An aircraft with positive spiral stability tends to roll out of a turn by itself if the controls are centred. Some light aircraft with little or no wing dihedral and a large fin tend to have strong static directional stability but are not so stable laterally. If a sideslip is introduced by turbulence – and left to their own devices – such aircraft will gradually start to bank and turn, with increasing slip and hence increasing turn rate and rapid increase in height loss. The condition is spiral instability and the process is spiral divergence which, if allowed to continue and given sufficient height, will turn into a high speed spiral dive, which often occurs when a pilot without an instrument flight rating strays into thick cloud. Neutral spiral stability is the usual aim of the designer.
It is evident that directional stability and lateral stability are coupled and to produce a balanced turn, i.e. with no slip or skid, the aileron, rudder and elevator control movements and pressures must be balanced and co-ordinated.