A flight investigation of pilot-induced oscillation due to rate limiting

Actuator rate limiting has been linked to the occurrence of pilot-induced oscillations (PIOs). Evidence from many of the events suggests that the airplanes were prone to PIOs before rate limiting was experienced. There is a lack of experimental data to determine if rate limiting causes PIOs, or to what extent it contributes to severity of PIOs. This paper presents the results of a limited flight test investigation of PIO due to elevator rate limiting. Three aircraft dynamics, seven rate limits, and two Head-Up Display (HUD) tracking tasks were evaluated on the US Air Force NT-33A variable stability aircraft by three pilots. The results show that: 1) for airplanes that possess inherent PIO resistance, even the most severe level of rate limiting does not consistently lead to PIO; 2) addition of forward-path filters increases the susceptibility to PIO; and 3) use of augmentation to stabilize a highly unstable design can lead to rapidly divergent PIOs when rate limiting is encountered.