I thought only close to envelope fighter types were inherently unstable. Aren't most civilian planes fairy stable naturally? Or not?
A spiral dive is not a spin.
Aircraft such as the one described are inherently stable. However, the interaction between directional stability (e.g. tail fin & rudder) and lateral stability (e.g. rolling left or right) is such that it will enter a spiral dive if left alone for a period. This is common even today for non-jet aircraft.