What must be done to determine the aircraft's empty weight if the aircraft has been weighed with fuel in its tanks?

• A. Subtract only the fuel weight from the weight.
• B. Add the fuel weight and its moment to the weight and moment.
• C. Subtract the weight of the fuel and its moment from the weight and moment of the aircraft as weighed.
• D. Ignore the fuel and use the as-weighed weight.

Answer: (C)

Removing the fuel from the weighed condition is how you get the aircraft’s true empty weight. When you weigh the airplane with fuel in the tanks, the fuel adds both weight and a moment (because it sits at a certain distance from the datum). To find the empty weight and empty moment, you must subtract the fuel’s contribution from the as-weighed weight and the as-weighed moment.

Use the simple relations:

W_empty = W_as_weighed − W_fuel

M_empty = M_as_weighed − M_fuel

For example, if the aircraft was weighed at 12,000 lb with a moment of 1,800,000 in-lb, and the fuel weighs 1,200 lb with a fuel moment of 110,000 in-lb, then:

W_empty = 12,000 − 1,200 = 10,800 lb

M_empty = 1,800,000 − 110,000 = 1,690,000 in-lb

Subtracting only weight, ignoring the moment, or adding the fuel’s weight and moment would give an incorrect empty weight and moment; you must remove the fuel’s full contribution to both weight and moment to obtain the aircraft’s true empty condition.