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AF447 Thread No. 3

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AF447 Thread No. 3

Old 28th May 2011, 01:08
  #241 (permalink)  
 
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Cool

Hi,

It's just my opinion .. but I always thought that the independence between the two sticks was a mistake or at least a contributing factor of misunderstanding
The Boeing system seems more relevant from a security standpoint and offer better communication between the two pilots (visual and sensitive)
Was this a contributor factor in this case ? ... I can't say....
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Old 28th May 2011, 01:23
  #242 (permalink)  
 
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Originally Posted by Graybeard
We also know that the computers were not robust enough to accommodate erroneous airspeed.
OK - one very-last post then...

You can't make a computer perform tasks with data that it known to be erroneous, it's just a logical impossibility. As such, the design of the system - on paper, before a single line of code was written - clearly defined that in a case where it is unable to do its job, the best hope lies with the pilots. So issue a "stop" command to the relevant module (beyond which nothing other than a full reset can bring it back up again), notify the pilots of the failure and leave the flying to them.

As soon as "Alternate (NO PROT)" mode was triggered, the pilots were effectively in control of a regular airliner with no protections and full control authority in the pitch axis. What isn't clear at this point (and what I suspect the BEA will be spending months trying to determine) is how the crew reacted to the situation they found themselves in, and what their intent was.

Just so we're clear - one last time. Based on the evidence we have so far, flight control protections and the software that defines them are unlikely to be directly relevant in this accident sequence, because they were disabled very early on in that sequence, precisely as they were designed to.
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Old 28th May 2011, 01:42
  #243 (permalink)  
 
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In today's report (and in this thread, yet) there is no mention of overspeed and especially its alarms (if any).

Before this report I always assumed that they might have had both stall and overspeed alarms. Similar to what happened to the 2 crashes due to clogged air-pressure instruments due to paint and insects (i.e. near Peru, & Dominican Republic).

This prel. report gives the impression (possibly wrongly) that the main alarms they got were stall warnings (amongst others) and not overspeed ones. And yet the pilots may have thought they were overspeeding. Oh my.

Anybody have any ideas about overspeed ?
.
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Old 28th May 2011, 01:42
  #244 (permalink)  
 
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Alber Ratman wrote:

Low speed / high alpha / rapid descent = Stall.. Not a dive. So why continue to hold a nose up attitude? We will never know unfortunately.
(my italics)

I don't agree. As is most often the case, the reasons for the actions of the participants will very likely be made clear though the CVR (possibly even aided by the DFDR). The info released by BEA is extremely sparse (for many reasons) but one can be quite certain that a great deal of discussion and interaction was occurring throughout those final minutes.

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Old 28th May 2011, 01:57
  #245 (permalink)  
 
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Dozy...

Looks like he was perfectly aware from where I'm sitting...
I believe your conclusion is not justified by the facts as we know them.

There is nothing we know yet that tells us whether the "dual input" was complementary or contradictory. Nor do we know what other discussion surrounded that action. Nor even, at this point, which two crew members were invloved.

And Conf's point stands: Sidestick inputs by one crew member may not be obvious or clear to another crew member. Whether you consider that is a "good thing" or not, it is something that AI is (so far) satisfied with.

grizz

Last edited by grizzled; 28th May 2011 at 02:16. Reason: clarification
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Old 28th May 2011, 01:59
  #246 (permalink)  
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Dozy Wannabe


...Just so we're clear - one last time. Based on the evidence we have so far, flight control protections and the software that defines them are unlikely to be directly relevant in this accident sequence, because they were disabled very early on in that sequence, precisely as they were designed to..."

I finally see your point of view. Some one needs to explain your Faith to some few qualified pilots, who are obviously of a different Faith.

No harm no foul, there is always room for your opinion well within whatever the experts decide...

See? Foolproof......

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Old 28th May 2011, 02:23
  #247 (permalink)  
 
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A lot of posts expressing surprise that they didn't realize they were in a stall. A lot of aircraft will stall and have no phugoid oscillations, so once you are in the stall there are really no "seat-of-the-pants" feelings of being in the stall. (You only have a falling sensation at the stall when you go from level flight to descending in the stall.) You have to be aware of AOA, airspeed, or reserve lift. Remember, this was at night in likely bumpy conditions so relying on the instruments was the only way out. You can bet that someone has already run a simulator with this accident scenario to see what a simple push would have accomplished.
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Old 28th May 2011, 02:29
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Dozy Wannabe:
You can't make a computer perform tasks with data that it known to be erroneous, it's just a logical impossibility...

