Figure 1-16.  Pilot Operating Handbooks from manufacturers.  • Standardize implementation of a regulation or  harmonize implementation for the international  aviation community  • Resolve a general misunderstanding of a regulation  • Respond to a request from some government entity,  such as General Accounting Office, NTSB, or the  Office of the Inspector General  • Help the industry and FAA effectively implement a  regulation  • Explain requirements and limits of an FAA grant  program  • Expand on standards needed to promote aviation  safety, including the safe operation of airports  There are three parts to an AC number, as in 25-42C. The  first part of the number identifies the subject matter area  of the AC and corresponds to the appropriate 14 CFR part.  For example, an AC on “Certification: Pilots and Flight and  Ground Instructors” is numbered as AC 61-65E. Since ACs  are numbered sequentially within each subject area, the  second part of the number beginning with the dash identifies  this sequence. The third part of the number is a letter assigned  by the originating office and shows the revision sequence if  an AC is revised. The first version of an AC does not have  a revision letter. In  Figure 1-17 , this is the fifth revision, as  designated by the “E.”  The FAA, in concert with other government agencies,  orchestrates the publication and changes to publications  that are key to safe flight.  Figure 1-18  illustrates some  publications a pilot may use.  Figure 1-17.  Example of an Advisory Circular in its fifth revision.  1-11  Flight Publications 