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AF447 Thread No. 3

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AF447 Thread No. 3

Old 10th Jun 2011, 14:54
  #1741 (permalink)  
 
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PJ2:

That would say, to me anyway, that the PF had no idea he was in a stall, correct?

Additionally, is it conceivable that the climb from FL350 to FL375 was caused by an updraft. (Yes, I know this is all conjecture.)

As per this:



Figure 10. Cross-section of Air France 447 flight track through thunderstorm cluster, based on satellite imagery analysis and conceptual MCS models (created June 2009, not updated since then but still considered valid). Light shading is precipitation near the surface; medium shading is cloud material, and dark shading is suspected updraft areas. The flight may have deviated several miles west to avoid the SALPO storm; whether they did or not is unknown; but they almost certainly went through the bulk of the MCS as shown here.

Last edited by rgbrock1; 10th Jun 2011 at 15:09.
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Old 10th Jun 2011, 15:28
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Cool

Hi,

The flight may have deviated several miles west to avoid
We know by BEA note that they go to left for avoid something ..
Unfortunately .. due to the constant turn to right after the upset (see BEA graphic) seem's they headed to what they wanted avoid .......
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Old 10th Jun 2011, 15:32
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rg, there may or may not have been a vertical airmass movement contributing to the altitude increase. What the BEA reports show is that the nose up command was made and for a period of time, sustained.

The faster your fly, the more force (up or down) a given control deflection will apply to your lift versus gravity vector summation. A number of the heavy pilots on this board have pointed out how gentle control inputs ought to be at high speeds and high altitudes.

Let's say you and I are travelling at 180 knots, and I make a control deflection of the elevator of an inch in a T-28B Trojan. (Built like a Grumman F-8-F Bearcat, more or less, high performance WW II fighter).

Nose moves a certain amount, I pitch up, airspeed will change, I'll trade energy for altitude at a certain rate.

Then, let's say we are traveling at 300 knots, same altitude, and I make the same control deflection. The resulting force will be significantly higher than the one I made at 180 knots. I am likely to get more nose up and much more altitude traded for my airspeed, unless I make a quick countercorrection. If I am used to flying about at 180 knots most of the time, I may put in what "feels" to be the same input in at 320 knots, and get a significantly different pitch attitude. (heh, you may ask how I learned this ... for another time).

Most enlightening has been some of the explanations in the past few pages of how the control laws in some FBW aircraft sense G load or set G load to govern and control control surface deflection to achieve "X" peformance. Part of the reasoning behind that seems to be to prevent overcontrolling, per my illustration above.

With that in mind, consider the following: what if the most common hand flying task undertaken by a modern pilot flying FBW aircraft is the takeoff or landing configuration and airspeed range. What "feel" is he used to? That seems to be an industry wide concern, at least on the pilot side, if what I read on PPRuNe is any indication.

Then here's the kicker: is FBW "feel" a good indication of what the aircraft is doing? (That's been addressed as well, some pages back). Regardless of whether it is or not, pilots develop a feel for their aircraft. It's part of being human. Our senses inform a great deal of what we do.

Back to your question.

If you get turbulence and updrafts, you will "feel" its effects in your seat and your body, but often you can't rely on "feel" flying to counteract it, particularly if you aren't flying via visual reference. In that case, you have to deal with it using instrument flying skills regardless of what you feel, or what you don't feel.
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Old 10th Jun 2011, 15:49
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Thanks for the explanation Lonewolf. Makes things clearer for this non-aviator!
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Old 10th Jun 2011, 15:49
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Sorry for the long quote, but it will explain things better than I can. Its from Boeing's take on upsets and may help us understand why the climb happened. (Yes I know Airbus FBW is different from Boeing nevertheless a-lot of what is said reads accross)


