CONF iture said:
Three actually, and that slush could effectively block all of them at the same rate, so as nobody disagree : NO warning NO caution ...
Slush? Is not the problem an ingestion of ice crystals over time within Cirro-Stratus/CirroCumulus cloud rather than a "heater OFF" style freezing of moisture at a drain hole (followed by a blockage, partial or complete of the pitot tube itself)? i.e. those high-level cloud types are just composed of ice crystals. That's the important difference.
Concentrate upon the possible difference in the two blockage processes and it may become clearer as to what actual effect it had upon automation.... and why the Thales tubes' heaters were more prone to being overcome. i.e. they were never designed to combat being hit by
already frozen ice particles. Think in terms of calories/BTU's in and out - and it's not hard to figure why the pitot heaters finally were overcome. That's the design deficiency.
It would have to be something unusual yet simultaneously common to all three pitot heads (as CONF iture said:"no alarm-raising disagree" to provoke a triple redundancy-based comparative alarm).
........a. a given concentration of ice particles (i.e. fairly thick and continuous cloud at quite a low temperature)
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........b. a TAT low enough that the accumulation rate of ice particles would ultimately overpower the heating capability of the pitot heaters.
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........c. If the accumulation/compaction rate was sufficiently subtle (i.e. slow) then the effect would be insidious. Thinking here that the speed loss seen by the system
would not be actual but
would be acted upon by autothrottle incremental increases, thereby putting the constant airspeed/mach (but
actually accelerating) aircraft ever closer to the corner of the coffin corner envelope at that height.
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When, in turbulence perhaps, the aircraft actually hit that critical mach (that you must avoid), what happened then?
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Did the autopilot disconnect? Did the pilots assume a coffin corner aerodynamic stall (because they were seeing a low IAS), and then take stall recovery action (lower the nose/cob the throttles?).
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What would the effect be of doing that? Last time I did "alt and comp" (and had a mach-induced pitch-up), I was intentionally inverted (because I had a very inquiring mind in those days). It was an interesting (and quite aerobatic) ride and I had much greater respect for MCrit after that.
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Can you ever (and have you ever?) ran this scenario in a simulator? If so, does it faithfully replicate what would happen in the airplane? Or is this subtle and insidious pitot blockage scenario a totally unforeseen circumstance, despite all the prior instances of Thales pitot induced upsets?
I have the gut feeling that to eventually determine with some certainty what happened to cause AF447's loss of control, we must first
completely understand the precipitating event.
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