Just Cause

To do this quickly, first find a helicopter and gain some altitude. When you are high enough in the air to be clear of obstacles should the helicopter drift, hit A to assume the stunt position. This should cause you to jump out and grab a hold of the tail. While here, hit B to let go, then immediately hit A to grab it again.

Hit B then A again 19 more times to unlock the achievement. You do not have to re-enter the craft.

The air vehicle stunts can be done slightly different than the Land and Sea vehicles, because you are already in the air. Get yourself a helicopter, and get into the air. Find a nice open space and go quite high up. When ready, press to assume stunt postion. While holding onto the back of the helicopter, press to leave the vehicle, which will cause you to let go. Immediately press to grab back onto the vehicle, when it says you can only. Repeat another 19 times for the achievement to unlock.