i believe it depends at what location/airport you intend to take pictures. So you cannot say for example a 300mm lens would do the job. If the runway and/or the aircraft is further away you might need a 400 mm lens or more. Concering my own experience, i often use a tele-lens between 200 and 300 mm. On some circumstances, when aircraft is very close, even a 80mm lens will do. Of course it is also a question of how much money you want and can afford to spend. I for myself would like to have a larger lens in the near future.
Let´s see what other photographers suggest.
Lot´s of luck picking the right lens and much success on aviation photography.