Which regulation governs the application of registration numbers on aircraft in the U.S.?

The application of registration numbers on aircraft in the United States is governed by FAR Part 45. This regulation outlines the specific requirements for identification markings, including the registration numbers that must be displayed on all civil aircraft. It specifies the size, location, and visibility of these numbers to ensure they can be easily read for identification purposes. This regulation is essential for maintaining an organized system of aircraft registration, which enhances safety and accountability in aviation.

FAR Part 23 deals primarily with the airworthiness standards for normal category airplanes, which is not directly related to registration numbers. FAR Part 91 outlines general operating and flight rules but does not focus specifically on registration markings. FAR Part 43 addresses maintenance, preventive maintenance, and alterations, which also does not pertain to how registration numbers should be applied. Thus, FAR Part 45 is the correct choice as it directly addresses the subject matter concerning the visibility and application of aircraft registration numbers.