FAA Advisory Circulars
Advisory Circular Index -- Advisory circulars are listed on the FAA's website at Advisory Circular Index with links to PDF copies. See also airport-related advisory circulars.
Numbering System -- Each advisory circular has a two-part publication number, e.g., AC 150-21. The first number corresponds to a "Part" of the Federal Aviation Regulations (published in the Code of Federal Regulations), and thus serves to indicate the general subject matter of the AC. In the example, "150" is the subject number for "airport planning" (corresponding to 14 Code of Federal Regulations Part 150). An A, B, C, etc., following a circular number indicates a revised edition. Sometimes changes are made to a circular without revising the entire document; in these cases the revised pages are issued separately, and the revised pages are indicated by the circular number followed by "Chg. 1", "Chg. 2", etc. (e.g., AC 20-84 Chg. 1).