The 20D with that lens is good enough for the job when used within its limitations. As you say, it's a relatively wide zoom lens so you need to be pretty close to something in order to fill the frame for aviation applications. You need to decide on the focal lengths you need for the airports you visit and start from their really. There are plenty of good lenses out there to suit most budgets, but you need to consider your needs first, eg is a 70-200 not long enough? a 100-400 too long at the short end? or is a 70-300 about right? Then you need to factor in your budget as lenses are anything from £100 - several thousand!
Equipment: 2x Canon EOS 50D; Sigma 10-20 EX DC HSM, 50-500 EX APO DG, Canon 24-105 f/4 L, Speedlite 430EX