Spiral Instability: A particular case of lateral instability, in which the banking angle is always too great, causing side-slipping.
Spiral instability
An aircraft with positive spiral stability tends to roll out of a turn by itself if the controls are centred.
When an aircraft has too much static directional stabiles (or weathercock stability) in comparison to the dihedral effect spiral instability arises. In spiral divergence the airplane disturbed goes into an ever steeping spiral dive.
Because it is more desirable for the aircraft to have "spiral instability" than Dutch roll tendencies, most aircraft are designed with that characteristic.
 See also: Directional stability, Instability, Longitudinal axis, Leading, Lateral stability
  
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