Which statement defines datum in CG problems?

• A. The datum is the reference point from which CG is measured.
• B. The datum is the color of the aircraft.
• C. The datum is the tail position.
• D. The datum is the wing span length.

Answer: (A)

The datum is the reference point from which CG is measured. In weight and balance, you calculate each weight’s moment by multiplying the weight by its arm—the horizontal distance from this fixed reference point to where the weight acts. The center of gravity is the total moments divided by the total weight, which depends on those arms. The datum is an imaginary, fixed point chosen by the designer to keep calculations consistent and simple; it isn’t a physical part of the aircraft. Distances can be forward or aft of the datum, and the datum itself doesn’t change with loading.

The other options don’t fit because they describe physical characteristics (like color, tail position, or wing span) rather than the reference point used for measuring moments in CG calculations.