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AF447 Thread No. 3

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AF447 Thread No. 3

Old 8th Jun 2011, 10:36
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macdee re: tight turn to starboard

The graphic you refer to in the BEA update of 27 May shows that the turns to starboard occurred after the plane had stalled and had already descended more than 3,000 feet, at which time "The airplane was subject to roll oscillations that sometimes reached 40 degrees." The turns also appear to have started shortly after the captain re-entered the cockpit, which may or may not be a coincidence.
Between disengagement of the autopilot (point 4 on the BEA graphic) and stalling (about 15 seconds after point 5), the plane shows no deviation in course.

Last edited by GraemeO; 9th Jun 2011 at 06:22. Reason: references to BEA graphic added for clarity
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Old 8th Jun 2011, 11:29
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cwatters writes
Has the possibility of a fault with the PF stick been ruled out?
With an AoA of greater than 45 degrees, neither the wings nor the horizontal stabiliser will be acting as aerofoils, but simply as drag surfaces. Therefore it is unclear what the effect of any stick input would be.

It is certainly difficult to visualise how there would be any roll authority, and pitch authority could conceivably be the reverse of normal. Therefore theorising a fault with the stick seems a step too far.
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Old 8th Jun 2011, 13:26
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@ mm43 (post #1567) : Thanks for pointing the FlightGlobal article. Wasn't aware of it, and based my hypothesis on the sole FCOM.
Thanks also to A33Zab
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Old 8th Jun 2011, 13:27
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Thanks GraemeO, just wondered.
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Old 8th Jun 2011, 13:43
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[cwatters writes
Quote:
Has the possibility of a fault with the PF stick been ruled out?
With an AoA of greater than 45 degrees, neither the wings nor the horizontal stabiliser will be acting as aerofoils, but simply as drag surfaces. Therefore it is unclear what the effect of any stick input would be.

It is certainly difficult to visualise how there would be any roll authority, and pitch authority could conceivably be the reverse of normal. Therefore theorising a fault with the stick seems a step too far.
Can't be ruled out, just like SS armrest failure.
You need a proper adjusted and fixed base to perform coordinated SS inputs.

In such case PNF could take control but what if you are dealing with an emergency situation and fighting to get control while PNF is in the middle of QRH? Would one change roles?
All they had was only 210 seconds.
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Old 8th Jun 2011, 14:09
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Has the possibility of a fault with the PF stick been ruled out?
Can't be ruled out,
True.

But if one cannot be sure of the effects of a correctly operating stick in extreme AoA conditions how can one begin to conjecture on the effects of a faulty one?
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Old 8th Jun 2011, 14:24
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Cool

Hi,

In the world of pilots of airliners ... is what the word panic is forbidden to use?
Do airline pilots are men with capabilities far superior to other men and therefore they would not be subject to the phenomenon of panic that often occurs in situations of great stress .. even among men the most seasoned
Panic is known to decrease the power of reason in a logical manner.
Note that the panic .. when it is externalized can be contagious (can influence people in contact with the person who panics)
Panic .. Perhaps an explanation as any other to explain the behavior of the PF and the sequence of events.
This can't be ruled out ... so far.
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Old 8th Jun 2011, 14:41
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Not a bus driver.

There must be 500 pages on all the threads by now and I'm buggered if I'm going to read them all. Can someone bring me up to speed on this quickly with definitive answers, I'm trying to get my head round this:

The stall warning is inhibited below 60kts IAS, as per my understanding.

1. Does WoW factor in this decision?

2. Someone please assure me the logic is sane enough NOT to inhibit the stall warning at any IAS in the event of AIRSPEED UNRELIABLE?!

Thanks
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Old 8th Jun 2011, 15:05
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Hi Ranger One,

Sorry - I can't assure you of that despite trawling through FCOM. It seems ground/flight logic has not been included with the Stall Warning - just the 60kt switch.
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Old 8th Jun 2011, 15:12
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Contributions from A33Z - attaboy

I think all of us here appreciate the inputs from A33Z concerning more detail of the control laws, etc. So ATTABOY!

Some of us are still not clear what is taking place with the "trim", however. My understanding is that the 'bus attempts to achieve one gee ( roll and pitch corrected gear up, not sure when gear and high lift devices deployed). In other words, let go of the stick and the system tries to get to one gee. Pilots can't 'trim" for a gee, only command something other than one gee. So only reason I can see for the THS to move is to allow for dynamic pressure changes ( speed) that limit the elevator's effectiveness and/or allow it to operate from a "neutral" position versus an extreme angle. In other words, the THS doesn't move to achieve anything other than one gee with hands-off stick position. So I can see that as speed decreases or increases, the THS would have to change in order that the pilot gee command would be the same amount of stick displacement, regardless of speed.