3 Maneuvering Stability
Maneuvering stability, like static stability, is influenced by CG location. However, when the CG is aft and near the neutral point, then altitude also has a significant effect. Since air density has a notable impact on the damping moment of the horizontal tail, higher pitch rates will result for the same elevator deflections as altitude increases. From the flight crew's perspective, as altitude increases, a pull force will result in a larger change in pitch angle, which translates into an increasing angle of attack and g. While a well-designed flight control system, either mechanical or electronic, will reduce the variation of stick force with CG and altitude, it is very difficult to completely eliminate the variation due to design limitations.
For example, for the same control surface movement at constant airspeed, an airplane at 35,000 ft (10,670 m) experiences a higher pitch rate than an airplane at 5,000 ft (1,524 m) because there is less aerodynamic damping. The pitch rate is higher, but the resulting change in flight path is not. Therefore, the change in angle of attack is greater, creating more lift and more g. If the control system is designed to provide a fixed ratio of control column force to elevator deflection, it will take less column force to generate the same g as altitude increases.
This principle is the essence of high-altitude handling characteristics for RSS airplanes. Unless an RSS airplane has an augmentation system to compensate its maneuvering stability, lighter column forces are required for maneuvering at altitude. Longitudinal maneuvering requires a pitch rate, and the atmosphere provides pitch rate damping. As air density decreases, the pitch rate damping decreases, resulting in decreased maneuvering stability (see figure 2 and "Maneuvering Stability" below).
An additional effect is that for a given attitude change, the change in rate of climb is proportional to the true airspeed. Thus, for an attitude change for 500 ft per minute (fpm) at 290 knots indicated air speed (kias) at sea level, the same change in attitude at 290 kias (490 knots true air speed) at 35,000 ft would be almost 900 fpm. This characteristic is essentially true for small attitude changes, such as the kind used to hold altitude. It is also why smooth and small control inputs are required at high altitude, particularly when disconnecting the autopilot.
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Old 10th Jun 2011, 15:56
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@ PJ2

"...At the MSA [Minimum Safe Altitude around the departure airport], and the aircraft is well away, either in the climb or in cruise and above FL100, one doesn't "wait" for a while to do the drill...one does not do the drill at all. One "levels off", and begins troubleshooting, which means get out the checklists for pitch and power to stabilize the airplane in cruise. There is not sufficient data to determine if this was done or not - we just have the back-stick and the climb*...." (
italics added)

This statement links the back stick to the climb. On the face of it, that seems fair. Nothing of the sort is stated in the BEA release. It is suggested, and inferred in the post above.

From the BEA release, the PF inputs left roll and NU. He then hears 'STALL', twice. I am suggesting that he released back stick, perhaps even let the stick migrate to neutral. BEA makes no statement pertinent to this action.
ND input is not eliminated in the report.

It is only later that BEA reports "the a/c climbs progressively beyond ten degrees (NU), and *STARTS* to climb..." (emphasis mine)

It takes an assumption on the part of the reader to conclude the PF was the reason the a/c "continued" (or even 'began') to climb. What can be taken as fact, is The aircraft may not have been responsive to his original input... It can only be considered so if one believes he maintained NU inputs..... NOT "......."an" input.

The sequence of events I take as fact, but the timing, and any intermittent activity, is not given, and for that reason, is speculation. REMEMBER that BEA reports "FROM 2:10:05". They could have assigned a time to each following event, but they chose not to. I am sure that is inadvertent.

If in roll direct, and subject to "twitchy ailerons" he may have tailored his "enthusiastic" input back a notch, and quickly switched to "several, minor" inputs to conform with his training to ALAW? Also, in Pitch, the cg was aft (per plan), something he would need quickly to conform his stick work to.

So, Roll Direct, aft cg, turbulence, (severe), STALL, STALL. Still, at eleven seconds after assuming control, "...so, speeds are gone.." (PF)

Cool customer? +1


update... "...You asked if an unraveling altitude could "explain" the full back-stick..." ...PJ2

Let's be specific. The first NU was not "full back stick"

again... (PJ2)

"The fourty-five second period between disconnect and apogee requires concentrated, detailed examination.
.."

