What is meant by the arm of an item installed in an aircraft?

• A. The distance, in inches, between the center of gravity of the item and the datum.
• B. The vertical distance between the item's CG and a reference line.
• C. The weight of the item at the datum.
• D. The item's stiffness relative to the datum.

Answer: (A)

In weight-and-balance work, the arm is the lever that shows how far the item’s center of gravity sits from the aircraft’s reference point, called the datum. It is the distance, in inches, between the item’s CG and the datum. This distance is what turns weight into a turning effect about the datum: moment equals weight times arm. The datum is just a chosen reference, so the arm can be forward or aft of it, giving positive or negative moments as needed for balance calculations. This is why the arm is defined as that specific distance to the datum, not a vertical distance, not the weight at the datum, and not a measure of stiffness.