Datum (Reference Datum) in aircraft weight and balance is best described as which of the following?

• A. A real physical point on the fuselage used for balance calculations.
• B. The center of gravity itself.
• C. An imaginary vertical plane or line from which all measurements of arm are taken.
• D. A fixed altitude reference used for performance calculations.

Answer: (C)

The main idea is that the datum is a reference reference line used for weight and balance calculations. It’s not a real point you can touch. Instead, it’s an imaginary vertical plane or line chosen by the aircraft designer from which the positions (arms) of all weights are measured along the aircraft’s longitudinal axis. Each weight’s arm is the distance from this datum to where that weight acts. By multiplying each weight by its arm, you get moments; summing all moments and dividing by the total weight gives the center of gravity location relative to that same datum.

So this is best described as an imaginary vertical plane or line from which all measurements of arm are taken. It’s not a physical point, not the CG itself, and not an altitude reference.