"The stall warning sounded twice in a row. The recorded parameters show a sharp fall from about 275 kt to 60 kt in the speed displayed on the left primary flight display (PFD), then a few moments later in the speed displayed on the integrated standby instrument system (ISIS)."
This is what I'd expected and earlier predicted here in this thread as being the "onset" (an eventual total pitot clog - see explanation at
post 335 (page 17) on this thread ). The DFDR was of course recording exactly what the pilots were seeing but meanwhile the aircraft's autothrust had actually been increasing power to maintain that programmed speed (and as a result of the gradual ice-crystal pitot blockage, actually exceeding that programmed speed by a considerable margin, whilst headed towards Mach Crit). But what triggered the autopilot disconnect? Was it a Mach Crit encounter or was it that the autopilot couldn't hold the increasing elevator force gradient of a system-driven mis-set THS (hoz stabilizer)? Or was it the sudden total clog of the pitots (see hail formation "exponential" analogy at my previous post).
"At 2 h 10 min 51s, the stall warning was triggered again. The thrust levers were positioned in the TO/GA detent and the PF maintained nose-up inputs. The recorded angle of attack, of around 6 degrees at the triggering of the stall warning, continued to increase. The trimmable horizontal stabilizer (THS) passed from 3 to 13 degrees nose-up in about 1 minute and remained in the latter position until the end of the flight." Over time, as they cruised in the ice crystals of Cirrus cloud (a known "pitot heat capacity" anomaly for that mark of pitot tube), the gradually clogging pitot system resulted in the autothrust incrementally applying power to stop the "apparent" speed decay. Similarly, the auto-trim maintained the nose-up trim for that programmed speed - and the autopilot offset the elevator (via "fwd stick") to hold height - as the aircraft was actually flying faster than shown. When it reached its design pitch-holding limit (i.e. the max nose-down force gradient it could hold), the autopilot gave up, and the handling pilot now had an instant unalerted surprise handful of an aircraft in Direct Law with nearly full nose-up trim and near to full power. So did the DFDR faithfully record this or did the BEA just construe (and misrepresent) it as the pilot's aft sidestick input? i.e. in the absence of any better/more logical explanation?
When it comes to high speed protection, should this crew have received wrong airspeed info indicating a high speed situation, you have protection where, once Mmo + few kts has been exceeded, you will get an auto pitch-up to try and maintain Mmo + few knots, so should this happen at slow actual airspeed, it will not be too hard to see why the pilot may have continued to pull back and continue increasing the acft's pitch angle. But my theory was that they were actually at an initially higher speed than indicated. Here (most importantly) we have to consider that after their involuntary zoom climb (due trim), the static pressure changes would thereafter have had a considerable additive (and further confusing) effect upon the blocked pitot systems and the displayed airspeed/mach. i.e.
( "The speed displayed on the left side increased sharply to 215 kt (Mach 0.68). The airplane was then at an altitude of about 37,500 ft and the recorded angle of attack was around 4 degrees.")
"the angle of attack exceeded 40 degrees" later saith the report. You have to close your eyes to this, because it's not anything those AF447 pilots would have known (i.e. no AoA display for them).
"By the time it reached the apex of the ensuing pitch-up and subsequent " bunt" (around 38,000ft), the aircraft was ACTUALLY entering into a deep stall with a forward speed of around 60kts and a high angle of attack...ultimately resulting in the 10,000ft +/minute Rate of descent at high AoA. But they'd initially responded correctly to the stall warning with TOGA power? - however that response was soon to change. Why? In Direct law, which they should now have been in, holding the stick back will maintain that stall. But why would the pilot do that back-stick thing? Perhaps they were attempting to attain level flight - and unaware that they were in Direct Law? But was there another reason and why did they then idle the TOGA thrust? Who knows for sure?
But here's a clue. In the subsequent descent with static pressure increasing and the pitots still blocked?, even though the airplane was actually stalled (complete with stick-shaker) the indicated airspeed would be increasing alarmingly - courtesy of increasing static pressure. That's my guess - and it's anyways a physical fact, Been there and done that trick with frozen trapped water in the static lines (i.e. the opposite effect of trapped dynamic pitot pressure). There's also a report on the Irish Accident Board's site about a 747 on a test flight with uncapped static lines due maint error. It's an elucidating gaelic tale that shows just how confusing the pitot-static scenario can be. See below for how much a 1000feet of altitude change is worth in terms of additional "displayed knots". Ask any instrument technician. That's what I did. He'll demo it for you on his test-bench.
As somebody said: "All this will probably come down to crew composition, very high workload, in adverse weather conditions, having to manually hand-fly an aircraft which suddenly found itself in alternate law at high altitude due to spurious information being fed to not only the flight display computers, but also the flight control protection and guidance computers, simultaneously."
Suddenly? Don't underestimate the power of surprise.
Spurious info? Maybe, but when it's what you are taught to believe (your instruments), that's what you react and respond to. You see a high and increasing airspeed and you apply backstick to attempt to control it - and you idle the throttles..... but instead you are (unbeknownst to you) embedding yourself in a deep-stall condition. Will the stall warning cease once embedded in deep-stall at 40 degs AoA?. That's my guess. That they were non-plussed by developments is obvious from the limited dialogue. Even the captain was struck dumb by what he saw. No solution was obvious in the time available - as the airspeed was seen to be much more than just "adequate" (i.e. even high - and even higher as the static pressure increased inexorably upon descent) i.e. so how could they be stalled? Unthinkable - so it wasn't even considered. It was perhaps a meteorological phenomena?). They just ran fresh out of ideas. Freeze-framed twilight zone? Been there and done that too.
