What formula determines index units?

The formula that determines index units involves calculating a moment about a reference point, which is generally known as the 'arm' in the context of weight and balance in aviation. The correct calculation for index units is derived from the relationship between weight, arm, and a factor that helps to standardize the result, in this case, the reduction factor.

When you multiply the weight by the arm, you're determining the moment about the reference point, which gives insight into how that weight distribution affects the aircraft's balance. Dividing this product by the reduction factor adjusts the moment to a standardized index unit, making it easier to use in calculations related to weight and balance. The reduction factor allows for consistency across different aircraft and configurations, ensuring that the index units are meaningful for operational use.

Understanding this formula and its components is crucial for weight and balance calculations, essential for maintaining the safe operation of an aircraft. The other options mix up the product and quotient relationships or utilize incorrect factors, which do not correctly express the intended relationship needed to determine index units.