Just so we're clear - one last time. Based on the evidence we have so far, flight control protections and the software that defines them are unlikely to be directly relevant in this accident sequence, because they were disabled very early on in that sequence, precisely as they were designed to.
It's PRECISELY the DESIGN that needs a review. The AP/AT were no doubt in Alt Hold and Spd Hold modes. The A/P has inputs from 3 A/S sources for good reason: they vote. If two or more go wonky, the AP should have enough intelligence to reject them all and go into flywheel mode, holding the last power and pitch setting. It should then send an SOS to the pilots, giving them the decision to take over, or let the flywheel run awhile.

See, a really capable AP could do that smoothly without going off in a pout.

GB
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Old 28th May 2011, 02:33
  #249 (permalink)  
 
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Back in the Loop?

spagiola post #117 (27/1733z):
"Did the resumption of the stall horn once the speeds once again become valid after commanding pitch down (at 2:12:02+15) confuse the PF into thinking he was doing the wrong thing?" [my emphasis]
Quote from MurphyWasRight:
"Until today were you aware that the stall warning could go away if a stall developed to the point that indicated airspeed dropped below 60KT?
And that it would return as you recovered?"

I think it may well have done, particularly considering the situation he had found himself in. The irony is that the AoA-based stall warning had been inhibited, indirectly, by the system perceiving an IAS below 60kts. On the occasions when a reduction of AoA was achieved, this may have cruelly coincided with a recovery in the (false) IAS to have enabled the (warbling, audio) stall warning to resume.

(In any case, because of the under-reading of IAS, the threshold of stall warning would have probably been too high for the altitude, as has been discussed by HazelNuts39 and others on previous threads [see Page 46 Of BEA Interim Report No 2]. So it is arguable that the stall warning, even when not inhibited, may have been inactive at times when it should have been active.)

This accident seems to demonstrate an Achilles-heel (weakness) in the stall warning system for Pitch-Alternate Law and Direct Law that needs to be addressed. As I wrote in my previous post, why is it considered necessary to inhibit AoA-probe data IN FLIGHT when the IAS is measured below 60 kts? (On the GROUND, it must be.) It seems to me that there are sufficient indications available to establish GROUND/FLIGHT status automatically, as used by numerous other systems on the aircraft.

Quote from MurphyWasRight:
"Until today were you aware that the stall warning could go away if a stall developed to the point that indicated airspeed dropped below 60KT?
And that it would return as you recovered?"
Although what you say in your last sentence may have happened, it seems to have been by (tragic) coincidence of the vagaries of the UAS. If the erroneous IAS reading had remained near zero, the stall warning would not have returned.

Checkbard,
post at 27/2129z, quote:
"[....] aircraft which crashed (or very nearly so) because the pilots failed to realise that the trim had been set full nose up, and couldn't understand why they lost pitch control."
This seems possible, if unlikely. The limited pitch-down commands from the PF may have not been sufficient to cause the THS to run to a less nose-up trim-state than the 13 degrees. In my
post at 27/2011z, however, I asked:
"is there any possibility that the THS motor stalled during down-elevator inputs?"
This has been known to happen in older types, but seems improbable in this case.

Quote from TyroPicard, re my question as to why the AoA data is inhibited IN FLIGHT below 60kts IAS:
"...without much airflow at really low IAS gravity affects the position of the AoA vane just as it does on the ground? The designers had to choose a speed - they chose 60kts."
You may be right, but, if so, surely they could be balanced and damped. G-forces could be the problem? (The VC10 has non-mechanical AoA probes, as you may remember.)

Yellow_Pen (27/2222z),
Agree that pitch and thrust can be relied on in fairly level flight. Once you are climbing or descending steeply, particularly in a stall, I guess it gets tough as a recovery tool? Maybe AoA is the only answer, but it would be a very radical step in airline ops.

Checkboard (27/2226z and 2302z).
Think it was me who started a discussion on use of FPV as an indication of AoA last year. It's not that easy to use in changing bank, and inaccurate when wind-to-TAS ratio is significant. But it's much better than nothing. Unfortunately, AF447 seems to have lost FPV-capability during 0211z.
Re the FPV VS data, I also used to think it was inertial, but remember being corrected by one of my copilots! Too near bedtime to check: sorry.

Svarin ("The
Last Effort"?),
Your posts have always given much food for thought. Don't go off mushroom-picking at this stage, please, just because you want a life...

Chris
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Old 28th May 2011, 02:37
  #250 (permalink)  
 
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As a curious SLF, if the altimeter is screaming downwards, why wouldn't you pitch the nose down into the vector ?
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Old 28th May 2011, 02:38
  #251 (permalink)  
 
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THS position Indications.

This post by PJ2 gives you the indications of THS position.

http://www.pprune.org/tech-log/39510...ml#post5571587


The F/CTL System Display is normally called and must have been in view due to following failures:

ATA 27.23 F/CTL RUD TRV LIM
ATA 27.90 F/CTL PRIM 1 FAULT
ATA 27.90 F/CTL SEC 1 FAULT

A33Zab
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Old 28th May 2011, 02:41
  #252 (permalink)  
 
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in the sim, if you pull back the stick, auto thrust locks to max power and the nose go down to avoid stall speed.

