The keel refers to the side area of the aircraft, or the total surface area exposed to airflow from the sides. This includes components such as the vertical stabiliser, fuselage, floats, engine nacelles, and other side-facing surfaces.

During flight, the keel area contributes to the aircraft’s directional stability and lateral stability by generating aerodynamic forces when subjected to sideslip or crosswind conditions. The size and distribution of the keel area influence the aircraft’s tendency to align itself with the relative airflow and resist unwanted yawing and rolling motions.

In general, an aircraft with greater lateral stability (stability about the longitudinal axis) will exhibit a stronger tendency to resist rolling disturbances, making it more difficult for external forces such as turbulence or crosswinds to induce an unintended bank angle.