Pilot induced oscillations (PIO) or more general aircraft/rotorcraft pilot couplings (APC/RPC) have always been a critical issue for the flight safety of both fixed and rotary-wing aircraft due to their unpredicted and catastrophic nature. Up to the present, the criteria employed in the prediction of rotorcraft PIO's have been largely drawn from the fixed-wing aircraft criteria. Therefore, their appropriateness to the design of civil or military helicopters may be questionable. The aim of the present paper is to obtain new insights into pilot behavior during dangerous helicopter PIO's by applying the so-called "boundary-avoidance tracking" concept from the fixed wings. The main idea of this concept is that, in dangerous situations, pilots concentrate more on avoiding hazardous boundaries threatening their safety instead of controlling point parameters as usually assumed for flying in normal conditions. The paper concentrates on modeling the pilot behavior during longitudinal axis Category I linear PIO and shows that the PIO problem can be detected much earlier by imposing tight boundaries within which the pilot can fly and by modeling the pilot awareness for such boundaries. The time available to reach the boundaries is used as a measure of pilot aggressiveness. The paper proposes new so-called boundary-avoidance charts for design purposes giving the PIO boundary as a function of pilot aggressiveness for a given configuration and flight condition. The application of these charts to the Bolkov Bo-105 and Blackhawk UH-60A helicopters shows that the UH-60 configuration is more susceptible to PIO's and requires more attention and anticipation from the pilot.

This paper is located in the AHS International 62nd Annual Forum (TL716 .A1 A512 2006 VOL.2).