Remove ability to repair helicopters, or remove 'cool down' time on heat seeker missiles.
Due to the ability to repair and the time it takes between being able to lock on with heat seekers, a decent pilot and an engineer can keep a helicopter for an entire game because they can survive a couple of hits, which gives enough time to fly out of range, repair for 15 seconds, and return.
Personally I find the little bird and Apache add very little to the game. They are fun for the person flying but bad for everyone else in the server, but I can understand why people want them. However having to spend the entire game looking up with a heat seeker just to try and keep them flying away to repair and give your team the ability to pay normally for seconds is a bit ridiculous.