I see a strong horizontal wind shift plus icing involved in this crash(what I told you since the beginning) , made enter the plane into a stall. after the A/P disconnect automatically.

you can not recover from a stall by just pushing the stick, and adding power.


then the long descend in a flat stall until impact.

few passengers not sleeping or waking up due to ears' pain and boom, all dead!



conclusion: don't fly in the red!
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Old 28th May 2011, 02:42
  #253 (permalink)  
 
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So I guess it's completely beyond all the experts in here to fathom the impossibility of an Airbus of the coast of SOUTH AMERICA to be in IMC from 38000 ft to the ground?

Well so much for the pilots trying to 'read instruments' when in fact they were probably in VMC conditions, which begs to ask the question if anyone in that cockpit could, would, or have the ability to simply stop looking at the panels and fly the plane straight and level...which would have required, against all training, hiring practices, and airline SOPS, to DISREGARD the instruments, disengage the autopilot, and just fly the plane...you guys remember that? When flying actually meant holding the yoke and not just moving a heading bug?

And even if, at best the A/S indicator was messed up, you know like the Dallas accident where someone forgot to turn on the pitot heat...in the end, I am sure, they either had an A/I to look at or the ocean for reference, so what exactly would have stopped them from leveling the nose, the wings, and going to a known power setting until they could figure out what was wrong with the tubes they were looking at?

I am almost wondering if another tail came off...
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Old 28th May 2011, 02:43
  #254 (permalink)  
 
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Trim

Just a few thoughts on trim.

I've never flown even the 'Bus simulator, but I have studied the systems. (Does the Caravelle simulator count?) I'm a retired Naval Aviator and both old and bold.

Yes, the THS is a 500 pound gorilla that can overpower the elevator position.
The THS is controlled by the Prim computers, theoretically any one of the three Prims can do it. Without Prims, it won't move except by manual input.

Recently, I was attempting to clarify some issues regarding THS manual trim (in the last thread), and asked the question, "How does the system know the pilots are done applying manual trim?" Apparently, once it senses the crew playing with manual trim, it gives up and says "Your trim for the rest of the flight." In a normal flight context, this is bad since the THS can overpower the elevator and cause potentially dangerous loss of control or interfere with proper functioning of protections.
The airlines/training establishments apparently consider this to be a very serious safety problem and they apply draconian penalties for touching the trim wheel. As a result, aircrews fly the majority of their careers without touching the manual trim wheel after it is set for takeoff. They almost forget it is there once airborne.

The trim wheel moves as the aircraft trims itself. There are marks alongside the trim wheel indicating its position, and these marks are illuminated so that they are visible at night in a dark cockpit, but it appears aircrews are not even thinking about looking at the wheel for information.

Part of the problem seems to be that the aircraft does not have the ability to resume automatic control of the trim once the microswitch that senses manual override is activated. As a result, crews are taught to keep their paws off the thing. That would result in bad training habits. (Real Airbus guys feel free to correct these statements.)


Next issue. The trim running up to 13 degrees makes sense due to the prolonged nose up stick input. Once a high AOA was achieved, however, the AOA readings and airspeeds were considered invalid (due to airspeed below 30 knots). The PRIMS could not have liked that and were no doubt all 3 disabled by that point. Once the PF started making nose down inputs, the Prims were still disabled and thus the THS did not move from its 13 degree position.. The crew did not get the proper feedback from their nose down control inputs (the stall warning actually came back) and they never saw the trim position due to their past training.

The only way to have saved the day was to get the trim rolled down manually to get the plane flying again.

Apparently the A330 pitching moments were not so bad as to prevent a stall recovery with proper control inputs.
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Old 28th May 2011, 02:43
  #255 (permalink)  
 
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But... It should go nose down anyway... Independent of sidestick inputs

Cg in the right place + excessive AoA should always equal pitch-down and a consequent reduction in AoA. Naturally.

Otherwise margins are so narrow...
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Old 28th May 2011, 02:47
  #256 (permalink)  
 
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dont u read my posts...

u can not get out of a stall in the airbus....it s an airbus , not a a caravel or a cessna.
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Old 28th May 2011, 02:55
  #257 (permalink)  
 
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the independence between the two sticks was a mistake or at least a contributing factor of misunderstanding
The Boeing system seems more relevant from a security standpoint and offer better communication between the two pilots (visual and sensitive)
..and would also have helped the captain, when he appeared, to see what control inputs were being made, even leaning over and grabbing the prong himself. I certainly would have.

