It's written for the D7000, but it's equally applicable to the D7100, the only difference being 51 rather than 39 focus points to choose from.
The D9 and D21 are the best modes for following aircraft or birds in flight. The fewer AF points in use, the less hard the AF system has to work. You can improve your keeper rate massively by practising following birds in flight. The more you practise, the better you get at keeping your subject within the relevant group of focus points. I'd recommend starting with D21, and then tightening down to D9 as your skill improves.
I'm currently perusing the D7100 manual whilst waiting on the postman ringing the doorbell to announce the arrival of mine.
In the meantime, I'm wringing the neck of my D7000's AF system chasing birds in flight.