Stability about the aircraft’s longitudinal axis, which extends from the nose of the aircraft to its tail, is called Lateral Stability. Positive lateral stability helps to stabilize the lateral or “rolling effect” when one wing gets lower than the wing on the opposite side of the aircraft.

Stability about the aircraft’s vertical axis (the sideways moment) is called yawing or Directional Stability. Yawing or Directional Stability is the most easily achieved stability in aircraft design. The area of the vertical fin and the sides of the fuselage aft of the CG are the prime contributors that make the aircraft act like the well known weather vane or arrow, pointing its nose into the relative wind.

Aircraft designers must ensure Positive Directional Stability by making the side surface greater aft than ahead of the CG. To provide additional Positive Stability to that provided by the fuselage, a vertical fin is added. The larger its fin size, the greater the aircraft’s directional stability.

Longitudinal Stability (pitching) is the quality that makes an aircraft stable about its lateral axis. It involves the pitching motion as the aircraft’s nose moves up and down in flight. A longitudinally unstable aircraft has a tendency to dive or climb progressively into a very steep dive or climb, or even a stall. Thus, an aircraft with Longitudinal Instability becomes difficult and sometimes dangerous to fly.

Static Longitudinal Stability, or Instability in an aircraft, is dependent upon three factors: Location of the wing with respect to the CG, location of the horizontal tail surfaces with respect to the CG and area or size of the tail surfaces are the factors providing Longitudinal Stability.

The function of the horizontal tail (among other things) is to stabilise the aeroplane around the lateral axis.