Ed; found some M0.82 cruise numbers:
Some reasonable but hypothetical speeds:
At FL350, with an MAC of 37% at 205k kgs, the IAS is 279kts, 271kts for M0.80.
Green dot, high and low speed buffet onset - A330, no center tanks:
Best lift/drag ratio speed is "Green dot", named for its symbol on the Primary Flight Display airspeed tape for 205k kgs at 350 would be 245kts, 26kts below cruise speed.
VLS, (lowest selectable speed) known as "the hook" because of the shape of the symbol, is 1.23 VS in the clean config and would be approximately 236kts or 35kts below cruise speed.
For a M0.82 buffet onset it would take a bank angle of 54deg (in a coordinated turn) to produce a 1.75g load factor for a 330 weighing 200k kg with a 40% CG
The buffet onset charts indicate that at 1g, mach buffet is well above MMO at about M0.87 perhaps a bit less, for an IAS of about 296kts or about 25kts above cruise speed.
The low speed buffet would be M0.58 at 1g, (level flight) or an IAS of about 192kts at FL350 or about 80kts below cruise speed.
...or about 100kts or so between the two buffet boundaries.
singpilot made a salient comment the other day - "trying to get into the cockpit" or something to imagine what was going on. I think a lot are trying. I'm trying to imagine what circumstances would cause the loss of 80kts or so. From 272kts, (roughly) to 192kts, (roughly), it would take a very long time even with engine thrust at idle, maybe a knot every second or two, or about a minute and a half to slow from 272 to 190.
Speed reduction occurs much more quickly with speed brakes out. With the boards up, both the VLS and stall speeds go up with speed brake use. In my experience, I have seen VLS increase by 30kts when the boards are at their maximum deflection, (usually in descent - they are rarely used if ever, in cruise unless one needs to slow the airplane down very quickly - it's not an issue, it's just rare). There is a slight pitch-up with use of speed brakes and a corresponding pitch down when they are stowed. The rate of deployment and retraction are computer-controlled and quite slow, (3-5" for up, 5-8" for down), giving lots of time to adjust pitch attitude.
While the speed calculations are from the AOM and are not unreasonable, what was done with them is entirely speculative and not knowable at present.
HarryMann;
The incipient speed-rise situation above might explain it, as perhaps would extreme turbulence. It does sound like it would have to have been a combination of turbulence + poor speed control, from his judgement on the matter?
In the article to which "UNCTIOUS" supplied the link, I saw no direct analysis of the A330 performance numbers or direct references to speeds. While I think the article is correct in its statements about "mach crit" and other notions the statements which are generic, do not bear a solid (causal) relationship to the A330 or this accident. A 100kt "distance" between the two buffet boundaries bears this assessment out. I think the statement contained in the article about "surprise" is a reasonable statement, perhaps understatement, but that alone does not account for the loss of control.
The article does make some statements that cannot be verified and are pure conjecture,
"
It's becoming apparent that whichever pilot was PF in AF447 misinterpreted the ADIRS symptoms as an aerodynamic stall and added power (possibly also increasing AoA) - with a resultant coffin corner encounter with Mach crit (which rapidly leads to uncontrollable roll and pitch excursions - see definitions below in next box)."
and that, and the absence of any analysis as it relates to the performance numbers of an A330 regarding high and low speed buffet boundaries leads one to suspect other "conclusions".
Nor am I "arguing a case"! I'm just trying to use the best numbers and a bit of thinking to see "what fits" and what isn't reasonable.