Which document is unnecessary for non-international flights?

In the context of non-international flights, a radio license is often considered unnecessary because domestic flights typically operate under regulations that do not mandate the possession of a radio station license for aircraft. Non-international flights, which occur solely within a country's airspace, are generally governed by domestic aviation regulations that may not require radio communication to the same extent as international flights.

In contrast, the other documents listed play essential roles regardless of flight type. Registration documents are crucial as they verify the aircraft's ownership and its compliance with national aviation regulations. The airworthiness certificate is also vital as it certifies the aircraft's condition to ensure it is safe for flight. The pilot certificate is necessary to prove that the pilot has the appropriate qualifications and training to operate the aircraft safely.

Thus, while the radio license may be required for certain operational aspects or in specific circumstances, it is not typically a requirement for domestic flights, making it the unnecessary document in this context.