The study addresses the problem of reducing the probability of emergency situations involving unmanned aerial systems. Accidents are regarded as outcomes of combinations of events that are individually of relatively low hazard. Causal relationships are represented by fault trees, in which the root node corresponds to an accident, the leaves correspond to basic events, and intermediate nodes describe their logical combinations. Accident scenarios are associated with the minimal cut sets of the fault tree. To identify accident prevention strategies, the concept of successful-operation paths is employed. Each such path is defined as a set of nodes having a non-empty intersection with all minimal cut sets. It is assumed that preventing all events included in a successful-operation path renders the development of accident scenarios impossible.
The study additionally accounts for the fact that the same flight mission may be executed along routes of differing complexity. Route complexity influences the cost estimates of measures aimed at preventing the events that form accident-related combinations. A model example is provided that includes the assessment of the complexity of two routes, a table of mitigation costs for basic events, and the selection of an appropriate successful-operation path for accident prevention. The proposed methodology is intended to reduce the probability of the realization of accident-inducing event combinations to an acceptable level while adhering to operational constraints and the mission-specific requirements of the flight task.

Keywords: unmanned aerial systems, emergency combination of events, fault tree, route complexity, logical-probabilistic security analysis