Which characteristics contribute to spiral instability?

Spiral instability in an aircraft is primarily influenced by its directional stability and dihedral effect. Strong static directional stability means that the aircraft has a tendency to return to its original flight path after being disturbed, providing a stabilizing force when the aircraft is yawed. This can sometimes lead to a situation where if the aircraft yaws in one direction, instead of returning to straight flight, it might continue to yaw, leading to a gradual spiral descent.

On the other hand, the dihedral effect relates to the angle of the wings in relation to the horizontal plane. A weak dihedral effect indicates that the wings do not generate sufficient roll responsiveness to counteract yaw disturbances. In the case of strong static directional stability combined with a weak dihedral effect, the aircraft can have a tendency to enter into a spiral rather than correcting itself, as the lack of effective roll control fails to counteract the yaw.

This interaction can create a scenario in which the airplane continues to yaw and roll, making it increasingly difficult for the pilot to recover from a spiral dive, and thus exhibiting characteristics of spiral instability. Hence, the correct answer highlights the specific conditions that lead to this kind of instability in flight dynamics.