Someone also said (and theShadow said earlier in his 20 May post - and last year): "You are not only dealing with conflicting airspeed info, you are also presented with multiple spurious ECAM warnings and cautions which it is sometimes hard to ignore, also depending on the alternate law protection loss which itself can be further divided in two categories, or even direct law which would mean direct side-stick to flight control input without any load protection - leading to control overload." Isn't automation wonderful?
A pitot-static system's pneumatic airspeed data (the usable output product) relies wholly upon very accurate dynamic pressure and static (i.e. ambient atmospheric) pressure inputs - and the latter changes rapidly during a descent at 10,000fpm. No digitized sourcing of that info, it's all air pressure analogue. Falsify either one (via blockage or leak) and zoom or descend and the story will be ever more confusing. Birgenair and Air Peru 757's found that to be the case. For example, with a snap-frozen static pressure (at FZLVL) the airspeed indication will wind back from 250 knots to zero over as little as 3400 feet of climb at 250kts IAS. I think that the BEA is still trying to wrap their minds around that obscure fact here (Gallic and not Gaelic closed minds). They are also (possibly) assuming that the zoom was a result of pilot input and not an aerodynamic pitch-up..... i.e. as a result of (possibly) hitting Mach Crit with an A/P disconnect and a very nose-down trimmed horizontal stabilizer (@3 degs nose-up but increasing to 13 degs nose-up due to pilot's aft sidestick inputs after top of zoom climb). But do I actually think they hit Mach Crit? No, more likely it was the excessive elevator force gradient that kicked out the autopilot and kick-started the fatal zoom sequence..
Someone also said: "Direct law is there to give the pilot more direct control of the aircraft but it still has some protection to offer - BUT at the same time the protection on offer is only as good and accurate as the information provided to the computers involved. Much more info is needed before one can create a valid picture of what went wrong when it comes to the decisions the pilots made in the last few minutes of the flight." However the change in static pressure resulting from the zoom into ever more rarified air and the instinctive attempt to maintain level flight and use backstick to reduce the ever higher displayed airspeed indicated during the ensuing descent (subsequent to the zoom climb) are
key factors dictating an inevitable entry into the unrecognized deep-stall condition. Additive to this was the dearth of info that they had to work with and little prior exposure to degraded flight control laws. And all this in night and in cloud.....
Confirmatory (for me anyway):
Did the pilot zoom climb the acft or was it caused by the automated mis-trimming in pitch? Perhaps this next statement in the report is a clue:
[I]"The airplane’s pitch attitude increased progressively beyond 10 degrees and the plane started to climb. The PF made nose-down control inputs and alternately left and right roll inputs."[/I] Perhaps the left and right roll inputs were his insufficient attempts to get the nose to drop (airline pilots rarely use more than 30 degs angle of bank due to the pax sensitivities - and in an Airbus? Perish both the thought and possibility....). When you've got a stuck elevator, or an aircraft pitching up of its own volition due to a runaway elevator pitch-trim, that's the way to go (i.e. roll the beast onto its wingtip to get the nose to drop - and drop flap). Pity they didn't think of that during the Jan 2003 Beech 1900 stuck elevator take-off accident at Charlotte NC.(52 degs nose-up at 1200feet agl).
So having read all the above, please feel free to shoot it all down. But ultimately, whether it's right or it's wrong, you have to ask yourself:
"Is the training to combat automation anomalies and its inherent malfunction complexities adequate?" As someone else said: "In alternate law - is the amount of warning signals inhibited to the bare minimum necessary to keep the tube flying? i.e. you don't need a warning that the lights in the aft toilets aren't working - while busy with a stalling conundrum...?" Note how quickly the situation described above can become completely and incomprehensibly unglued. The debate yet to come is going to be ponderous and inherently evasive. The AF447 crew were caught out by a little known pneumatics phenomenon and reacting understandably to what they saw. They died clueless as to their actual predicament but I cannot bring myself to blame them. As they said:
"We have no valid indications". They were right. Man can easily be defeated by automation. It's a burgeoning and futuristic problem. I can't shame them for being cheated of life by a system that's too conscious of cost and inconsiderate of consequence. The engineers and designers? Well they live in Never Never Land. If only the twain should meet....
On another subject, my post SR-111 invention in 1998 of satellite-uplinked recorder data is back in the limelight and I hope, with a vengeance. Wish it wasn't. But if you want to familiarize, just Google Iridian/Roadshow. Like all similar solutions to the long-winded AF447 saga, it's not as if somebody somewhere wasn't prescient. If we could just stop those holes in the Swiss cheese from aligning...... or more easily and quickly determine why they did.
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Edited to add an afterthought:
a. I've heard two different qualified opinions as to whether the acft would have ended up in Alternate Law or ultimately transitioned to Direct Law. ???
b.
"Just 20s after the captain returned to the cockpit, said the BEA, the thrust levers were set to the 'idle' position, with the engines delivering 55% of N1." i.e. Did the captain, upon entering the flight-deck, see the high (but fraudulent) IAS on descent and order the throttles to idle, understandably assuming a LOC existed and everything/anything BUT a stalled condition. You tend to take in and believe what you see on a first scan..... when the matter is urgent.