Abstract: Conventional reliability theory is based on binary-state assumption and probability assumption . However in many cases the binary-state assumption is not acceptable ,i.e., the system failure cannot be reasonably defined in a crisp way but should be in a fuzzy way, and thus fuzzy-state assumption should be taken in place of the binary-state assumption. Profust reliability theory is a form of fuzzy reliability theory and based on the fuzzy-state assumption and the probability assumption. Therefore profust reliability is a new reliability index and can be used to comprehensively characterize system reliability behavior Failures of the longitudinal flight-by-wire system in an ACT test aircraft are divided into three classes: safety-fatal,task-fa-tal and maintenance-required,and do profust reliability analysis for this system. Further,a new index of failure criticality on top of profust unreliability is defined so that this new index is not only adequate to reasonably distinguish effects of distinct failure modes, but also able to directly account tor the effects of failure modes on system reliability behavior. The conclusion of this paper is that it is reasonable to take the profust reliability as the system design index.

Key words: failure mode, flight-by-wire system, profust reliability, coverage factor