"..By the time the stall had fully developed at the top of the descent (38,000) when the physical energy of the aircraft had largely dissipated and the AoA went from 16deg to >40deg as the descent began, there were no options remaining with the exception, possibly, if they had re-trimmed the THS set at 13NU, (the effects of which now verified by DJ77's observation & CONF iture's diagram)..."

I would suggest a more focused focus on the first fifteen seconds after a/p loss.

To muddy the field with a 'global picture' at this point is a disservice to the PF.

Without more explicit data, it is tempting to discuss that which the PF had perhaps no chance of recovering. In determining what caused the NU (initial and secondary, only THEN tertiary), the "second guessing" has some validity. PF had perhaps ten seconds to suss this, (with his F/O), and the Captain came back almost assuredly after the game is lost. Actually, more like ELEVEN seconds.

Last edited by bearfoil; 10th Jun 2011 at 16:20.
 
Old 10th Jun 2011, 16:15
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bear, it seems that in FBW, "return stick to neutral" might not necessarily remove the nose up/climb command. As I understand the sidestick function, your "back stick" command tells the plane to do "x" (a g command), which it keeps doing until you (gently?) move the backstick again to tell it to do "y." (different g command).
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Old 10th Jun 2011, 16:26
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Lonewolf

"...bear, it seems that in FBW, "return stick to neutral" might not necessarily remove the nose up/climb command. As I understand the sidestick function, your "back stick" command tells the plane to do "x" (a g command), which it keeps doing until you (gently?) move the backstick again to tell it to do "y." (different g command)..."

Yes, but the a/c is in ALAW (maybe?). PNF says (eleven seconds after) a/p drop, "alternate Law". To stop the climb would take a second ND input, No?
Could PF be fighting a (P/AB 'IO') battle? PITCH is not Direct, only ROLL.

Pilot/Airbus Induced Oscillation (PAIO) ....bear says.

No let's call it PilotAirbus Oscillation Induced Oscillation. Remember, the PF input "several" NU commands. Were these intended to counteract the Bus' argumentative ND? Each of her ND's of too little duration to cause the THS to relent in its travels to 13 degrees?
 
Old 10th Jun 2011, 17:02
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What would the a/c be doing after STALLSTALL? Would she be attempting to ND the airframe? With elevator? Then PF counters with NU? expressed in TRIM (THS). What does the Protection do, here? What does she do when Overspeed protection quarrels with STALLSTALL? Did PF get between duelling Protections?

Was this a battle between two stubborn inputs, one Elevators and the other THS? Did PF eventually "Win" when his surface overcame the a/c's? By that time was it too late to recover the NOSE? Not even with Manual Trim?
 
Old 10th Jun 2011, 17:56
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artificial horizon & nose up

Has anyone produced a A330 PFD/artificial horizon to show what it would look like with a 16° nose up ?
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Old 10th Jun 2011, 18:06
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Ile de sein

Beside all the ongoing more or less sophisticated discussion what might have happened, when and why, may I remember those that lost their life.
The ship Ile de Sein is shown in the harbour of Las Palmas de Gran Canaria on the site of "Live Ship Map".
Former message indicated they had raised more than 100 victims of the crash from the seafloor. Guess these remains are now flown from LPA to France.
What comes next ?
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Old 10th Jun 2011, 18:21
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bear;

On fifteen seconds, sure. Fourty-five takes the examination to the apogee but yes, we can look at a narrower time frame and make sense.

On the UAS, absolutely the notions are inferred, posited, speculated.

There is nothing in the BEA note to confirm/deny such notions. I posited the notion well before the recent BEA note, from knowing the UAS drill and the checklist items.

A year and half ago I timed the loss of speed in an A330 Level D simulator...it took about 4 minutes to lose the 70kts from cruise to Alpha-prot at idle thrust at FL350, 205T, 37%CG, autoflight engaged, normal law.

