try placing this in the waypoint where you want the heli to slow, name the chopper heli. You can set the speed by changing the value currently 10.
null = [heli,10] spawn {_air = _this select 0;_speed = _this select 1;airslow = true;while {alive _air and airslow} do {_air limitSpeed _speed;sleep 0.1;};};
when you want it to resume normal speed just place this in the next waypoint or trigger airslow = false