Directional stability refers to stability about which axis?

• A. The vertical axis; with the vertical tail as the primary contributor
• B. The longitudinal axis
• C. The lateral axis
• D. The yaw axis

Answer: (A)

Directional stability is the airplane’s resistance to yaw, a rotation about the vertical axis. When the aircraft sideslips or yaws slightly, the vertical tail surface generates a restoring yawing moment that tends to realign the nose with the relative wind. This weathercock behavior makes the vertical axis the key axis for directional stability, with the vertical tail providing the primary stabilizing influence. The other axes govern different motions: the longitudinal axis relates to roll stability, and the lateral axis to pitch stability. The yaw axis and vertical axis describe the same direction of rotation, but the common description emphasizes stability about the vertical axis and the vertical tail’s role.