How is spiral stability defined in aviation terms?

Spiral stability refers to the ability of an aircraft to return to level flight after entering a banked turn. In aviation, this is an important characteristic because when an aircraft is banked, if it has good spiral stability, it will naturally want to roll back to wings level once the input that initiated the turn is removed. This tendency helps prevent the aircraft from entering potentially hazardous situations, such as a spiral dive, where the aircraft could lose altitude rapidly and become difficult to recover from.

In this context, other definitions do not accurately capture the concept of spiral stability. The ability to maintain altitude during a turn relates more to the aircraft's performance and aerodynamic design rather than its stability characteristics. Stabilization provided by rudder input focuses on yaw stability and coordination during turns rather than the aircraft's propensity to return to level flight. Lastly, resistance to changes in airspeed pertains to the aircraft's handling characteristics and stability in maintaining a constant speed rather than its ability to recover from a banked turn. Thus, the correct answer reflects the essence of spiral stability in an aircraft's flight dynamics.