This paper will examine the frequency and distribution of central modals (can, could, may, might, must, shall, should, will, would) in aviation manuals, using a corpus-based, and predominantly quantitative, approach. For that purpose, a corpus consisting of three manuals for the BOEING 737 was compiled. The study will also explore the extent to which a mostly automated quantitative analysis of an unannotated corpus can yield helpful insights into the frequency and distribution of central modals in aviation manuals, and comment on the utility of combining quantitative and qualitative approaches. The examination will involve comparisons of frequencies of central modals, both as a group and individually, between the AC and a representative corpus of American English (FROWN), as well as between the three manuals comprising the AC.