What factor is crucial when calibrating fuel quantity indicators during flight?

When calibrating fuel quantity indicators during flight, considering the unusable fuel supply is essential because it directly affects the accurate measurement of the fuel available for use. The unusable fuel refers to the portion of fuel in the tanks that cannot be drawn by the fuel system due to the design of the tanks or the fuel system, and it is critical for pilots to know the precise quantity of fuel that can be actually utilized for flight operations.

If the unusable fuel is not accurately taken into account during calibration, the indicators may overstate the amount of fuel that can be used, potentially leading to unsafe flight conditions. This is particularly important for flight operations that depend on precise fuel management, ensuring that there is sufficient fuel for the entire flight and that the aircraft can operate safely without reaching critical low fuel levels.

The other factors, such as the aircraft's weight, fuel temperature, and fuel grade, while relevant in various operational contexts, do not have the same direct impact on the calibration of fuel quantity indicators. Weight and temperature can affect fuel flow and performance, while fuel grade is more about compatibility and efficiency rather than measurement accuracy in the context of indicators. Thus, the unusable fuel supply stands out as the most critical factor for accurate calibration.