What rule governs the continued maintenance and alteration of aircraft?

The rule that governs the continued maintenance and alteration of aircraft is indeed found in FAR Part 43. This regulation provides the standards for maintenance, preventive maintenance, rebuilding, and alteration of an aircraft. It outlines the requirements for who may perform such maintenance, the types of maintenance that can be performed, and the necessary record-keeping protocols.

FAR Part 43 is essential because it ensures that aircraft are maintained to a standard that preserves safety and airworthiness. It encompasses guidelines and acceptable practices for inspections and repairs, directly impacting the operational safety of aircraft. The regulations help in ensuring that any modifications or maintenance performed do not compromise the aircraft's integrity or safety features.

In contrast, the other parts listed address different aspects of aviation regulations. FAR Part 21 deals with certification procedures for products and parts; FAR Part 45 relates to identification and registration marking; while FAR Part 23 focuses on airworthiness standards for normal category airplanes. Each of these parts serves a distinct purpose within the regulatory framework but does not specifically address the ongoing maintenance and alteration practices crucial for aircraft operation and safety.