The damage model of helicopters is buggy beyond belief and could use improvements. For example, if you go fast enough and just gently scratch the ground, you either suffer massive damage or straight up explode. I found this out when I tried to "belly slide" on a runway.
Also, touching poles or trees with the body of your chopper causes damage way too easily while apparently the rotor can never suffer any damage no matter what you touch with it.
In addition, I once managed to explode a helicopter by just shooting about five times through it's glass window with a rifle. I never even hit the chopper itself.