Spiral instability is a condition that exists when the static directional stability of the airplane is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium.

• A. A condition that exists when the static directional stability of the airplane is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium
• B. A condition of poor directional stability
• C. A condition where dihedral effect is strong
• D. A condition with excessive yaw damping

Answer: (A)

Spiral instability happens when the airplane’s tendency to resist yaw (static directional stability) is very strong compared with the dihedral effect’s ability to restore the wings level in a sideslip. If a sideslip occurs, the strong directional stability resists the yaw, but the dihedral effect isn’t strong enough to produce a meaningful restoring roll. The result is a yaw that is not corrected by a roll in the opposite direction, so yaw and bank tend to continue in the same direction, creating a progressively tighter turn—a spiral descent. That’s why the statement describing directional stability being very strong relative to the dihedral effect best captures spiral instability.

The other scenarios describe different stability characteristics: poor directional stability, a very strong dihedral effect, or excessive yaw damping, each leading to different behaviors and not the persistent, turning spiral described here.