We know now that the accident began and ended in about the same time it took to lose just 70kts. We know why now.

The aircraft was stalled when it struck the water. When did it stall? Prior to the BEA Note, many here considered the notion of a pitch-up but none of us knew for sure, or if so, why.

A pitch-up could occur for a number of reasons which have been discussed at length. One was executing the memorized items in the UAS drill. Was it possible? Of course it was. That has nothing to do with whether it occurred or whether it is probable.

Shortly after the accident and the loss of airspeed information was becoming known, because it made complete sense and was in complete concurrence with what I knew about flying transport aircraft, I observed in the second thread on this accident that the only thing to do was to leave the aircraft alone, (change nothing, fly manually); it was stable before the loss, and would remain so while the pitch and power were manually controlled by the pilot. A loss of control is not an inevitable outcome of a loss of airspeed information.

On Alternate Law flight, the ailerons are no more "twitchy" than in Normal C* flight.

Normal law does not "soften" control response.

"Normal" and "Alternate 1/2" laws are about protections, not about the briskness of control response. In roll direct, (Alt 2), the ailerons behave the same way, BUT the aircraft no longer has bank angle protection, and the controls are no longer trying to maintain the last commanded bank angle or pitch angle. The aircraft is a DC8, and that is the way to think about it when laws degrade...one is required to "pilot" the airplane and think like a pilot, not a video gamer as some author ridiculously wrote.

Originally Posted by bearfoil
Without more explicit data, it is tempting to discuss that which the PF had perhaps no chance of recovering.
No, it is not "tempting", it is a reasonable point to discuss because at the apogee of the trajectory, the aircraft had a pitch of 16deg and an AoA of 6deg, rapidly increasing to 16deg then >40 while the pitch remained NU. The potential for recovery cannot be assessed as high and I have qualified this in my previous post.

The key as we know bear, is to determine the origin of or the reason for, the pitch up and then to examine the broader factors which contributed to making this a feasible response to the UAS and/or the AFS Disconnection. By no means, is any of this pleasant business, I assure you.
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Old 10th Jun 2011, 18:29
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bear: I think you may be mixing normal law protections with alternate law stability functions.

In alternate law, the pitch protections you mention do not fight as they aren't in operation. (Based on the much linked .pdf brief at smartcockpit, caveat, dated information). You do have high and low speed stability. Links here: http://www.smartcockpit.com/data/pdf...light_Laws.pdf
and a link to a PPRuNe discussion on same from March 2009, pre AF 447 crash.
A320 Low speed stability [Archive] - PPRuNe Forums, is here.

Low and High Speed Stability: can make a nose down (if slow) or nose up(if fast) input in alternate law, which the pilot can override. With double or triple ADR failure, you don't have either. (You are also paddling in a canoe up that proverbial creek ... ) But if you are in alternate law, it appears that you know you are required to fly by hand.

This goes back to the question raised a good while back: with airspeed signal being squirrely, did either stability function (not protection) activate in an attempt to prevent either overspeed or stall?

Is that what you were getting at? Given that airspeed appears to be a trigger for one, and AoA and airspeed the trigger for the other ... good question. It appears that this is not an input that can override the pilots commands. (Toss in "Abnormal Attitude Law" and things get more interesting, I suppose ... if it's active)

Note of interest: from the much linked brief at smartcockpit ... dated info ... slide 5.24 ... page 64 ...

The ECAM system F/CTL page (Flight Controls?) has in the middle of the screen, just below center, Pitch Trim depicted numerically. I think that means THS trim angle, not sure. I will for the moment assume that to be the case.

If a flight control malfunction was not the highest priority ECAM alert (and how many of them were active?) then the PNF wold not be seeing that on the ECAM screen, and thus not have the info (were he looking there for indications of just what's amiss) he needed to cue the PF that

"THS is 13 deg nose up, move it/trim it down"

or words to that effect.
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Old 10th Jun 2011, 18:38
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Super discussion of FBW control laws, yet....

