Finally i got the reason for speed 115 knots From Bell Engineer
The airspeed limitation mentioned in the flight manual with one AP inoperative, is associated to AFCS hard over situation that could happen during flight.
The autopilot design is very strict, and the autopilot flight control authority is limited by the regulation. The general rule for an autopilot in SAS mode is that the linear actuator must not have more than 10% authority over the total flight control authority. This 10% measurement is for safety reason in case of autopilot failure in hard over situation.
In dual autopilot operation each linear actuators are sharing the work (flight control displacement) and the each linear actuator will only displace 50 % of its travel (equivalent of 5 % total flight control authority). So in case of an AFCS failure in hard over, the failed axis will have a little effect on flight control (roughly 5 % of total flight control movement) and the pilot can easily take over the control without any risk for safety of the aircraft and its crew at any airspeed.
In single autopilot mode of operation, the linear actuator can be displace as much as 100 % of its travel (equivalent of 10 % total flight control authority), So in case of an AFCS failure in hard over at an higher airspeed (over 115 kts), the failed axis will have a greater effect on flight control (this time roughly 10 % of total flight control movement), so the pilot in command will have less time to re-take control of the aircraft.
So the airspeed limitation (115 Kts) is only a safety measure allowing the pilot to take command of the flight control before the aircraft attitude become uncontrollable in the event of an AFCS failure in a hard over situation.