Am I close?
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Old 8th Jun 2011, 15:23
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Eee by gum Gums. That's so close - I can't spot the error.
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Old 8th Jun 2011, 15:25
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gums

As in, trimming for stable climb rate? Some version of 'cruise' climb? As energy depletes, the THS increases Pitch Up to maintain climb? Something in this climb suggests a maneuver that the pilot felt was OK? I am lost how "trim" has any place in what one would think an emergency maneuver? The BEA say, "FROM 2:10:05, ".... They leave out a sequence of pilot displays. What information was the PF operating on to continue this climb draining energy merely to gain altitude? Was he trying to dump overspeed? Or was this THS movement unknown to him, he believing he had plenty of energy, and felt his AoA was reasonable? Had he thought he was merely initiating a "Little bit to the left" turn? Adding just a bit of NU that the THS decided to chase? When did he Know he was without A/THR, and auto trim wouldn't mess him up? It was eleven seconds after the a/p that both of them made note of the sitch.

Halp!
 
Old 8th Jun 2011, 15:34
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2. Someone please assure me the logic is sane enough NOT to inhibit the stall warning at any IAS in the event of AIRSPEED UNRELIABLE?!
First of all, AIRSPEED UNRELIABLE may only be, indeed probably is, "crew assessed" not aircraft (if the aircraft assesses it it will "deal" with it).

Next, even if the crew do assess it, they do not, and cannot "tell" the aircraft.

So now we have a stall warning, driven by a airflow "vane". When would you like the stall warning to go off? Even at airspeeds below which the vane can work?? These vanes are substantial pieces of metal that can withstand the best part of 500K/1.0M. They are not little feathers that will read reliably at 20K IAS

Your post expresses indignation at a warning that was not present, when maybe it should have been. The designers also have to consider not presenting warnings when they are not valid - indeed "Stall" is one of the highest level wanrings in the Airbus, and the consequence of an incorrectly presented one is potentially very hazardous.

The airframe / software designers have to work to some parameters. Knocking off AoA interpretation below 60K IAS seems valid enough to me - and also at that sort of airspeed you are not just "stalled" in the normal sense of the word - you are a falling leaf. I cannot see elevators having any effect at 50KIAS. The idea of the Stall Warning, and recovery actions, is to never get anywhere near that situation.

I am not relating the above to the accident as much as we know it, just the facts in the posts above.
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Old 8th Jun 2011, 15:55
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NoD:
Your post expresses indignation at a warning that was not present, when maybe it should have been. The designers also have to consider not presenting warnings when they are not valid - indeed "Stall" is one of the highest level wanrings in the Airbus, and the consequence of an incorrectly presented one is potentially very hazardous.
On the other hand there is a huge difference between inhibiting initiating a stall warning at <60Kts and arbitrarily -stopping- an existing stall warning when the (sensed) speed goes from 60 to 59 Kts and then even worse
-resuming- the alarm just as corrective action is taken (nose down) and the sensed speed rises above 59Kts.

Do totally agree with you that consequences of screwing up the stall alarm can be very bad.
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Old 8th Jun 2011, 15:56
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Hi NoD,

When would you like the stall warning to go off? Even at airspeeds below which the vane can work??
If I'm airborne - I'd like the warning to be available all the time please, especially with blocked up pitots showing Zero IAS.

the consequence of an incorrectly presented one is potentially very hazardous.
not as hazardous as the warning being inhibited when stalled - apparently.

The idea of the Stall Warning, and recovery actions, is to never get anywhere near that situation.
I agree. But if you walked onto the flight deck and found your colleagues were descending like AF 447 - do you think you might have had a better chance if the stall warning was still sounding?

I cannot see elevators having any effect at 50KIAS
The fully trimmed up elevator was doing a very good job at holding that trimmed attitude. Nose down input accelerated the aircraft - but the stall warning started again - which must have been very confusing. The real airflow air speed was much greater than 50 kts - it's the angle of attack at the entrance to the pitot which made them show so slow.
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Old 8th Jun 2011, 16:02
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The BEA report places emphasis on the point that the unreliable airspeed lasted for no more than a minute. However it was in that minute of understandable confusion that the major upset occurred.

