Empirical Approaches to Pilot-Induced Oscillations

Figure 21.1 is a time history of the pilot-induced oscillation that occurred during landing of the Space Shuttle Orbiter Enterprise in 1977. Pilot-induced oscillations (PIO), or airplane-pilot coupling (APC) incidents, in which pilot attempts at control create instability, are a natural subject for pilot-in-the-loop analysis and a major motivating factor for the method’s development. However, pilot-induced oscillations appeared long before advanced pilot-in-the-loop methods were in place. Engineers were obliged to improvise solutions empirically, so that airplane programs could proceed.

One cause of pilot-induced oscillations was apparent without much deep study If con­trol surface rate of movement is restricted for any reason, such as insufficient hydraulic

fluid flow rate into actuation cylinders, the pilot is unable to reverse control motion quickly enough to stop an airplane motion, once started. A late correction drives the airplane too far in the reverse direction, requiring ever-increasing control motions. Describing func­tion analysis of rate limiting does indeed show destabilizing phase lag. Thus, one empir­ical design rule for pilot-induced oscillation avoidance is high available control surface rates.

In unpublished correspondence W. H. Phillips comments on other empirical findings on pilot-induced oscillations:

We found that very light control forces together with sensitive control were very likely to lead to pilot-induced oscillations. Viscous damping on the control stick was not the answer as this put lag in the response to control force as well as the recovery. What was needed was a large force in phase with deflection for rapid stick movements, which could be allowed to wash out quite rapidly. This could be obtained with a spring and dashpot in series. Grumman

called this a “sprashpot” and used it successfully in the feel system of the F-11F________________ The

negative Cha of flap-type controls causes the control force to fall off after the airplane responds.

An additional empirical approach to solving longitudinal pilot-induced oscillation prob­lems is the double bobweight system described in Chapter 5. An aft bobweight provides heavy stick forces to start a pitch maneuver, by applying pitching acceleration forces to the stick. Stick force falls off as the airplane responds.