Don't tell anybody on this site... but my favorite hobby is building/flying/crashing remote control airplanes. I would assume shooting a real plane is about the same as shooting R/C (at a slower speed )
I would recommend AF-C with Dynamic 21 point focus. 21 point gives you a little more room for error over 9 point. 51 seems to confuse the camera too much.
I would also recommend using 'Back button focusing' (AF-ON). This moves your focus button from the shutter button to the 'AE-L/AF-L' button on the back of the camera (hence the name.. back button focusing). You'll either love it or hate it. AF-ON (for me) separates the task of focusing AND targeting the plane. On the D7100 its super easy to switch back and forth (AF-ON vs. normal shutter button) with the 'i' button.
Then you need to get a tall monopod <--- This is a MUST!!