The big question is whether the flight crew where then already locked in a trap of no escape or did they miss a trick? As this is so far beyond the envelope of simulators, it is a question only an Airbus aerodynamicist could answer with any authority.

I cannot help feeling that the strong implication of inappropriate control input (in the later stages of the incident) from the PF may be misplaced if pitch and roll authority were all but gone and we know so little about the response he was getting to his inputs.
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Old 8th Jun 2011, 16:23
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Don't Hang Up

Not only that (inappropriate inputs later on), who is to say that the pilot's initial (NoseUp, Left Roll) were anything but proper? The BEA has not expanded on the most important maneuver, the first one, flown after loss of AutoPilot.

He may have been under the mistaken impression that he could pull, roll, (for gums, bank and yank) with impunity, the a/c would PROT itself. Also, his climb (whether intended or no) may have been instigated whilst he was confident in AutoThrust. It was at least eleven seconds after A/P loss that the pilots acknowledged, "lost speeds, Alternate Law", (which Alternate Law? Did they know?).

A loss of autopilot does not automatically drop the a/c into ALTLAW. Loss of speeds does. A vanilla loss of A/P leaves the a/c in NORMAL LAW, full protections. Were the crew instantly aware of ALT LAW? NO, they acknowledged it eleven sedconds later. This is important.

One cannot merely assume that BEA released its sparse product in good faith. Given the Gallows attitude of the Press, was it not somewhat expected that "Blame" might be appropriated? Something their mission prohibits?.

Whether conscious or not, the BEA have some explaining to do, re: the entry into the Public record of such paltry data.
 
Old 8th Jun 2011, 16:29
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DHU

With an AoA of greater than 45 degrees, neither the wings nor the horizontal stabiliser will be acting as aerofoils, but simply as drag surfaces. Therefore it is unclear what the effect of any stick input would be.
Strictly speaking, both are still airfoils, producing both lift and drag as long as there is relative wind, which there was. Control surface movement will still alter their lift and drag albeit with significantly less effectiveness approaching inadequate, particularly with all the nose-up stabilizer trim introduced. For example, recovery may require that the engines be brought to idle to minimize their contribution to the nose-up pitching moment, certainly not an intuitive response when descending out of the assigned cruising altitude. Lowering wing flaps (and landing gear) should provide a nose-down pitching moment. In any event, nose-down elevator shouldn't have been necessary for recovery. Absent a too rearward cg, T-tail, TOGA thrust from under-slung engines, and/or nose-up control input, a conventional airplane trimmed for level flight should be pitching nose down even at this angle of attack.
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Old 8th Jun 2011, 16:32
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If I'm airborne - I'd like the warning to be available all the time please
OK - even when the spec/manufacturer of the vane says the IAS is way below the validity of the vane's spec? I'd rather not...

Please go back to your basic training... if you are a pilot? The Stall Warning is a symptom (one of) the incipient stall - a "warning sign" as such. You react to it, but preferably prior to it, by flying the aircraft away from the stall. You, I hope, do not, when the warning goes away, say "oh that's all right then".

not as hazardous as the warning being inhibited when stalled - apparently
Cannot see where you get that from at all? We have no analysis from the BEA, and if the stall warning went off once, if only briefly, it brought the crew's attention to the situation and they should have been 100% sorting that out.

If we have go airline crews who recover from a stall purely in reaction to, and only while, the warning is going off, and consider recovery complete when it silences, we have a bigger problem that I thought NB this is not saying this what the AF crew did, just a reaction to your post.

do you think you might have had a better chance if the stall warning was still sounding?
No idea... but that is not relevant. We need to design as robust and reliable a system as experience shows we need - not redesign whole systems based on a release (unanalysed) of some data on one accident that will, inevitably, have a multitude of causal factors and recommendations.
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Old 8th Jun 2011, 16:39
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But she was trimming for climb, automatically, so much so that the a/c stalled, having lost autothrust and assumedly, "Stall" (aPROT). So this is the deadly combination. A/P drops, a/c climbs, and Stalls. Thirty seconds over Oraro.

This time grab can be parsed at this point to weight a conclusion of wildly general and biased opinion. What happened after the Stall is extremely interesting, but as has been pointed out, minimally important. This a/c should not have Stalled, ever.

For instance, the initial maneuver was accompanied by two separate and brief Stall Warnings. What was the AoA the a/c had trimmed in at a/p drop? What was her Speed? Local airflow? Is ICE absolutely necessary, Willem Occam?
 

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