In software, "stable" often means that, given certain assumptions (OS interfaces, working hardware, etc), the code will not crash even if given bad inputs.

Based on my reading of this thread, in aviation, "stable" means that, given certain assumptions (fuel supply, etc), the plane will tend to return to equilibrium after pretensions along one particular axis, if you give it no inputs. Pilots in the thread, please correct me if I'm wrong.

You may think to yourself "that's stupid, stable means it doesn't crash!" But if you want to communicate with people in the field, you have to learn their language. If an astrophysicist said "what do you mean that program is stable?? It doesn't have enough hydrogen to roast a marshmallow, much less initiate and sustain fusion!" you would roll your eyes. Sure it's all the same English word, and there is even a common semantic root, but they all mean very different things in their contexts.

Planes are stable by design. Instabilities have causes:

>a rudder power control unit malfunction led to a Dutch roll oscillatory instability

>a Dutch roll incident following structural failure of the rudder

>a trainee pilot's actions violently exacerbated the Dutch roll motion and caused three of the four engines to be torn from the wings

Stability, in a technical sense, means if you push something off of equilibrium, it will return to equilibrium after some time has passed. It is unstable if it does not return to equilibrium. Less stable means it takes longer to return to equilibrium, more unstable means it diverges quicker from equilibrium.

My understanding of dutch roll is it takes only a small perturbation to push it off of equilibrium and into oscillation, the oscillation is sickening to people, and it doesn't return to equilibrium without active correction (by the pilot or an active system). A small perturbation means it happens more or less continuously. Correcting the oscillation rather than making it catastrophically worse is something only a well trained pilot can do successfully.

The stable point is not straight and level, it's lurching around sickeningly. I mean that literally, at Boeing I was told about a 707 crossing the Atlantic when the yaw damper failed. The pilot didn't crash it, but the airplane was full of vomit by the time it arrived.

To be technical, it is not stable flying straight and level. The stability point is dutch roll, like a carbureted car does when the engine rpm "hunts" at idle.