To get on top of a airplane go to the airport and get a chopper.

Fly above the plane when it's about to take off. When you're right above the plane with your chopper jump out and you will fall on the plane.

Then you can have some fun on top of a plane as it flies.

Another way to do it is to get a MBK Sanchez and when a plane comes drive across the ramp that's closest to the plane and fly off it aiming towards the plane.

The only way you can do it is if you really go fast off the ramp.