What is the maximum allowable difference in fuel between left and right tanks that can trigger the FUEL BAL annunciator?

The FUEL BAL annunciator is designed to alert the crew when there is a significant imbalance in fuel between the left and right fuel tanks. The maximum allowable difference that can trigger this warning is a critical aspect of aircraft fuel management, ensuring that both tanks are balanced to maintain optimal performance and safety.

The correct answer indicates that a 20-pound difference between the fuel levels in the left and right tanks is the threshold for activating the FUEL BAL light. This maximum allowable difference is established to prompt timely corrective actions, such as transferring fuel or adjusting fuel consumption rates, to avoid potential issues with aircraft handling and balance.

In contrast, the other options—25, 30, and 35 pounds—are beyond the threshold that would activate the FUEL BAL annunciator. These higher values would not trigger a warning, which could lead to an unrecognized fuel imbalance that may affect flight safety. Thus, 20 pounds represents the critical limit for balancing fuel between tanks, providing a necessary margin for effective flight operations.