If an alteration or repair substantially affects an aircraft's operation in flight, what must occur before passengers can be carried?

When an alteration or repair significantly impacts an aircraft's operational capabilities in flight, it is necessary for the aircraft to be test flown by a qualified pilot before it can carry passengers. This requirement ensures that any modifications have not adversely affected critical flight characteristics such as handling, stability, and control.

The test flight serves as a final check to confirm that the aircraft operates safely and meets all regulatory and safety standards following the modification. This process helps identify any unexpected issues that may arise from the changes made, thus providing an additional layer of safety for passengers.

In contrast, a safety inspection may not necessarily address specific alterations or repairs, a new maintenance log is not required simply for carrying passengers post-modification, and not taking any action would compromise safety and regulatory compliance. Therefore, conducting a test flight by a qualified pilot is a crucial step to ensuring the aircraft is ready for safe passenger transport after any significant alterations or repairs.