Helicopter Damage.

[narwan]

Being damaged means receiving SIGNIFICANT damage, even if it is just one point. Not every rifle bullet that physically hits the helicopter results in a point of damage. Personally I've regularly seen helicopter get hit by (small arms) fire resulting in no damage points.
As soon as you do receive one or more points of damage that means that some degree of significant damage has been done to degrade the combat effectiveness of a very expensive and complicated piece of machinery to the point it is considered wiser to return to base and have the trouble fixed than to run the risk of not getting the chopper back to base. While the fight in SP lasts for a relatively short time (minutes or hours at most), in 'reality' that chopper would preferably have to live and fight another day. So when damaged it flies back to base to get checked out/repaired.

As to damaged choppers staying on the map, it is possible if you manage to rally them each turn and fly back to the action. They'll still try to retreat off map at the end of each turn but if the choppers stay on map (they're either to slow or the map too large) it's possible to keep them on map for a while. Maybe that's what you're seeing in your game?

Narwan