If the reference datum line is placed at the nose of an airplane rather than at the firewall or some other location aft of the nose, what happens to all measurement arms?

• A. All measurement arms will be in positive numbers.
• B. All measurement arms will be in negative numbers.
• C. Arms may be zero.
• D. Arms become undefined.

Answer: (A)

The measurement arm is the distance from the reference datum to where the weight is located, with a sign convention that distances aft of the datum are positive and forward are negative. If the datum line is placed at the nose, every component of the airplane lies behind that datum, so each weight is located aft of the datum. That makes all measured arms positive numbers. The arms would only be zero if a weight were exactly at the datum line, and they would be negative if any weight were forward of the datum, which isn’t the case here.