In weight and balance calculations, what does "arm" refer to?

In weight and balance calculations, "arm" specifically refers to the horizontal distance from the reference datum to the center of gravity of the aircraft. The reference datum is an imaginary vertical plane chosen by the aircraft manufacturer from which all horizontal measurements are made. The center of gravity is the point at which the total weight of the aircraft is considered to be concentrated.

Understanding the arm is crucial because it helps determine the moment, which is the product of the weight of the item (or the aircraft) and its arm. This information is essential for ensuring proper balance and stability of the aircraft during flight. Proper weight and balance calculations help prevent adverse effects on aircraft performance and handling, such as stalls or control difficulties.

In contrast, other terms in the question do not accurately define "arm." The vertical distance concept does not apply to the arm; thus, options relating to vertical measurements are not relevant. Usage of this terminology is critical in ensuring pilots can perform necessary calculations for safe operation and compliance with regulations.