I noticed that whenever an airplane passes over our house, my dog starts howling and doesn't stop until the plane is a very far distance from our house.
Is this a normal behaviour?
If not what can I do to keep my dog from howling at planes?
In addition to John Cavan's answer, if you've ruled out any medical problems your dog most likely howls at the plane to make it go away - and it's working!
If planes pass over your house frequently enough, you'll want to desensitise your dog to the noise.
Try searching for airplane noises on YouTube and play them at a volume that your dog is comfortable with. Reward with lots of treats and praise.
Increase the volume in very small steps, as you will undo a lot of the work if you misjudge your dog's limits and he starts howling.
Have lots of patience, it'll likely take a while for your dog to get used to the noise and to associate it with good things happening(TM).