Loss of tail rotor effectiveness (LTE), also known as unanticipated yaw, has been recognized as a major contributing factor in several helicopter accidents where directional control is lost. Through the helicopter flight data monitoring (HFDM) program, pilots receive constant flight evaluation reports to support LTE risk mitigation. Nevertheless, the existing LTE safety metric presents several pitfalls that hinder the reliability of the detection of violations of boundaries for flight safety. This paper presents a new framework for the development and evaluation of an expanded safety metric to enhance the detection of proximity to LTE within the HFDM program. The new metric combines three different underlying phenomena that can lead to an LTE event, that is, loss of weathercock stability, running out of pedal (tail rotor collective) for trim, and tail rotor vortex ring state. A parametric surrogate model for the detection of proximity to an LTE event is formulated by combining results from physics-based simulations with machine learning techniques. The new framework allows for operator fine-tuning of proximity to LTE boundaries, and, hence, it provides flexibility in risk management from postflight analyses. Finally, the framework is compared against the method currently used within the HFDM program, confirming its enhanced detection of the proximity to LTE.
Read full abstract