Capturing the switch between point tracking and boundary avoiding pilot behaviour in a PIO event

Pilot induced oscillations (PIO) are dangerous phenomena known to have been the cause for several aircraft and rotorcraft accidents. Usually, the primary method for PIO prevention focuses on fixing the aircraft. However, a key contributor to PIO’s is incorrect pilot behaviour. Properly understanding and modelling the pilot characteristics in the moments immediately before and during a PIO is therefore of paramount importance. The present paper examines a new theory of pilot behaviour in PIOs, the so-called boundaryavoidance tracking (BAT) or more general boundary reactive control (BRC). The concept, defined initially for fixed wings, hypothesises that, in critical cases, pilots often engage in a boundary-avoidance tracking task where the goal is to avoid a hazardous parameter, such as ground impact, or a routine limit, such as an assigned minimum attitude. The BRC concept is applied to helicopters to predict a Cat I PIO event in the longitudinal axis induced by an excessive time delay between the pilot input and system response. The paper demonstrates that the pilot control aggressiveness is inversely proportional to the time it would take him to exceed a given boundarythe so-called time to boundaryand that this time threshold is proportional to the system delay. The paper proposes a hybrid pilot model for studying PIOs combining traditional pilot modelling with boundary reactive control. The hybrid pilot model is successful in reproducing the pilot behaviour observed during a PIO test in the simulator at the University of Liverpool.