Active Inceptor Failure as a Trigger for Pilot-Induced Oscillations

The use of active inceptor systems allows for control of the aircraft even following mechanical failures within the control inceptor. For the specific case of isometric failure, whereby the inceptor "freezes" in position, a virtual force-displacement model is used to continue to provide control input. Testing on DLR's experimental helicopter (active control technology/flying helicopter simulator) has shown the potential to encounter pilot-induced oscillation (PIO) tendencies when flying using this mode. This paper presents results from a simulation campaign undertaken to determine whether PIOs could be exposed through this use of control and/or the resultant severity. The results show that isometric failure caused severe PIOs. Unacceptable failure characteristics were reported by all four pilots. PIO incipience was predicted through the use of offline tools. It is recommended that specific PIO investigations be undertaken during the evaluation of active inceptor failure modes.