Which fuel tank is required to be filled first during refueling?

The correct choice is to fill the main tanks first during refueling due to operational and safety protocols associated with fuel management in aircraft. The main tanks are critical for the aircraft's immediate fuel supply, as they are typically the primary source of fuel for the engines during flight.

By filling the main tanks first, pilots ensure that the aircraft maintains balance and stability during the refueling process, which is crucial for safety. The main tanks also serve as a first line of defense against potential fuel contamination or other issues. If the main tanks were filled after the auxiliary or center tanks, there could be complications in managing fuel weight distribution and ensuring that the aircraft remains within operational limits.

Furthermore, the fueling procedure is designed to optimize fuel flow and minimize the risk of spillage and system pressures when adding fuel. Thus, adhering to the practice of refueling the main tanks first is integral to safe flight operations and maintaining compliance with specified refueling protocols.