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AF 447 Thread No. 11

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AF 447 Thread No. 11

Old 20th Nov 2013, 17:23
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@Dozy

Nearly but not quite:

The logic to trigger deployment of reverse thrust and lift dumpers required either both mainwheels to be on the ground [signalled by position switches on the landing gear torque links], or one mainwheel on the ground and a rotational wheelspeed equivalent to a ground speed of at least 72kts. Brake application depended on mainwheels spinning up to 80% of a computed reference speed, which is normally the rotational equivalent of ground speed, with special treatment during the transitional touchdown "spin up" phase. Because of the excess airspeed, the aerodynamic lift was much greater than normal, so that the ground reaction on the landing gear was very low. Taken in conjunction with the flooded runway, this provoked aquaplaning of the tyres in contact with the runway, so that they never got up to the required rotational speed, and brake application and the application of spoilers/ reverse thrust was delayed
Checking back on my source data another AI mod was to apply a small amount of dumper deflection immediately at touchdown, which killed enough lift to trigger braking but which was not enough to hamper a go around.
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Old 20th Nov 2013, 18:00
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Thanks for the quote, OG. Yes, one mainwheel has to suffice. Landing with residual bank (e.g., deliberately on the upwind main gear in a strong crosswind) provides routine ground-spoiler deployment, encouraging the downwind wheels to touch down in short order, even if the pilot leaves a lot of into-wind aileron applied (in the transition to ground law). The sudden loss of roll-spoiler effect is partly responsible, of course. With the nosewheels down, IIRC, nosewheel steering is not available above the 72 kt ground-speed, whether rudder-fine or tiller.

(Excuse the off-topic memoir.)
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Old 20th Nov 2013, 18:55
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OK465,

I would like to know. I believe the green band is indicative of 3º at nominalized conditions on a B-737. The tick line near the top represents stick shaker AoA, approach to stall.
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Old 20th Nov 2013, 19:26
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TD & Dozy,

it's sort of a trick question in that....

ALL AOA data, both HUD (BAE) & PFDs is independent of the 2 ADR modules. (However there are other HUDs that have that annoying ADR/AOA dependence for AOA display.) Same with FPVs. This aircraft PFDs always display it and have FPV capability regardless of type HUD.

The green band is the non weight based AOA for the weight based approach speed with appropriate flaps out. The tick is as TD says the onset of the shaker. The PFD also has a PLI (pitch limit indicator) that corresponds to the 'tick' AOA.

The maximum digital readout one can see in an FFS is around 30.5 degrees, this with BOTH ADR modules failed, no air data. I think it's limited by the fact the aircraft just won't sustain more than that, unlike the 330. However that could just be the point it pegs for any AOA above 30.5, until AOA is reduced, a system somewhat I believe like RetF4 envisions. I wouldn't trust the sim much beyond this point however....yet.

IMO, contrary to some opinion, it is emminently easy to utilize as a performance instrument, not a control instrument, the data is already in the box and it just takes some OPC pin activation and CDS enabling to display it.

And you don't have to ever look at it if it's otherwise objectionable to you. We know who you are.
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Old 20th Nov 2013, 20:29
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Some good points, Okie.

My aero understanding and actual college courses and actual flying with AoA indicators tells me that the "stall" or max L/D is pretty much the same as long as you don't have all the flaps and leading edge devices and such taken into account. Weight is not a factor, as the wing doesn't care according to the configuration. It's the lift curve for the configuration, not the weight.

IMHO, there's no reason to display the "bracket" as we had in the A-7D displayed all the time in the HUD. Wanna put it someplace on the crowded MFD, then fine. So in the Viper, we only got the AoA bracket with gear down for approach. And since we didn't have a lot of extra aero devices than flaps, the bracket gave a super reference for approach AoA that was fully compensated for weight, as we should realize that AoA for stall or such doesn't care about weight or gee.

Not to say that the AoA indication is a panacea, as it has to take into account of aero configuration. The display can be raw or it can be biased depending upon configuration. But it sure as hell tells you where you are and where you should be. Simple cross check with actual speed that you or your right seater calculated/looked up will help. Bit me in the butt one night on a slick runway when my trailing edge flaps were not fully deployed, but leading edge flaps were fine. So optimum AoA was higher speed than I should have been flying and I almost ran off the other end of the runway. BFD, lesson learned, as I had placed my trust in the "optimum" AoA for the approach.

