Phase compensation: a means of preventing aircraft-pilot coupling caused by rate limitation

Abstract

The non-linear behaviour of rate limiting elements in flight control systems could lead to severe Pilot-Induced Oscillation (PIO) or Aircraft-Pilot Coupling problems (APC). The additional phase lag created in rate saturated conditions is the primary cause for aircraft-pilot instabilities. This problem could be reduced by compensating the phase lag via specific signal filtering. The paper describes a simple phase compensation algorithm for a rate limiting element which is derived directly from rate limiter describing function relationships. The proposed compensator uses no feedback or logic and gives adequate results for all types of inputs. The development of this new algorithm will be discussed and the performance evaluated. Further, PIO protection capability will be demonstrated on a simulated pilot-in-the-loop bank angle control task. Finally, this new phase compensator, connected to rate-limited actuators, was flight tested on DLR's In-Flight Simulator ATTAS in order to prove the capability under fly-by-wire system operations and under real flight conditions. Flight test results are presented and discussed.