Which document is required aboard an aircraft?

The required document aboard an aircraft is the Airworthiness Certification. This certification is essential as it indicates that the aircraft meets specific safety and design standards established by the aviation regulatory authority. It signifies that the aircraft is in a condition for safe operation and has passed all necessary inspections. Without this document, a pilot would not be able to operate the aircraft legally, as it ensures compliance with airworthiness standards.

While a pilot's logbook, flight instructor certificate, and flight plan are important documents within the context of flight operations, they are not required to be onboard the aircraft itself during flight. The pilot logbook is primarily for the pilot's personal record-keeping; the flight instructor certificate is relevant for qualifications but does not pertain to the aircraft’s airworthiness; and the flight plan, while useful for operational planning and communication with air traffic control, does not need to be physically present in the aircraft. Therefore, the Airworthiness Certification is the only document that is legally mandated to be on board.