What must each fuel quantity indicator read during level flight when the quantity of fuel remaining is equal to the unusable fuel supply?

In aviation, the fuel quantity indicator is designed to provide accurate information about the amount of usable fuel available in the aircraft's fuel system. When the quantity of fuel remaining is equal to the unusable fuel supply, it indicates that there is no fuel left that can be utilized for the operation of the engines. As a result, during level flight, the fuel quantity indicators should read zero.

This is critical for flight safety, as pilots rely on these indicators to ensure that there is enough fuel for the flight. Understanding the distinction between usable and unusable fuel is important; unusable fuel cannot be drawn from the tanks and therefore does not count towards the fuel that can be used for powering the aircraft. Therefore, in this scenario, a reading of zero on the fuel quantity indicator signifies that the aircraft is at the threshold of fuel availability, essentially alerting pilots that immediate action concerning fuel management is necessary.