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• 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.” 

## Flight Publications 

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. 

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