@ ashling:

The snippet I posted of the Viper laws shows function after function related to dynamic pressure/static pressure) so read IAS or CAS or EAS, your call. Not true airspeed. So using the area and deflections of the control surfaces, the confusers change their deflection angles and rates to get about the same "feel" at any altitude or speed for the "gee command" logic of the Airbus and the little jet I flew long ago. Because the actual airframe is moving at true airspeed thru the atmosphere/over the earth versus IAS, the rate gyros come into play. So the FBW systems have rate functions to help with this.

Honest to God, I would have to strap some of you into the front seat in a family model and show you what it 'feels" like and what it actually does. Sorry, but those days are gone forever, guess I should let them go ( cheap reference to Don Henley's "Boys of Summer", heh heh).

@ wolf and bear:

My point about the climbing ( pitch up attitude) condition is that the Airbus appears to command a gee that is compensated for the pitch attitude ( unlike my system). e.g. at a 30 degree angle, the 'bus would trim for 0.87 gee or so. You would get a nice, stable climb angle with a neutral stick after your initial nose up gee command to that attitude.

The problem is you are slowing down, and the jet is trying to maintain that gee. So the THS is being" trimmed" so that the neutral position of the stick is gonna command 0.87 gee. If you hold just a little bit of back angle/pressure for any length of time you will get a trimming THS that results in a command of 0.87 gee if you let go ( return to neutral) at 30 degrees pitch, less gee if at a higher attitude, more gee if at a lower attitude ( think 15 or 16 degrees nose up).

Secondly, the Airbus compensates for bank angle to maintain the gee required for level flight, so I don't have to "trim" a bit back in a turn. A climbing turn with a positive pitch attitude is another thing, and I would have to look at the functions and how they are integrated.
++++++++++++++++++++++

One thing that has bugged me is the role of AoA in the Airbus control laws/modes. Instead of "limits" throughout, I see warnings and advisories and such for some modes versus "limits" . It also bugs me that some of the documents state that "speed" is involved in the "laws" versus a pure AoA, and that some of the AoA warnings are inhibited according to speed. I fully understand this when on the ground, but otherwise, the jet flys like the ones I learned to fly in a dim past. Increase AoA and the nose pitches up. Keep demanding that AoA or increasing it, and the nose pitches down and maybe some bad things occur.
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Old 10th Jun 2011, 19:12
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As SLC, that is a very frightening perspective.
You are in cruise, albeit in bumpy weather.
Depending on the breaks, you are 11 seconds from an unrecoverable stall.
That is the message that bearfoil is articulating in message 1747. ( any subliminal Seattle affect??).
Is he wrong? if so, why?
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Old 10th Jun 2011, 19:20
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gums, Lonewolf

"bear, it seems that in FBW, "return stick to neutral" might not necessarily remove the nose up/climb command. As I understand the sidestick function, your "back stick" command tells the plane to do "x" (a g command), which it keeps doing until you (gently?) move the backstick again to tell it to do "y." (different g command). "
(lonewolf, "stick command").

First of all, at the time noted by BEA of a/p loss (2:10:05) there is "Cavalry Charge", and "Master Caution". FCOM directs use of autopilot in turbulence, and so we assume that the Pilot is well trained in handover.

If the a/p quit due limit overwhelm, or rate<aspect, it is important to know the stick logic. In A/Law (1&2), Pitch is protected? In A/Law 2, ROLL is DIRECT. Direct means one tweak, or tweak and hold, or intermittent tweak?

"Twitchy" is not necessarily an a/c fault, it may mean simply that the a/c appears "touchy" in ROLL to the pilot, if in DIRECT.

lonewolf:
"...Is that what you were getting at? Given that airspeed appears to be a trigger for one, and AoA and airspeed the trigger for the other ... good question. It appears that this is not an input that can override the pilots commands. (Toss in "Abnormal Attitude Law" and things get more interesting, I suppose ... if it's active)..." yep.

