In this paper, we show that modal logic is a valuable tool for the formal analysis of human errors in aviation safety. We develop a modal logic called Agent Safety Logic (ASL), based on epistemic logic, doxastic logic, and a safety logic grounded in a flight safety manual. We identify a class of human error that has contributed to several aviation incidents involving a specific kind of pilot knowledge failure, and formally analyze it. The use of ASL suggests how future avionics might increase aircraft safety.