TechJunkie wrote:I'm still trying, and failing, to learn more about how a stall could be caused by excessive speed at a flat angle of attack. I must have misunderstood something at some point because that doesn't seem possible.
A stall is when airflow separates from the wing. Jet aircraft (and some WWII fighters) also have what's called a "critical Mach number," which, for stall discussion purposes, refers to the speed where the shockwave ahead of the wing moves back onto the wing and begins to disrupt the airflow, and eventually, if speed continues to increase, will cause airflow to separate from the wing.
If you look at the bottom wing in this graphic, you see that the aircraft is only doing M.80, but the airflow over the top of the wing is moving as high as M1.0 and starting to separate from the wing.
If you want to know WHY the airflow separates, there's some pretty serious JPL-level Fluid dynamics behind it that are way over my head.
It can happen at a very low or even a zero angle of attack. The approach to both conditions (low-speed and high-speed stall) feels exactly the same to the pilot. It begins as very light ripples of turbulence, then increasingly larger bumps...then, depending on the aircraft, very violent shaking and loss of control.
The WWII Hawker Tempest, Typhoon, and the Lockheed P-38 all had problems with this because pilots would roll into a steep dive and their speed would increase to the point that the tail surfaces would stall out and there was no way for them to pull out of the dive. The P-38 would get "control reversal," which means exactly what it says...you turn the wheel to the left, you go right. Not what you want to see when you're screaming toward the Earth at 500kts. They lost a bunch of aircraft before they figured out what was happening.
Modern transport category aircraft are required to not exhibit any violent reactions during a stall, but that requires engineers to design a wing that is less fuel efficient than it otherwise could be. So, as you might guess, they are designed to meet the regulations, but not much more. I don't really recall if they are required to meet the "no violent reaction" standard on the high-speed side though. They might, I just don't remember.
In this Air France accident, the guy apparently thought he was facing a high-speed stall situation since he was getting "the onset of boundary layer separation" which is engineering terminology for "shaking like hell," AND his indicated
airspeed was steadily increasing (assuming this pitot system failure was like every other one I've ever seen), even though in fact his speed was decreasing due to his own actions.
So that's why I assume
he keep trying to pull the nose up to stop the speed increase he was seeing and prevent an impending loss of control...so he thought.