The thing I keep harping on is that neat HUD that shows actual flight path vector based upon inertial data ( could be actual or a blend of pitch and some gyros). If I pulled and the FPM didn't move "up", then I was pulling too hard or was close to a stall. The pitch lines were also very comforting, especially for a night takeoff or for a carrier shot, because if that FPM was not above the "horizon line", then you were descending regardless of your basic attitude indicator display. I was taught, and flew many hours in bad weather and had to use the basic attitude and power specified/recommended. Then cross check for altitude and vertical velocity. Anyone here that didn't fly that way?

In almost 2,000 hours of HUD flying, I only had the FPM invalid once ( translate to 10,000 hours for you heavies). I knew it instantly and used the steam gauges to continue the takeoff. That was in Viper, as the A-7D used TAS and gyro attitude to provide a poor man's FPM.

I also experienced a static port freeze one day upon descending for a TACAN approach. Airspeed went to hell as I changed altitude, and altitude was frozen. BFD. I had that neat FPM and those pitch lines to tell me I was doing fine. Comforting. Set power per normal and come on down. I would have done the same thing without the HUD and inertial FPM, just as most here would have done. Basic airmanship and flying skills, ya think?
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Old 20th Nov 2013, 21:35
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Nope, it didn't work.

Originally Posted by Machinbird
Do that in roll direct and I suspect you will learn gentleness, and there might even be an additional mess in the cockpit.

Alternate1 and Alternate2 laws might work similarly in pitch, but they bear no resemblance in roll.
What relevance it has to AF447? For Finnegan's sake it didn't crash in spiral dive or rapid roll to ocean but in upright stall! That's what happens when you try to pull the aeroplane where she just can't go perfomancewise.

Originally Posted by gums
So if the FCS doesn't have an absolute AoA limit, then at least show the pilot the stall AoA.
Herein lies the rub; on 330 it is very much mach and configuration dependent. No airspeed - no mach - no meaningful alpha limit.

Originally Posted by gums
If you want some kinda "magenta line" to helpsteering
Who gives a rat's stern about my wishes? I have commented on the way I performed visual (no steering cue) and CAT3a (cue required) approaches using head-up guidance system I.A.W. what my employer expected of me.

Originally Posted by gums
We had one important backup mode - Standby Gains, which is extremely relevant to our discussions here the last three years. If air data was deemed unreliable the FCS used a fixed value for "gains".
Thanks, I stand corrected: Viper had degradation modes, different in details but principally similar to Airbus FCS laws.

Originally Posted by gums
We did not worry about a myriad of alpha stuff
Neither need Airbus pilots as long as they don't bust the lift limit.

Originally Posted by gums
Both limited the jet to stall AoA ( roughly).
...on aeroplane supported by cropped delta of relatively low aspect ratio i.e. with far larger useful AoA span (per design and intended use) than moderately swept, high aspect ratio, supercritical, cruise optimized wing of 330.

Could still run outta energy in a steep climb and momentum would allow the jet to exceed the limts because the flight controls could not react quickly enough ( very hard for our jet, and even more so for the heavies, IMHO)
It's hard to climb heavy steeply but if combat aircraft controls can not react quickly enough, what chances have the civil transport ones?

Originally Posted by gums
Otherwise, much of my views could be misunderstood.
As someone who was using HUD in airliner for a while I agree it is quite useful but I don't think it would have saved the AF447.

Originally Posted by BOAC
not sure what 'vent' means here
Typo for "went".

Originally Posted by BOAC
it does appear that the designers forgot this very important 'trigonometry' and that a jet in a fully developed stall can and did, in fact ?'vent'? below 60kts (IAS).
20/20 hindsight. No one has proven before that it can.

Originally Posted by Owain Glyndwr
With hindsight the logic is faulty, but I don't see any need to alter the logic governing validity of AoA signals which might well bring on other problems we know nothing of. Instead I would propose a change of stall warning logic so that if triggered by a valid AoA signal it should stay latched on until it received another valid AoA signal that stall conditions were a thing of the past.
Spot on. Instead of introducing WoW signal into stall warning logic and creating another possible point of failure, not completely dissimilar to one affecting Spanair 5022, this one could be done by simple software code change. However...

Originally Posted by Owain Glyndwr
That would have avoided the confusion found in the AF447 cockpit for a minimal change to the aircraft systems
...if there was confusion about everything else, at least actions on the stick show that CM2 firmly believed that stall warning can be resolved by pulling.

