Spiral instability exists when the static directional stability of the airplane is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium. When the lateral equilibrium of the airplane 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, will result in the bank angle becoming steeper and steeper. At the same time, the strong directional stability which yaws the airplane into the relative wind is actually forcing the nose to a lower pitch attitude. We then have the start of a slow downward spiral which, if not counteracted by the pilot, will gradually increase into a steep spiral dive. Usually the rate of divergence in the spiral motion is so gradual that the pilot can control the tendency without any difficulty.
All airplanes are affected to some degree by this characteristic although they may be inherently stable in all other normal parameters. This tendency would be indicated to the pilot by the fact that the airplane cannot be flown "hands off" indefinitely.
Much study and effort has gone into development of control devices (wing
leveler) to eliminate or at least correct this instability. Advanced stages
of this spiral condition demand that the pilot be very careful in application
of recovery controls, or excessive loads on the structure may be imposed.
Of the inflight structural failures that have occurred in general aviation
airplanes, improper recovery from this condition has probably been the
underlying cause of more fatalities than any other single factor. The reason
is that the airspeed in the spiral condition builds up rapidly, and the
application of back elevator force to reduce this speed and to pull the
nose up only "tightens the turn," increasing the load factor. The results
of the prolonged uncontrolled spiral are always the same; either in-flight
structural failure, crashing into the ground, or both. The most common
causes on record for getting into this situation are: Loss of horizon reference,
inability of the pilot to control the airplane by reference to instruments,
or a combination of both. Specific instructions on preventing loss of control
and the recovery from spirals have been explained in preceding chapters.