Fault tolerant behavior of redundant flight controls as demonstrated with the sure and assist modeling programs

Abstract

A comparison and evaluation between a triple redundant and quadruple redundant flight control system (FCS) is performed through the use of NASA's ASSIST and SURE software programs. To enhance the productivity of these programs, the development of a user friendly, front-end, menu driven interface routine was initiated. The front-end tool provides a user environment based on a system block diagram concept and automatically converts the system inputs into ASSIST code. Two test case models were used to compute the Probability of Loss of Control (PLOC), the Probability Of Mission Abort (PMA), and the Probability of Hazard Condition (PHC) values for the two different architectures. The solutions obtained by these tools were compared against existing solutions obtained from approximate equations calculations for a certain set of system requirements. The data produced by these software tools displayed more accuracy then the conventional approximation approach; however, the overall system predictions remained consistent for both approaches. The quadruple architecture design surpassed the triple architecture design for the given system parameters by a significant safety factor margin. The software tools approach also demonstrated how the coverage percentage values, which represent the voter capabilities in recognizing and handling failures, significantly affected the system performance. The ability of the software tools to provide user friendly interactions that produce accurate results in a quick and easy manner make this method of predicting fault-tolerant reliabilities for flight control systems the preferred choice of analysis.