When should an aircraft be removed from the national registry?

The correct answer highlights the significance of an aircraft's Certificate of Airworthiness in maintaining its status on the national registry. An aircraft should be removed from the national registry if its Certificate of Airworthiness has expired because this certificate signifies that the aircraft meets the necessary standards for safe operation as set by the relevant aviation authority. Without this certification, the aircraft is deemed unfit for flight, and its registration is no longer valid, as it no longer complies with regulatory safety requirements.

The first option, concerning complete destruction or loss, is significant as well, as it would typically lead to a removal from the registry. However, the presence of an expired Certificate of Airworthiness is a more immediate cause for removing an aircraft from the registry, making it the primary focus here. Therefore, the most complete understanding of aircraft registration leads to the conclusion that an expired Certificate of Airworthiness necessitates removal from the national registry.