An Application of the Functional Resonance Analysis Method (FRAM) to Risk Assessment of Low-Speed Helicopter Operations to Avoid the Aerodynamic Phenomenon Known as Vortex Ring State

Abstract

This paper presents an application of the functional resonance analysis method (FRAM) to the risk assessment of low-speed legs of a helicopter flight mission. A combination of low forward speed and a specific rate of vertical descent could lead to meet a region of the helicopter’s flight envelope that shall be avoided to prevent the vortex ring state, an aerodynamic phenomenon which could cause uncontrolled high rates of descent. A fully developed vortex ring state (VRS) is characterized by an unstable situation in which the helicopter experiences uncommanded pitch and roll oscillations, with a reduced or no collective authority, and a helicopter descent rate that may approach more than 5000 feet per minute. If the recognition of VRS signs is performed early by the pilot in a condition of sufficient altitude, then the required recovery may be performed. However, some mission legs require to descend close to the ground, as the case of a firefighting mission, in which one of the fundamental task requires to fill a tank with water. This type of mission has been selected as the case study of this paper, and a qualitative risk assessment is performed using FRAM. The model developed using FRAM is systemic, where both accidents and successes are seen to emerge in the system from combinations of normal variability. Moreover, the model is complex and non-linear, providing a complete overview of functions and tasks performed by the whole system and the related couplings. The paper presents the potential events related to the mission selected and provides results in terms of possible safety barriers.