Yea no lowering hp is a bad idea. That will be unfun for Tech and FOXTROT and will just result and more going back to base.
Bisons solution is the best solution I believe.
Let the vehicle use resources to repair just like Bison said, it's always the case when you have people who run away when a bullet scratches their paint, there are people too scared to fight with low HP, maybe that will fix them running away. Lowing the hp of helicopters is not the way to go in this situation it will only make this dreadful.
Rebels have to deal with a 3-5 hour HC on the daily basis because the repairing is abused so hard.
Bisons solution is the best solution I believe.