A nonlinear simulation of a helicopter carrying a fin stabilized underslung load was developed. A human pilot model was incorporated in the simulation, for the study of the dynamics and stability of the combined helicopter-slung load-pilot system. Simulation results and their subsequent analysis show that the underslung load is stabilized by the helicopter. Analysis using extended bifurcations confirms observations from flight tests regarding the critical role of the piloting technique in determining the system's stability. Pilot attempts to counteract load lateral oscillations by applying lateral stick commands, turn the system unstable and result in pilot induced oscillations. Freezing the lateral stick causes load oscillations to completely decay, or damp down to very low residual amplitudes. Pilot time delay is found to be the main factor affecting the system's instability.