If all index units are positive when computing weight and balance, the location of the datum would be at the:

When determining the location of the datum in weight and balance calculations, it's crucial to understand the implications of having all index units as positive. The datum is a reference point used for measuring the distance of various components of the aircraft, including payload and fuel, to ensure that the center of gravity (CG) remains within acceptable limits for safe operation.

When the index units are all positive, it indicates that all measured components are positioned forward of the datum point. In this scenario, positioning the datum at the centerline of the nose or tailwheel is logical because it allows for measurements taken from a consistent reference point along the longitudinal axis of the aircraft. This ensures that the calculations yield meaningful results and maintain a clear relationship between different components.

Placing the datum at the centerline of the nose or tailwheel also simplifies positioning for various configurations and helps effectively evaluate the impact of loading changes on the overall balance of the aircraft. Any other reference point, such as the centerline of the main wheels or the tail wingtip, may lead to confusion or complications when calculating the resulting balance and CG locations, especially if all loads are concentrated in the forward section.