What must fuel quantity indicators read during level flight when fuel is equal to the unusable supply?

When fuel quantity indicators show that the available fuel has reached the unusable supply, the correct reading during level flight is zero. This is because the unusable fuel is that portion of fuel in the tank that cannot be used by the engines, typically remaining in the tank due to design constraints, such as the shape of the tank or the location of fuel pickups.

Once the usable fuel is fully depleted and only the unusable fuel remains, the aircraft can no longer rely on any remaining fuel for engine operation, hence the fuel indicators should reflect a reading of zero. This is critical for safety and operational awareness, as pilots need to be fully informed that no usable fuel is left for the engines.

Understanding this situation helps pilots recognize when they can no longer operate the aircraft safely, reinforcing the importance of accurate fuel management and monitoring throughout flight operations.