If the bi-directional crossflow pump fails, what is the alternative method to balance fuel?

When the bi-directional crossflow pump fails, the alternative method to balance fuel is through gravity crossflow. This method relies on the natural force of gravity to allow fuel to flow between the wing tanks, ensuring that the fuel levels are balanced during flight.

Gravity crossflow is particularly useful in situations where powered systems are inoperable, as it does not rely on electrical power or mechanical systems. By allowing fuel to flow from a higher tank to a lower one, operators can manage fuel distribution effectively, even in the event of pump failure. This practice is crucial for maintaining the aircraft's center of gravity and ensuring safe flight operations.

The other methods listed may require functionality from powered systems or additional equipment that would not be available in the case of a crossflow pump malfunction, making them less viable options. Therefore, gravity crossflow is the most effective and immediate method for addressing fuel balance when the bi-directional crossflow pump is out of service.