# Spiral Divergence:
Spiral divergence or spiral instability is a condition where an aircraft is directionally very stable, but laterally very unstable. It is characterized by low angle of attack and high airspeed. When the lateral equilibrium of the aircraft is disturbed by a gust of air and a sideslip is introduced, the strong directional stability tends to yaw the nose into the resultant relative wind while the comparatively weak dihedral lags in restoring the lateral balance.
Due to this yaw, the wing on the outside of the turning moment travels forward faster than the inside wing and, as a consequence, its lift becomes greater. This produces an overbanking tendency which, if not corrected by the pilot, results in the bank angle becoming steeper and steeper. At the same time, the strong directional stability that yaws the aircraft into the relative wind is actually forcing the nose to a lower pitch attitude. A slow downward spiral begins which, if not counteracted by the pilot, gradually increases into a steep spiral dive. Usually the rate of divergence in the spiral motion is so gradual the pilot can control the tendency without any difficulty.
Many aircraft are affected to some degree by this characteristic, although they may be inherently stable in all other normal parameters. This tendency explains why an aircraft cannot be flown “hands off” indefinitely. A spiral divergence is not a type of spin because neither wing is stalled. In a spiral divergence, the aircraft will respond conventionally to the pilot's inputs to the flight controls.
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Table of Contents:
- What Is Spiral Divergence?
- Example - Spiral Divergence
- Recovery From Spiral Divergence