Originally Posted by
surplus1
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Over time I’ve learned a few things about stalls but unfortunately for me that does not include any knowledge of anything known aerodynamically as a “stable stall”. If you would be kind enough to tell me what that is, I would be grateful.
In those four decades of flying the line I’ve never heard of anything known aerodynamically as a “stable stall” to which you referred in all three of your posts. Of course that doesn’t mean that it does not exist, it could well be my ignorance. Again, would you please tell me what a “stable stall” is?
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I am extremely confused as to how you could define it by the term “stable” as well as by how that aircraft would be kept in a full stall, of both wings, wings level, over a descent of some 35,000 feet in a [near] vertical trajectory until impact. Please help me to understand what would keep one of the wings from gaining some lift and the other from rolling off during the descent. Something other than ALTN law, please.
If what you’re going to tell me is “ALTN law did it”, then tell me also why, when ALTN law makes the required control input to keep a wing from rolling off, would that not be likely to induce rotation of some type [what we call a flat spin]. What control surface(s) would you expect the computer to move in its effort to keep the wings level?
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If I may be so bold as to attempt to answer these points.
An aircraft can be in a "stable stall" if it possesses adequate elevator power to hold the AOA above the critical value, retains enough roll control authority to allow the crew to maintain wings level, retains enough directional stability and/or rudder power to keep sideslip relatively constrained and, in such circustances, if the crew elects to remain in the flight condition. (I'm excluding a "locked-in" stalled condition, such as a T-tail deep stall, where the crew may not have the option, at least in the pitch axis).
Without specific knowledge of A330 high-AOA aerodynamics, and just looking at the configuration, with a low set tailplane I would expect there to be plenty of elevator power - both to pitch down if desired, but also to stay post-stall as well. Assuming AB tried to get reasonable handling characteristics for the natural stall (always a good thing to do, aerodynamically, whatever "protections" you plan on building in) then there may well be a inboard/midwing stall - perhaps from the pylon/wing area? - which would imply that some (outboard) roll control should still exist.
The question then becomes, why would the crew not recover, since I believe that the configuration shouldn't be vulnerable to the locked-in case. The answer might lie in the nature of the upset and the pitch/power unreliable airspeed procedure.
Speculation: once in the post stall regime, with a high AOA, a pitch attitude near the horizon (as the procedure calls for) might not have been enough to reduce the AOA if the flight path had dropped 30 degrees or more below the horizon. Similarly, the intermediate power (I believe) called for might not have been enough for the very high drag of the developed stall. In that scenario the crew actions of flying pitch and power and maintaining wings level could, conceivably, lead to a long, stable, stalled condition with a near-zero pitch attitude, a relatively high rate of descent and a fairly low forward speed.
In the context of that speculation I'll note that fixed pitch, apply power, is the typical low altitude "recovery from stall warning" FAA training but does absolutely nothing for you at high altitudes, where you don't have the power to just drive out of the stall and need to lower the nose. The unreliable airspeed procedure is more designed to maintain you in a stable flight condition and avoid an upset, not necssarily to provide recovery from an upset once it develops.