This paper presents the design and verification of a state estimator for a helicopter based slung load system. The estimator is designed to augment the IMU driven estimator found in many helicopter UAV's and uses vision based updates only. The process model used for the estimator is a simple 4 state acceleration driven pendulum. Sensor input for the filter is provided by a vision based system that measures the position of the slung load. The estimator needs no prior knowledge of the system as it estimates the length of the suspension system together with the system states. Finally the estimator is verified using flight data and it is shown that it is capable of reliably estimating the slung load states.