Keep Your Back to the Sun
Airshows are especially tricky because you're trying to shoot a relatively small object against a large, bright background. If you're not careful, the camera will read the sky and say to itself, "Hey! That's bright. I'll lower the exposure to get a beautiful blue image." In the process, the camera underexposes the real subject: the airplane.
There are a couple things you can do to mitigate this problem. First, avoid shooting into the part of the sky that contains the sun. Wherever the sun happens to be, shoot with your back to it so the sun helps illuminate the plane, not create a brighter backlight behind it. As a rule of thumb, I never press the shutter release unless the plane I'm photographing is at least 90 degrees away from the sun. Next, try out different camera settings.