Nonlinear filter for pilot's remnant attenuation

The configuration of a particular nonlinear filter which does not introduce any phase shift and has good attenuation characteristics is studied for possible applications in fly-by-wire pilot/aircraft systems. The filter generates a variable coefficient with which the pilot's output signal is multiplied before it is used for the control surface actuation. Under no circumstances can there be any phase shift in the signal. In principle, the bandwidth can be made adaptive to suit a particular flight phase. A rolloff rate of 40 dB/dec can be easily obtained. The proposed filter was tested using a five degree-of-freedom, nonlinear aircraft model on an analog computer to simulate a typical high-wing single-engine aircraft. Test resuts with and without the filter are shown for a situation where the pilot is applying a step displacement localizer correction in a path-following task using an instrument landing system