It depends a bit on what camera you have, but many have a mode (Pentax calls it "catch in focus") where the shutter will automatically fire when it is in focus. You might try that---use manual focus mode and pre-focus on a spot the plane is going to fly through, then let the camera fire on its own when it thinks it has focus.

Also, use a faster shutter speed than you probably think you need to freeze the motion of the plane. If you have enough light to use 1/1000 or 1/2000 or above, try that. A little trial and error may be needed here.