The ratio that determines the spinning characteristics of an aircraft. The mass of an aircraft can be distributed about its three axes (i.e., longitudinal, lateral, and vertical). The distribution of mass about the longitudinal axis determines the moment of inertia in the rolling plane and is denoted by A. The distribution of mass about the lateral axis determines the moment in the pitching plane and is denoted by B. The distribution of mass about the normal, or vertical, axis determines the moment of inertia in the yawing plane and is denoted by C. The ratio of the amount of inertia about the lateral axis to the moment of inertia about the longitudinal axis is called the B/A ratio
. An aircraft with a B/A ratio higher than unity will be reluctant to spin and will recover more easily. Conversely, incorrect use of controls may delay or even prevent recovery if the B/A ratio is high.