1.) Warning shots are intended to be used in situations where a player is unable to contact a heli pilot (If they are in combat/pilot wearing a mask)
2.) It signify s you need to leave the area, if they fail to leave the area or attempt to land you may shoot them.
3.) Helicopters should fly away if they do not wish to be initiated. However after receiving warning shots should they choose to land they can immediately engage in combat without further initiation (Much like if someone shoots your car tires).