What is the 'arm' when dealing with weight and balance in an aircraft?

In the context of weight and balance calculations for an aircraft, the term 'arm' refers specifically to the distance from a reference point known as the datum to the center of gravity (CG) of a particular item installed in the aircraft. The datum is a reference line established by the manufacturer, and it is crucial for determining how the weight distribution affects the aircraft's performance and stability.

Understanding the arm is vital because it helps in calculating the moment, which is derived from the weight of the item multiplied by the arm distance. This information is essential for ensuring that the aircraft remains within its weight and balance limits, which is critical for safe operations. The arm measurement is typically expressed in inches or centimeters, depending on the standards used.

Other options relate to the weight and stability of the aircraft in different ways but do not accurately define what the 'arm' is in weight and balance terminology. Hence, the concept of the arm is central to weight and balance calculations and essential for maintaining aircraft control and safety.