Originally Posted by BOAC
However, why WoW was 'abandoned' I do not know
It was never there. If you already must have speed and mach input to calculate critical angle of attack, it makes sense to simplify it by just putting in a couple of lines of code that cut alpha readouts at speeds that are completely unsustainable.

Originally Posted by BOAC
but I cannot see why posters are going on about 'non-swivelling pitot tubes'
I am just mocking the inability of some to understand pitot tubes reliably measure only total pressure resulting from airstream approximately parallel to aeroplane's longitudinal axis. Such a design has been with us since they have first been attached to aeroplane and everyone except a couple of PPRUNers finds it fine.

Originally Posted by BOAC
Surely we all understand why the IAS reading was invalid?
Those who think IAS reading below 60 kt was really invalid - not.

Originally Posted by Dozy Wannabe
In response to that, Airbus reduced the required pressure on the WoW sensors
I don't recall anything about this. What I remember is that post-Warsaw spoiler logic on 320 was changed inasmuch selecting reverse cracked open the spoilers thereby killing a bit of lift and compressing both struts. The rest remained unchanged and full deployment was still only available when both MLG WOWs detected landing. Way the Airbus spoilers designed, WOW signal is a must to prevent airborne activation of ground spoilers.

Originally Posted by Dozy Wannabe
OG, I'm almost certain you're correct in that the <60kts AoA NCD flag has nothing to do with determining air/ground mode - it's purely an attempt to enforce the AoA vane manufacturer's specified limitation of minimum 60kts airspeed for valid data, and prevent junk data from being used in FCS calculations.
Interesting... I'd like to hear more about about junk data when Airbus flies slower than 60 kt, I mean why is this problem not solved yet by introducing the alpha probe that can work reliably below 60 kt?

Originally Posted by gums
Basic airmanship and flying skills, ya think?
Yes.
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Old 20th Nov 2013, 21:59
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Originally Posted by Clandestino
Spot on. Instead of introducing WoW signal into stall warning logic...
WoW is probably not part of "stall warning logic" outside of air vs. ground mode operation. The argument seems to be based on the misapprehension that the <60kts NCD flag was implemented instead of WoW to determine AoA vane validity, which is a fallacy. It is far more likely that both inputs are used.

I don't recall anything about this. What I remember is that post-Warsaw spoiler logic on 320...
I was going on the Wiki page, which stated:

Originally Posted by Wiki
As a result of the accident, Airbus Industrie changed the required compression value from 6.3 tons to just 2 tons per main landing gear.
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Old 20th Nov 2013, 23:20
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Me thinks that Cland and Gums are on the same page for most of the philosophy here.

I can also appreciate the enginnering inputs from Doze, but saw many well-intentioned "engineers" suggest inputs to our FBW system and other stuff that we operators threw out. They were mainly "protections" and such, including a FCS "pull up" if Hal thot we were gonna auger if we exceeded pitch and altitude and such trying to hit our target. (99.9% of we pilots threw the notion out. There comes a time when "pressing" just a little bit makes the difference, and ya take your chances.) So we opted for a big flashing "X" in the HUD when HAL thot we were pressing too hard. Ignore it at your peril, and I have HUD film of a friend that ignored and paid the the price.

The mentality ( attn Doze) that the system acted like it was designed has some problems if the aircrews don't fully understand the "system". And as I have oft-repeated, the 'bus reversions modes are complicated and should be more straightforward. The basic FCS system should be basic enough in the reversion modes to HELP the pilot and not confuse the pilot about what protection is still here and what isn't. And as Cland and I seem to agree, there's a huge role for basic airmanship.

@ Cland:

- AoA is, indeed, a factor at mach approaching "critical" mach and such. The airfoils nowadays are much smoother than the old ones I flew. Only flew three that had significant warnings you were gonna get into trouble. And I admit that I flew airfoils that could exceed the mach ( M=1.0) without losing control of the jet.

- Aspect ratio and the airfoil shape of four of the jets I flew definitely made thos jets more tolerant of doing strange things when flying at "the limit: as the AF447 were doing when things went south.

- Running outta "E" is still a player, and the Russian crash might show that when the crew did a go-around to an extreme pitch attitude. But that's another thread.

It took us some very extreme pitch attitude/power combinations to run outta "E", and then exceed the aero surfaces' capabilities to keep us under control. I just can't see such with the heavies without serious airmanship problems.

