A couple of weeks ago I was out on the west side of the Sacramento Valley shooting the beginning of the almond bloom. I noticed an Air Tractor crop duster working the orchards and found a good (and safe) place to park so that I could photograph him as he made his runs through the orchards.
It wasn't really part of the shoot I was working on, but I thought it would be fun to take a little time to try to catch the crop duster as he went by.
Fortunately, I was in a great spot, so I put my 100-400mm zoom on my 7D, giving me an effective focal length of 640mm, and started firing away. It was a lot of fun and the pilot waved at me several times as he went by.
I'm certainly not an expert when it comes to photographing airplanes in flight, but it was a fun bonus to the shoot.