The Use of System Theoretic Process Analysis (STPA) onNovel Tiltrotor Aircraft to Prevent Mode Confusion

Abstract

Initiatives are underway to develop tiltrotor and vertical take-off and lift (VTOL) aircraft that enhance commercial and military aviation’s autonomy, capability, and survivability. These designs integrate rotary and fixed-wing elements, introducing distinct safety considerations. These safety concerns are largely due to the differing mental models of operators trained in either rotary or fixed-wing aviation, alongside the rising reliance on autonomy. The traditional hazard analysis techniques (e.g., Fault Tree Analysis and Failure Models and Effects Criticality Analysis) do not adequately account for system component interactions or human factors in complex new aircraft designs. System Theoretic Process Analysis (STPA) is a powerful new hazard analysis technique for novel tiltrotor aircraft that includes their unique safety requirements. It is a top-down system hazard analysis technique that identifies loss scenarios (N. G. Leveson and J. Thomas Mar2018). It satisfies the tasks described in MIL-STD-882E (Department of Defense 2023). This research demonstrates the use of STPA to identify and mitigate potential instances of mode confusion between the operator’s mental model and the autonomy’s decision logic in the uniquely dynamic tilt-rotorcraft environment. Two previous tiltrotor aircraft accidents are analyzed utilizing Causal Analysis based on System Theory (CAST) to help set a framework for the importance of human and machine collaboration in systems. These accidents show a trend in the dangers of aircraft system mismanagement between various controllers. The CAST results for these accidents help provide information about how to prevent these types of incidents in the future, setting the stage for the use of STPA on novel tiltrotor aircraft, as demonstrated in this thesis. STPA can be used before design, implementation, and fielding, allowing for better early design of systems and reducing the cost of later redesign or modification.