What point is used for measuring moment (balance) arms?

The datum is the reference point used for measuring moment or balance arms in aviation. It is an established point from which measurements are taken to determine the positions of various weights in an aircraft. By defining a datum, all weight positions can be expressed as distances from that reference point, thus facilitating the calculation of moments and ensuring the aircraft's balance.

Using the datum allows pilots and engineers to accurately assess aircraft loading and ensure that the center of gravity remains within the acceptable limits for safe operation. The moment is calculated as the product of the weight and its distance from the datum, thus determining how far a weight will act in terms of causing rotation about the aircraft's center of gravity.

In contrast, the center of gravity refers to the balance point of the aircraft, while the axis typically relates to rotation points for maneuvers. The focal point, although a term used in other contexts, does not have a specific application regarding moment arms in aviation. Thus, the datum is crucial in understanding weight distribution and balance in aircraft operations.