How should fuel be accounted for when weighing an aircraft?

The correct approach to accounting for fuel when weighing an aircraft involves ensuring accuracy and compliance with weight and balance requirements. By draining the system until only unusable fuel remains, or alternatively filling the tanks full and then subtracting the weight of the usable fuel, the actual weight of the aircraft can be established with precision.

This method takes into account that fuel weight can significantly impact the aircraft's overall weight and balance, which are critical for safe operation. On one hand, by leaving only unusable fuel, you're ensuring that the weight measurement reflects the empty or minimal operational state of the aircraft; on the other hand, if you choose to fill the tanks, you create a standard condition from which any operation can be assessed.

Other methods of measuring fuel weight, such as only measuring the fuel in tanks, do not provide a comprehensive understanding of the aircraft's weight when it comes to takeoff and performance calculations. Similarly, using a standard weight for fuel ignores variations in fuel density, which can lead to inaccuracies in the overall weight calculation. Instead, the chosen approach accurately reflects the actual conditions which need to be accounted for in real-flight scenarios.