I appreciate the oppo to add to this discussion, and have learned a lot from the heavy pilots here.
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Old 21st Nov 2013, 00:25
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Originally Posted by gums
The mentality ( attn Doze) that the system acted like it was designed has some problems if the aircrews don't fully understand the "system".
The "system" makes the A330 behave like any other aircraft. If you keep pulling up and let the speed decay, you will stall. F/O Bonin was a sailplane pilot of considerable skill - at some fundamental level, he understood this. If some folk want to make an argument that he was pulling up in the hope that Alpha Protection would keep him safe, they're welcome to it. But that argument chafes against the fact that as a sailplane pilot, he had to understand that if the aircraft was stalling and the increased thrust didn't help, then he had to get that lift from somewhere.

But he never acknowledged that the aircraft was stalled, despite the warning blaring in his ear for more than a minute. The horror afflicting the piloting fraternity is that he had all these indications and yet continued to pull up - a response for which there is no logical explanation. This accident is as inexplicable as it is horrific - something inside him kept telling him to pull up in the face of all the data he was presented with. I suspect that psychologists will be tasked with trying to provide an explanation until long after I'm gone, but it's not a case of being confused by the systems, it's a case of a pilot - for reasons that we'll never understand - pulling up consistently in the face of a situation where the precise opposite action was required.
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Old 21st Nov 2013, 00:43
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Only problem I have with your "analysis", Doze is that the jet flies like all the rest.

That is true if all you want to do is have auto-trim corrected for pitch attitude, roll limiting to 30 deg or so, and so forth. Mission requirements, I guess, as you have pointed out to me and others about my personal mission/operational requirements. All I can say was the thing flew like everything I ever flew until at the edge of the envelope, and then it excelled.

I shall never understand the continued back stick, so I guess I have to wait until we meet in that hootch bar in the sky one day where all we pilots wind up. Still makes me cry.

I also understand that many heavy pilots have never flown their jets close to the performance envelope WRT mach, AoA and so forth. So not interpreting mach/overspeed indications versus incipient stall is not a surprise to me.
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Old 21st Nov 2013, 00:56
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I also flew sail planes and enjoyed the feeling of silent flight but it,as Sully and I feel, it doesn't help you much flying airliners. He landed in the river because he knew how to fly and saved everybody and was a hero.

I would hope all of us have the piloting ability to do what he did. Button pushers would not have a chance in his condition.

Kind of sad, isn't it? This is the new generation of pilots sadly.
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Old 21st Nov 2013, 01:09
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The new pilots should force themselves to make them as good as the old guys and not rely on automation if it all fails. We could do it, why can't you?
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Old 21st Nov 2013, 02:33
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Originally Posted by Clandestino in response to Chris Scott
You suggest that A330 is sensitive beast that has to be treated gently. I say that it is exaggeration and you can throw her around hamfistedly at cruise altitude, in alternate law for 4 minutes with normal acceleration varying between +1.96 and -0.26G with end result being no worse than just utter mess in cabin and galleys.
Originally Posted by Machinbird
Do that in roll direct and I suspect you will learn gentleness, and there might even be an additional mess in the cockpit.

Alternate1 and Alternate2 laws might work similarly in pitch, but they bear no resemblance in roll.
Originally Posted by Clandestino
What relevance it has to AF447? For Finnegan's sake it didn't crash in spiral dive or rapid roll to ocean but in upright stall! That's what happens when you try to pull the aeroplane where she just can't go perfomancewise.
This the relevance.
Bonin applied excessive control inputs that the aircraft would have tolerated had it been in Alt1 law, but since it was in Alt2b, a roll direct law, the sensitivity of the roll channel was far higher than he expected and he set up a roll oscillation that lasted 30 seconds. During this same 30 seconds, he inexplicably pulled up into a climb that used up virtually all the aircraft's energy reserve above stall speed. In my opinion, this roll oscillation fits the general definition of a PIO.

What most readers here do not seem to appreciate is that a PIO event will grab 100% of your attention while it is in progress. My only experience with PIO was in an early visual simulator and it was brief and without consequence. Nevertheless, it made a deep impression. The best analogy to it was like unexpectedly tripping and beginning to fall and looking around desperately for a handhold.

In effect, while piloting an aircraft, the aircraft is your body. Would you be able to execute a checklist or maintain a scan while your body is falling out of control? This type of 'emergency' will cause a tremendous flow of adrenalin in those without prior experience. Will you be able to pilot rationally immediately after such an event? This is my take on the cause of AF447's loss.
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Old 21st Nov 2013, 07:11
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'WoW' is involved in the FWC stall warning logic but here it is called flight phase [IN FLIGHT = flight phases 5,6 & 7).

