground a little bit.
As requested, the maps with scout choppers on are:
Kiasar Railroad
Operation Riverside
(need confirmation whether it needs to be a scout chopper)
Re-edit: Hanrec2, MuddyAmoeba & LockieF have all stated that this achievement is sightly glitchy, in a good way though. They have stated that you can pick up someone who has held the flag and it will still unlock. Use the above method if this 'glitch' doesn't work for you.