Which description correctly characterizes spiral instability?

• A. It occurs when the airplane's static directional stability is very strong relative to the dihedral effect
• B. It occurs when the dihedral effect is dominant
• C. It occurs when the vertical stabilizer is damaged
• D. It occurs when the airplane's static directional stability is very strong relative to the dihedral effect

Answer: (D)

Spiral instability happens when the airplane’s tendency to point its nose into the wind (static directional stability) is very strong compared with how much the wing dihedral can create a counteracting yaw when the aircraft banks. In this situation, a disturbance in yaw isn’t resisted enough by the roll-yaw coupling produced by dihedral, so yaw and roll begin to reinforce each other. The result is a tightening spiral where the aircraft banks and yaws into a descending turn rather than stabilizing back to straight-and-level flight. If dihedral effect were stronger, or directional stability were not so dominant, that coupling would tend to damp out the motion and avoid a spiral dive. Damaged vertical stabilizer is a different issue and does not describe spiral instability.