When does a pilot's duty period terminate after domestic flight, if clearing customs is required?

The correct answer is associated with the specific regulations concerning pilot duty periods following a domestic flight that requires customs clearance. After a domestic flight, if customs is a necessary step, the pilot's duty period does not simply end when the aircraft lands. Instead, it remains in effect for a specified duration after the aircraft has reached its parking position, commonly referred to as "block in."

In this scenario, the duty period is extended to allow for the time required to complete customs formalities. Therefore, the termination of the duty period is officially recognized as occurring 30 minutes after block in when customs clearance is involved. This takes into account the necessary procedures and potential waiting times that could arise during the customs process, ensuring that regulations around pilot duty time are adhered to and that pilots have adequate time to fulfill their post-flight responsibilities without exceeding legal limits.