Spiral stability describes the tendency for the aircraft not to roll back to level flight from a banked attitude. It occurs with the opposite conditions to those that induce dutch roll, weak lateral stability and strong directional stability.

If the controls are centralised in a banked attitude, the aircraft will develop some degree of sideslip. Stable sideslip forces will try to roll the wings level, but stable directional forces will try to yaw the helicopter to line it up with the new relative airflow and the yaw will produce a rolling moment opposing the sideslip-induced roll.

The definition of spiral instability then rests on how the aircraft, trimmed for straight and level flight will behave if the cyclic control is centralised in a turn. If the aircraft rolls out, it is positively spirally stable. If it holds the same bank angle it is neutrally stable and if the bank steadily increases, leading to a spiral dive, it is spirally unstable.