!But that is not the issue!

All 3 ADIRUs did send an AoA SSM NCD (No Computed Data) signal, for the FWC this means there was no valid AoA to compare against a threshold which depends on Mach, S/F and SB config.
(Max AoA threshold 16.3º in S/F Config FULL (ALTERNATE & DIRECT ) and a fixed 23º in NORMAL)

Actually this philosophy is identical to Boeing T7 only the values are different.

Now if you want the system to be changed the solution has to be found in the ADIRUs NCD state within ARINC specification.
(in AF447 the measured air speed was sometimes 0kt - and even negative q -, just setting a lower value or 0kt would not have solved the issue here.)

For accuracy of the vane:
The AoA vane accuracy is shopchecked in a laminar air flow windtunnel at 100kt.
At an offset of 3º(+/- 0.2º) from original value the vane should return to original position within 0.25º with a max. overshoot of 0.5º to pass this accuracy check.
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Old 21st Nov 2013, 07:15
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@ Clandestino, Dozy

I don't recall anything about this. What I remember is that post-Warsaw spoiler logic on 320 was changed inasmuch selecting reverse cracked open the spoilers thereby killing a bit of lift and compressing both struts.
That agrees with my understanding of information obtained from the horse's mouth.
The 'wiki' comment probably arises because the A320 oleo design was changed to reduce the breakout 'stiction', but that was done to improve the harsh taxy ride that occurred on some surfaces. As a byproduct it reduced the vertical force necessary to start the torque links moving from their fully extended position (used to signal WOW) but that was not its primary purpose.

A classic case of 2+2=4.5!
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Old 21st Nov 2013, 13:34
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BEA Recommendations Follow-Up AF447

The forgotten report:

Suite Données Aux Recommandations De Sécurité

Hardly any of the BEA's recommandations have been implemented.
The report makes a scandalous read with the DGAC and EASA still passing the buck.
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Old 21st Nov 2013, 18:03
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Why not a redundancy-lost message?

While considering system updates, how about the generation of a message something like:
pitot redundancy lost, review UAS procedures
before the autopilot dropped out.

I don't know how long before autopilot drop-out it could/would have been generated, but it would surely have reduced the startle factor.

My guess would be 10s of seconds from the rejection of the unfrozen pitot to the divergence in readings from the other two pitots leading to the declaration of UAS.
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Old 21st Nov 2013, 18:35
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Originally Posted by Peter H
My guess would be 10s of seconds from the rejection of the unfrozen pitot to the divergence in readings from the other two pitots leading to the declaration of UAS.
Nope, it's pretty much instantaneous in human terms. The software is checking and cross-checking several times a second - you'd be talking about maybe 2 or 3 seconds from initial icing-over to the systems detecting an extended discrepancy and disconnecting AP.

In the case of AF447 the point is somewhat moot, as the PNF is supposed to monitor ECAM and action things accordingly. The PF started pulling up almost immediately the AP disconnected without first assessing the situation. The cause of AP disconnect was right there in the ECAM - but it seems that the PF's instincts overruled his reason and training, and neither the PNF or the Captain could break through his panic.
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Old 21st Nov 2013, 18:57
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Peter H
My guess would be 10s of seconds from the rejection of the unfrozen pitot to the divergence in readings from the other two pitots leading to the declaration of UAS.

DozyWannabe
Nope, it's pretty much instantaneous in human terms. The software is checking and cross-checking several times a second - you'd be talking about maybe 2 or 3 seconds from initial icing-over to the systems detecting an extended discrepancy and disconnecting AP.

Sorry, surely there were two events involved, and it's the time between them we are discussing.

IIRC The first extended discrepancy resulted in the mis-diagnosis of the working pitot as faulty. The second extended discrepancy occurred when one of the two frozen pitots started to unfreeze.
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Old 21st Nov 2013, 19:16
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Originally Posted by Peter H
IIRC The first extended discrepancy resulted in the mis-diagnosis of the working pitot as faulty. The second extended discrepancy occurred when one of the two frozen pitots started to unfreeze.
Where are you getting that information from? My understanding was that all three pitot tubes froze almost simultaneously in a matter of seconds, thus making the time discrepancy somewhat irrelevant.

The point I was trying to make was that the instinctive reactions of the PF were not in response to any specific warning, but a panic reaction to the situation - attitude, turbulence and aural warnings.
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