PJ2:
"..The key as we know bear, is to determine the origin of or the reason for, the pitch up and then to examine the broader factors which contributed to making this a feasible response to the UAS and/or the AFS Disconnection. By no means, is any of this pleasant business, I assure you..." also yep, but with a "boy howdy" thrown in

gums:
"...It also bugs me that some of the documents state that "speed" is involved in the "laws" versus a pure AoA, and that some of the AoA warnings are inhibited according to speed..." So yeah, if unreliable, why would speeds inop the STALLSTALL? Mos' confusatory...
 
Old 10th Jun 2011, 19:31
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bonjours dans l'ecole

No, but BEA has released data that taken at face, certainly includes such a proposition. Perhaps as little as THREE seconds. Arguable, and if pinning one's hopes entirely on PILOT ERROR, then yes, three seconds to survive.

A little knowledge is a dangerous thing? Toujours. Aux'assemblage? Certainement, Nons?
 
Old 10th Jun 2011, 19:32
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étudiant

"Depending on the breaks, you are 11 seconds from an unrecoverable stall"

I would say "depending on the quality of the airline's training"
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Old 10th Jun 2011, 19:41
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Thanks, oldchina!
So where does one get insight into the "quality of the airline's training"??
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Old 10th Jun 2011, 19:55
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My response is from the PoV of a passenger.
Originally Posted by nervous SLC
You are in cruise, albeit in bumpy weather. Depending on the breaks, you are 11 seconds from an unrecoverable stall. That is the message that bearfoil is articulating in message 1747.
(any subliminal Seattle affect??).
Is he wrong? if so, why?
You forgot the ice. (Is it in your glass of bourbon whiskey? Too expensive by half, but a better value per dollar than the beer airlines serve. )

I'd say your read of his post is wrong.

Why? (It isn't the three second violation).
Depending on the breaks, you are 11 seconds from an unrecoverable stall.
No, unless you assume very anomalous values for "the breaks" on passenger flights. Depending on the breaks ... a whole host of strange things can happen. Some jackwad could set off a bomb in his underwear, and not screw it up this time. Some manureforbrains could drop his alertness and allow a suitcase full of explosives into the cargo hold, a la Pan Am flight 103. What breaks are you worried about?

I'll also point out that "unrecoverable" is a premature verdict, based on the idea that in this mishap, it wasn't recovered. You cannot make the a priori assumption that the stall was unrecoverable ... among other reasons, when? When in the event chain was recovery less, or more, likely?

Depending upon what operational lessons the BEA can filter out of this crash, the odds of a repeat go down considerably, and the odds were remote in the first place. (Obviously, greater than zero ... )

The problem isn't just bumpy weather, the way your rephrase his post, which is somehting that you or I can be subject to at any flight altitude. The problem is a cascading series of untoward factors combining together in anomalous fashion. A few of the preconditions included ice of some sort (presumably ice crystals found only at high altitudes).

Oh, by the way, a stall can also occur on a sunny smooth day. So that risk "depends" on a lot of factors. It just isn't very likely when you board your next 737 or A320 on your next trip, and by not likely, the odds are significantly smaller, by orders of magnitude, than you getting into an automobile accident, your fault or someone else's.

I invite you to look up the Swiss Cheese model of aircraft mishaps.

Swiss cheese model - Wikipedia, the free encyclopedia

Add a few crackers, and you can have a nice snack ... just switch from bourbon to vodka or gin, on the rocks.

By the way:
The Seattle Effect
Is that what you mean by Seattle Effect?
or this?
The Seattle Effect- Excerpt from "Networking Futures: the Movements against Corporate Globalization" (Duke Univ. Press, 2008) | The Real Battle in Seattle

The reference appears a non sequitur.
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