PILOT-AIRCRAFT INTERACTIONS

Pilot-induced oscillations (PIOs) are oscillations of the PAS occurring inadvertently. For high-performance aircraft with fully powered control systems, PIOs could occur rather frequently [5]. The pilot can be regarded as an adaptive controller (of the vehicle) whose capabilities exceed those of the most sophisticated unmanned control systems. The performance of the pilot and the system can be predicted under certain circumstances. The man-machine system is thus amenable to mathematical analysis. This is also because the system adjustments adopted by the pilot for uncoupled multiloop feedback systems are consistent with those of good feedback systems. For more complicated multiloop systems, the pilot model and analysis methods have a continuing practical usefulness. The mathematical models used in PAS analysis could be TF type. The inputs and outputs of the nonlinear elements (friction, breakout, nonlinear gearing) of interest in PIO can be approximated by a pair of sine waves. Before a PIO, the pilot adopts a quasistationary set of feedbacks and equalizations (phase lead/lag, gains) that amount to the performance of a good control system (stability, small error). After a PIO the airframe motion changes to a nearly sinusoidal motion. For pre-PIO adaptation, the simplified rule is |Yo| = |YpYc| ffi |y| in the vicinity of the crossover frequency 1-2 rad/s [5]. In the case of the actual PIO phase, the terminal phase of pilot adaptation is synchronous or precognitive behavior. The pilot essentially duplicates the sinusoidal with no phase lag and Yp ffi Kp. However, in most PIO cases the pilot has adapted the pre-PIO strategies. Before PIO the pilot uses visual and motion cues to perform the basic flying task. When the PIO limit cycle occurs, the dominant input is the acceleration felt by the pilot. The pilot outputs are the forces and displacements applied to the control stick/pedals. The pilot then has both force and displacement control loops within his or her neuromuscular actuator system.

While adapting to a flight operation/phase, some factors cause a sudden change in the situation dynamics [5]: (1) change in the pilot’s organization/adjustment of the system, (2) initiation of a large steady maneuver, or (3) a damper failure. Subse­quently, the oscillations build up and are sustained for a few cycles—a limit cycle exists. Soon the pilot realizes or feels this and starts controlling, and pumps in the stick commands.