What is spiral mode and how does its stability affect aircraft behavior?

Spiral mode is a slow, non-oscillatory lateral-directional mode representing the tendency of an aircraft to roll into or out of a turn following a disturbance. Positive stability (spiral convergence) returns aircraft to wings-level; negative stability (spiral divergence) causes the aircraft to slowly roll and tighten into a turn if uncorrected. Spiral stability depends on the ratio of directional to lateral stability - strong dihedral effect (lateral stability) relative to weathercock stability promotes spiral stability. Slight spiral instability is acceptable if the time to double amplitude is long enough (>20 seconds) for easy pilot correction.