If the pilot valves are inoperative, what action is not advisable?

When considering situations where the pilot valves are inoperative, pressure refueling is not advisable because these valves control the flow of fuel and maintain safe pressurization during the refueling process. If the pilot valves are nonfunctional, it raises significant safety concerns, as the system may not properly regulate the fuel, leading to potential over-pressurization or fuel spills. Pressure refueling relies on the successful operation of these valves to safely manage the fuel being introduced into the aircraft's tanks.

In contrast, gravity refueling can be conducted without the pilot valves being operational since it relies on the force of gravity rather than pressurized fuel flow. Visual checks for leaks are also prudent to ensure there are no fuel leaks during any refueling operation, while fuel flow testing could be assessed under controlled conditions, even with the pilot valves out of service. However, pressure refueling specifically necessitates fully functioning pilot valves to ensure the safe management of fuel under pressure, making it an imprudent action under these circumstances.