Machinbird said:
No guarantees that the Vmo/Mmo pitchup scenario was the cause, but if possible, it sure influences where to look for the aircraft and thus should be considered in the search plans.
Before you can even entertain the possibility of "a Vmo/Mmo pitchup scenario", you need to establish some premise supporting it. Otherwise it's just unfounded speculation (the worst kind). You say:
Yes, if you block the pitot inlet only, the indications will drop. But, if you block both the inlet and the tiny bleed holes drilled into the pitot tube to clear moisture from the tube, you end up with trapped air in the pitot system and that is a different ball of wax. Once you have trapped air behind the blockage, you can build pressure. Both kinds of blockages can and have happened.
Could you amplify exactly what these two propositions entail in terms of indications? (i.e. static/non-varying? / low and decreasing? / fixed? etc). I realise that a pitot tube (i.e. Thales BA) will be affected differently to a pitot/colocated static tube - and acknowledge the relevance of the drain-hole. IMHO the slowly accumulating ingested granulated super-cooled ice crystals would not coalesce inside the tube, except at the heated edges, where there'd be a boundary layer of water that would flow and keep the drain-hole clear. In each pitot, tube-central air-flow pressure sensed at the transducer would be increasingly impeded - but never blocked entirely - leading to a growing decrement of the order of a few knots initially, and many knots latterly (after about 10 minutes say). You could liken the ice-crystal blocked pitot-tubes' characteristic to blowing through a tube filled with those tiny polystyrene beads used to fill bean-bags. The fact that the autothrust and autotrim were both subtly responding to this apparent speed loss would not be apparent to a crew in normal ennui (cruise lethargy mode). Why would a crew be in lethargy mode? Nothing says that weren't just cobble-stoning along in dense high-level Cirrus, having successfully picked a path through, between any active painting cells. Nil angst, el Kapitan in crew-rest, everything ostensibly hunky dory and nominal. Such reveries are normally only ever interrupted by cabin happenings, engine faults, electrical glitches or fuel transfer problems. Haywire flight control induced upsets aren't in the syllabus.
The BEA (says CONFITURE at post 1661) has conceded (pg 53) that the automation can be duped by false airspeeds to misposition flight control surfaces (although it's a bit unclear from the fractured Franglais in the quoted BEA text below):
The reappearance of the flight directors on the PFD when two air speeds will be calculated that can lead the crew to rapidly re-engage the autopilot. However, these speeds, though of the same order, can be erroneous and low and thus lead the autopilot to command movements of the flight control surfaces that are inappropriate for the real speed of the airplane.
It's obviously the auto-trimming to a false premise (i.e. incorrect airspeed) that the BEA is discussing quite obliquely there. But when you (Machinbird) talk about bleeding off 35,000ft worth of kinetic and potential energy in around 4.2 minutes,
I'd agree that you need either very high speed/descent rate or a very high drag scenario/descent rate. Looking for continuity from my early Vmo/Mmo mach tuck scenario, I could amplify it thusly in a different direction. If you can't agree with an acceleration all the way into mach tuck, how about an acceleration to the point where the increasingly out-of-trim horizontal stabilizer (THS) finally exceeded the autopilot's force gradient holding ability to maintain FL350 (i.e. it just couldn't hold the increasingly deflected elevator loads beyond a certain point and declutched itself). Try to imagine what would've happened next.
The THS is trimmed for a lower speed and so the baro-hold is causing the autopilot (at the higher actual airspeed) to hold stick forward/elevator down pressures. At the point where the autopilot disconnects, there'll be a quite significant instant pitchup (in comparison with the shock-wave induced pitchdown that's the expected outcome of hitting mach tuck). If the speed discrepancy at AP disconnect was (say) of the order of 30 knots indicated airspeed, then the "zoomability" factor is a function of around 55kts TAS. In this case any fair animation would have to show something like a 60 degree nose-up attitude within 10 seconds - before the very surprised pilot's input could be initiated and take effect. That's an instant unusual attitude of a low IAS at high power at around 43,000ft.... and one that's quite beyond any normal airline pilot's experience. In any further imaginings into what might have happened next, think night, IMC, cascading visual and aural alerts, well out of pitch-trim (but unaware of it), in ALT2 and with engines operating at high power at high AoA in very thin air. That's almost an automatic N over root T compressor stall. Losing a donk on one side is all you'd need for a quicker entry into autorotation. But what kind of autorotation? Is the crew ever going to wake up to the fact that the THS is trimmed way out of whack? Not likely - because the pitch-up and entry into a stable flat spin took only about a maximum of 60 seconds. So the spin entry is with a very nose-up trimmed THS. Is that engine-out scenario / airframe configuration supportive of a stable flat spin? You betcha. Descent rates? Exactly equal to the TU154 at Pulkovo. Flt612 entered an area of severe turbulence, pushing up the airplane from 36,000ft to 42,000ft within just 10 seconds. The angle of attack increased to 46 degrees and the airspeed dropped to zero. (in fact the crew actually tried to outclimb the storm clouds and stalled it).
from: tinyurl.com/27acnvn
Based on various information, including the partially decrypted chat logs from a recovered flight recorder, crash investigators believe that the aircraft climbed to an altitude higher than the maximum for which it was designed, causing the aircraft to enter into a flat spin from which it never recovered.[7]
So using the BEA admissions on the THS mis-set, it's possible to logically extrapolate the likely step-by-step, very dynamic deterioration of their situation. Besides a possible engine asymmetry, further factors mitigate against recovery:
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...a. Any recovery would require the trim (THS) anomaly to be first detected then rectified.
...b. Centrifugal forces in the cockpit would be high and quite disorienting (and pax panic [back-rush aka movement aft], migrating drink carts etc in the cabin could change the CofG in a bad direction -aft). So too could any gear or flap extension.
...c. The one thing that sets a spin apart from other maneuvering within the average airline pilot's experience is that
correct anti-spin recovery controls must be set and held for quite a lengthy period in order to overcome the couples dictating the roll, pitch and yaw. That's an alien task for an airline pilot, so he's unlikely to clear that hurdle.
At this point disagreement between PF/PNF in a survival situation will only tend to further complicate and detract from extricating themselves from the predicament.
The mode of separation of the vertical stabilizer, the injury patterns and the crew compartment deformation would tend to support a high rate descent impact in a stable flat spin (about 7500 ft/min). The engines (and other bottom seeking debris mass) shouldn't be too far from the LKP.
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