ByteJockey,
I have no idea why your post would be deleted. That FAA link should be
required reading for anyone interested in thinking about possible causes for this accident and every driver operating at high altitude. For the uninitiated, it's filled with useful facts and info like this relating to high-altitude flying....
"In another high altitude situation, the crew decided
to use heading select mode to avoid weather while
experiencing turbulence. The steep bank angle that
resulted from this mode quickly caused slow speed
buffeting. The crew’s rapid inappropriate response
to disconnect the autopilot and over-control the
airplane into a rapid descent in poor weather exacerbated
the situation. These real world examples
provide evidence towards the need for more detailed
training in high altitude operations."
While it is all pertinent, of particular note should be areas of interest that could be directly related to the conditions the AF was operating at the time and possible factors; High Altitude Aerodynamics, Thrust Limited Condition and Recovery, Maneuvering Stability, Effects of Wx that could cause Stall or Slowdown at High Altitude, Use of Anti-Icing on Performance, Inflight Icing Stall Margins, Flight Techniques of Jet Aircraft, Human Factors and High Altitude Upsets, Multi-Engine Flame-out.
Understanding those basics of high altitude flight, and transposing what is known or probable about AF at 35,000' (
which was still relatively heavy operating near the top of it's weight-defined envelope),
that conditions were turbulent and possibly severe,
that the A/P disengaged itself and the crew were hand-flying (
without numerous protections), that there were airspeed anomalies or miscompares whether they be vertical gust, icing, turbulence or
a combination of all-induced within or (in the case of vertical gusts and turbulence)
in the clear above quickly developing convective activity, and the
possibility that the aircraft entered an area of rapidly increasing temperatures aloft as it traveled laterally through uplifted warmer moist air and it's evident that
there is no reason an aircraft need be broken, malfunctioning, or giving erroneous information to the crew to find itself in a perilous and rapidly deteriorating aerodynamic (and possibly untenable) situation if recovery is thrust limited and the attempted recovery executed in severe conditions of turbulence, icing, vertical gusts, and possible engine problems associated with loss of aerodynamic flow/control.
It doesn't matter if it's an Airbus, Boeing, or the latest whiz-bang corporate jet...no aircraft escapes these Laws of Aerodynamics
or magically isn't affected by what Mother Nature can hand it at those altitudes just because every system is working.
No aircraft is designed to fly at or near the upper limit of it's envelope without strict adherance to load-factor/bank, and AOA limits, and the difference of being near the edge of that envelope as opposed to even 5,000' lower in terms of riding through turbulence/limits on maneuvering inputs to observe them, and having excess thrust to overcome rising-temp or turbulence/vertical gust-induced airspeed degradations/fluxuations are night and day.
If the previously posted QRH numbers are anywhere close...that for that altitude and approximated weight, and assuming the crew slowed the aircraft to below its Maximum Turbulent Air Penetration Speed when the pilot messaged 10 minutes prior to the ACARS message flurry, the margin for control between overspeeding and min clean speed was merely 15 knots at that altitude and weight, and only 25 knots between overspeed and min speed/stall buffet. And within that small airspeed window load factor/
bank limits must be observed or the low-speed buffet boundary rises rapidly.
If the bank angle gets too excessive (whether turbulence or pilot-induced, and at high altitude "too excessive" is simply a
standard bank at low altitude) it's possible for the low-speed buffet boundary to rise (and it's rise corresponds to the rate of roll...i.e. very quickly in turbulence) the point it crosses over the aircraft's current speed
which will produce an immediate stall. If the bank angle continues this low-margin buffet can cross-over past it's Max Turb Speed and over the aircraft's Mmo/overspeed and the "corner" available for flight doesn't exist. At high altitude the pilot's ability to counteract turbulence-induced roll to stay within load-factor AOA limits is seriously hampered by sluggish response to control inputs. It's entirely possible that vertical gusts acting unevenly on the aircraft or roll axis occillations associated with varying turbulence amplitude can't be counteracted quicky enough by pilot inputs, and in severe turbulence, guaranteed,
because the aircraft experiencing at least momentary loss of control is part of what defines severe turbulence.