Well so much for the pilots trying to 'read instruments' when in fact they were probably in VMC conditions,
It was at night, they were in IMC (at least initially, why else did the probes ice up?). Look out the window of a big jet at night, in cloud, and recover from a deep stall with all manner of noise and warnings going on? Try it. Let us know how you get on.

The only way to have saved the day was to get the trim rolled down manually to get the plane flying again.
Mentioned in my just-issued new Boeing stall recovery...
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Old 28th May 2011, 03:08
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Per BEA initial report - in structured form for clarity.

01:35:15 Radio:TO ATLANTICO - @ INTOL
01:35:46 Radio:FROM ATLANTICO - Maintain FL350...eta TASIL?
01:55:xx Captain awakens #2 co-pilot
01:59:32-02:01:46 Turbulence & WX Briefing
02:06:04 PF notifies CC turbulence ahead
02:08:07 PNF "Go left a little"


11 Seconds elapsed:
=================================================
02:10:05
AP then AT disconnects.
PF I have the controls"
A/C rolls to right.
PF inputs left and nose-up
Stall warning 2X
A/S "sharp fall" from 275 > 60kts Captain's PFD
Same indications on ISIS "few moments later"
=================================================


34 Seconds elapsed:
=================================================
02:10:16
PNF "...we've lost the speeds" "...alternate law"
A/C attitude goes beyond 10 pitch up
A/C V/S indicates positive rate of climb
PF nose down inputs. L/R inputs.
V/S goes from 7000 fpm to 700 fpm
Roll angle recorded between 12 deg R and 10 deg L
Captain's A/S rises sharply to 215kts
ALT 37,500.
AOA 4 degrees
=================================================


02:10:50 PNF calls Captain to cockpit


49 Seconds elapsed:
=================================================
2:10:51
Stall warning
TOGA
AOA 6 degrees
PF "maintained" pitch up commands
Everything up to the point highlighted in red makes sense in relation to pilot inputs vs aircraft situation, even with loss of airspeed indicators. At 2:10:16 aircraft indicates positive rate of climb and angle of attack, pilot corrects by nose down input.

Stall warning sounds at 2:10:51, TOGA applied with AOA at 6 degrees, but PF MAINTAINS PITCH UP COMMANDS, and it pretty much all turns to shite from this moment on.

Why would you maintain a pitch up command?
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Old 28th May 2011, 03:09
  #259 (permalink)  
 
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As I read that report with tears in my eyes all I can think of is "why?" Why in (censored) did Hal decide the plane had stalled falling from 270 kts to 60kts in less than a minute. That's acceleration that would have shredded the plane. Hal should have disengaged COMPLETELY rather than force the trim ever upwards to try to maintain altitude. It's as if Hal forgot the inertial system was still there and still functional.

That is a very basic design defect that must be corrected. Simply putting on probes that supposedly don't ice is not enough. (side note, sensor_validation suggested an acoustic monitor on the probes to prove airflow - good idea.) The aircraft must recognize that airliners take more than a few minutes to slow from 270 to near zero absent an attitude shift to a very high angle of attack. If that happens auto-trim should be disengaged with the trim left at the last known good value. Otherwise Hal gives the pilots an uncontrollable plane. That means it must think the pilots are there simply to take the blame in an accident.

So that's my unsolicited highly depreciated two cents worth.
{^_^}
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Old 28th May 2011, 03:29
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I would like some input on the following excerpt from the BEA report:-
At 2 h 12 min 02, the PF said "I donít have any more indications", and the PNF said "we have
no valid indications". At that moment, the thrust levers were in the IDLE detent and the
enginesí N1ís were at 55% the angle of attack decreased, the speeds became valid again and the
stall warning sounded again.
My interpretation is that the aircraft was in a deep stall and passing through FL250 with RoD about 10,000 feet/min, and even though both engines N1's were 55%, the pitch-down inputs decreased the AoA and the airspeed data became valid. However, that IAS was low and the stall warning activated again but ceased with further pitch-up commands as the data again became invalid.

Without AoA information, neither pilot had any idea in what part of the stall regime they were, and seemed to react as if a return of the SW was indicating Vmo, hence the continuing pitch-up command. So failure at this time to persist with the pitch-down command effectively left them doomed, as even then with the time taken for the THS to readjust (if it was going to) and for the wings to start flying, meant that the chance of reducing the RoD to zero before FL0 was marginal.

It strikes me that no-one on the flight deck realized that the AoA data was only valid when IAS was greater than 60KTS. Even though they went looking for the FPV, it seems it was only after the SW stopped at 2:11:40 plus a few seconds, and therefore to no avail - see ACARS FPV messages timed 0212z. Their final chance to grab the FPV page was missed shortly after 2:12:02 when the stall warning sounded again - meaning valid airspeed!

Do others agree??
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