Part1 is on the Plane. Part2 is at the Parking Position(in the Air, Anchored and CanCollide off).
If Part1 touch Part2 it changes a GUI.
How it works?
Aircraft Guidance Docking System shows you if you need to stop.
I would reccomend magnitude. something like:
magnitude = (part1.Position - part2.Position).magnitude
http://wiki.roblox.com/index.php?title=Magnitude