First, and with respect to Towerdog, in neither case is the 'stall speed' zero. Why? In neither case is the wing 'flying' (other than in the legal sense), and as the wing is neither lifting nor altering the trajectory of the aircraft, it is neither stalled nor unstalled (other than inasmuch as there may or may not be some, more, or less, streamline flow - but then, what if there is streamline flow, but it's reversed, as in a tail-slide...?). So, stall speed is not zero because there can be no stall speed in the circumstances.. .. .Second (and with much respect to John Farley) the statement that 'All autopilots avoid the stall by using AOA not speed' is untrue. Autopilots do not concern themselves with flight close to the stall. They provide the human operator with a means of programming the aircraft's flight path, and rely upon the operator not programming an unsafe path which would lead to a stall.. .. .The closest we can get to an autopilot-induced stall is a V/S climb where the commanded rate exceeds the aircraft's capability, eg +9900fpm in, say, a medium jet at medium level. The autothrottles will drive to full thrust to attempt to hold the speed, the elevators will cause the nose to pitch up in an attempt to gain the RoC, but the AFCS will revert to LVL CHG and the aircraft will get no-where near the stall. If it did approach the stall, or if you did the same thing in a turboprop, for example, the autopilot would be thrown out by the stick-shaker or stick-pusher.. .. .In a modern 'bus, of course, things are a little different, as the aircraft will protect itself with reference to AoA (I'm talking of the stick-held-fully-back scenario), but here it is not the 'autopilot' which is doing this, rather there is an envelope protection system at play which happens to be part of the automatic flight controls - or, as there are no 'manual flight controls', I should say that it's simply part of the 'flight controls'.