What is a key requirement for an aircraft operation that has undergone substantial changes or repairs?

For an aircraft that has undergone substantial changes or repairs, proving that it has been test flown is essential to ensure its airworthiness and operational safety. This requirement is crucial as it confirms that all alterations or repairs have been properly executed and that the aircraft performs as intended.

When an aircraft undergoes significant modifications—such as changes to the primary structure, system alterations, or alterations that affect the aircraft's performance characteristics—it's vital to conduct a test flight to assess its performance and to verify that it complies with regulatory standards. This process not only helps to identify any potential issues that may have arisen from the modifications but also reassures both the authorities and the pilot that the aircraft is safe to operate.

In contrast, flying the aircraft directly by the manufacturer or having it endorsed by any rated pilot does not guarantee that the modifications were executed correctly nor does it ensure that the aircraft is ready for safe operation. Furthermore, operating under sport pilot rules is not relevant to substantial changes; these rules provide a different set of operational parameters that do not align with the requirements following significant modifications or repairs.