Momentarily losing control of the aircraft in severe turbulence at lower altitudes means that although for a moment the pilots couldn't counteract the environmental, outside force acting on the aircraft the aerodynamics remain far more sound, and once the outside force abates (if the aircraft isn't structurally damaged) control can be quickly regained with the help of authoratative control response and excess engine thrust availability. Since at low level the performance envelope between high and low speed buffet boundaries is far wider and bank angle/load factor limitations far less restrictive, the aircraft may never go outside the associated larger operating envelope.
This is not the case at high altitude where the aircraft is operating near to the top edge of it's envelope where speed buffet boundaries and roll limits small even in smooth air. Due to these thin margins, turbulence or vertical gust forces and fast temperature changes can quickly place the aircraft outside any or all of these limits (possibly at the same time) but when it abates it leaves the aircraft well outside it's envelope and fully in the realm of unsound aerodynamics where flight control inputs are degraded, and overcontrolling or wrong input can quickly exacerbate the problem. To make matters worse, engine thrust available to help regain the envelope is minimal or non-existant if a flame-out occurs.
For the non-pilots who have an interest in this;
It's hard to explain to the uninitiated just how differently an aircraft flies when near the top of it's operating envelope compared to just a few thousand feet lower, and what these degradations and thin margins mean. If you're near the top end of that envelope, even something as little as a unforecast, sudden rise in temperatures aloft of 5 to 10 degrees can require
immediate action (descent to a lower altitude because engine thrust isn't available in excess at altitude) because the top of the envelope is lowering...crashing down if the temp rise is quick enough.
The same goes for encountering even moderate turbulence with regards to bank/load factor limits. Turbulence at altitude is not simply a matter of comfort or breaking the airplane like it is at lower altitudes...its a matter of the aircrafts ability to remain flying as the low-speed stall margin rises up to approach your current airspeed every time the rolling action is induced. The greater/quicker the bank, the higher/quicker the rise. This is why at high altitude autopilots automatically limit roll to about 1/2 the bank (appx 15 degrees) of what it normally rolls the airplane to at lower altitude. This roll-limiting feature for most autopilots usually becomes active automatically when climbing through 30,000 to 33,000 feet and the aircraft enters the higher-altitude realm of small-margin aerodynamics.
Coupled with this of course is the whole necessity of coordination with using rudder and it's limits, dutch rolling and yaw tendencies at altitude for every type of aircraft, and how ocillations can develop during turbulence with receding and leading wing local velocities that (due to small buffet boundary margins) can actually approach low-and-high buffet at the same time on difference sides of the aircraft if they increase in amplitude.
When hand-flying, a pilot must still observe the critical limits even while counteracting turbulence/fluxuations that are trying to force (or has already forced) the aircraft past them. It's my understanding that with the A/P disengaging the way it did on the AF flight as indicated by ACARS message, even as a Fly By Wire aircraft that normally has overbanking protections built-in while being hand-flown, while operating in Alternate Law (as it was this case as indicated by another ACARS message) this particular overbanking protection (among others) is removed, and it will essentially fly and maneuver like every other aircraft.
At high altitude, heavy, and near the top of its evelope, that means "poorly". Basically, high altitude flying is potentially one of the biggest can of worms in aviation.....high altitude flying in severe weather quickly opens it, and nothing on the aircraft need be broken or malfunctioning for it to be perilous.
And if aerodynamic control is is lost and can't be regained, Air data readings...which are designed to read accurately when the aircraft is flying soundly, not in the middle of lost aerodynamics situations or possibly wildly fluxuating outside conditions as found in extreme weather/updraft/downdrafts.....will predictably be inaccurate, fluxuating, and conflicting/miscompare between systems which could easily produce some of the subsequent ACARS messages that people are assuming mean either component source failures or sensing malfunctions.