Which FAR Part states an aircraft's registration must be displayed in the aircraft?

Part 91 of the Federal Aviation Regulations (FAR) addresses general operating and flight rules for civil aircraft in the United States. Among its stipulations, it explicitly states the requirement for an aircraft's registration number to be displayed prominently on the exterior of the aircraft. This ensures that the aircraft can be easily identified for enforcement of aviation regulations, compliance checks, and enhances safety by allowing authorities to quickly ascertain ownership and operational status.

The other parts listed, such as Parts 121, 145, and 135, focus on specific aspects of aviation regulations. Part 121 governs the operation of scheduled air carriers, Part 145 deals with the certification of repair stations, and Part 135 pertains to commuter and on-demand operations. While they each have crucial safety and regulatory requirements, they do not specifically mandate the display of an aircraft's registration number, which is a unique requirement found in Part 91.