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AF447 Thread No. 3

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AF447 Thread No. 3

Old 27th May 2011, 21:36
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Originally Posted by kilomikedelta
Yellow Pen; I'm not in the aviation software business so I'm not familiar with the details of any particular aircraft system. I have some experience in software writing and crisis management. I'm just asking why the software would accept the THS being at its limit for so long.
KMD - I suggest you read through the previous thread on the subject, where a lot of people with experience of how real-time software works patiently explained what the software does and does not do.

The software is designed to give the pilot exactly what he or she asks for, except in certain circumstances where a dangerous situation is developing (extreme nose-up command without attendant power increase, or an incipient spiral dive). It does this by monitoring several parameters at once, and requires that those parameters be valid and the protections enabled in order to do so.

At no point will the computer override a pilot's command completely, it will simply mitigate the response of the flight surfaces to carry out what the pilot is asking of it as safely as possible. In the case of an incipient stall due to full stick-back, it will increase thrust to maximum power. In the case of an incipient spiral dive it will limit the angle of bank to an absolute maximum of 67 degrees.

The initial situation we have here is the flight protections falling back to Alternate Law in response to the loss of speed data. This removes some protections and at this point it becomes easier for a pilot to inadvertently put the aircraft into a dangeous attitude. The PF is appearing to command vigorous nose-up by pulling back on the stick in response to these indications, but because of the degraded control laws and lack of speed data, the computer is unable to determine whether such a command is in fact endangering the aircraft. The design pattern suggests that at this point the computer is not best-placed to determine whether the demand is unreasonable or not, and the control logic simply follows the pilot's commands without intervening.

Based on the information we have so far (which is admitedly sparse), it appears that the system behaved exactly as it was designed, and the crew was faced with an unenviable situation - unreliable instrument readings at cruise altitude in unfavourable weather in the middle of the night. Even with everything working I'm pretty sure that many pilots have many places they'd rather be.
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Old 27th May 2011, 21:39
  #202 (permalink)  
 
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kilomikedelta
I'm just asking why the software would accept the THS being at its limit for so long.
I suppose we can imagine a situation where a deteriorated airframe (e.g. DHL plane hit by missile) would induce a pitch down moment that would need to be compensated by THS being permanently held close to or at its pitch-up limit.
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Old 27th May 2011, 21:40
  #203 (permalink)  
 
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Yellow Pen; We understand each other then. I suppose it would have been helpful to notify the pilot that all his pitch control efforts via the sidestick were for naught and that he should have considered using the verboten trim wheel.
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Old 27th May 2011, 21:43
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And that it would return as you recovered?
..implying that you might have 'done' the 'wrong' thing

WIH is wrong with displaying AoA; WHY this 'committee' level decison making on filtering vital flight data using simplistic 'buzzer of doom' logic (NB: it used to be a yaw string, or a simple visible vane, on a hang-glider it's 'wind on the face', in many a/c its stick force... )

In an A330, it's, er... a heavily filtered 0-1 logic state
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Old 27th May 2011, 21:44
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I suppose it would have been helpful to notify the pilot that all his pitch control efforts via the sidestick were for naught and that he should have considered using the verboten trim wheel.
That wouldn't help - as the pilot's sidestick efforts were working perfectly. If he thought of the pitch trim wheel, he would have probably attempted to trim nose up, given that was the input he was applying to the stick.
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Old 27th May 2011, 21:45
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No time, confused data and three pilots possibly adding to the confusion.
We grieve for all on board.
Oh boy this is an understatement. There aren't many chapters in aviation history as sad as this one. One finds oneself grasping at straws to understand it
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Old 27th May 2011, 21:47
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@MurphyWasRight

A simple question for Airbus pilots:

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? 27th May 2011 16:10
It would be interesting to know if PJ2 is aware of this behavior of the stall warning. If he is not, that would seem to confirm it as being an exceedingly arcane behavior. OTOH, AB has said the FBW was working as intended. Was it so, or is this behavior an oversight... an unintended consequence of a situation not considered? --OE
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Old 27th May 2011, 21:48
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Murphy,

No I wasn't, maybe vaguely somewhere in the grey matter, and even if I was could I recall that info under pressure?

I make the same point that you are re the confusion that might have caused in post 159. Its a thought.

That said if they had responded correctly to the initial stall warning would they have found themselves in that situationand having found themselves there they had other indications, attitude, to help with the diagnosis. A pilot well versed in UPs/Upsets should have been able to recover the aircraft safely.
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Old 27th May 2011, 21:49
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Originally Posted by Rob21
Henra, maybe I got it wrong but I understood from the report that the aircraft was in a pitch up attitude of 16 when it reached 38.000 ft.
Agreed !
That is indeed a bit mysterious why the 16 NU at that altitude was allowed to develop.
Probably it is linked to the general question why NU was commanded.
Fear of Overspeed after some turbulent up and down compromising feeling for energy state?
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Old 27th May 2011, 21:54
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Checkboard; Would the cockpit displays tell the pilot that the THS was at maximum nose-up so that stick aft wasn't helping?
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Old 27th May 2011, 22:05
  #211 (permalink)  
 
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Rob21; Old Engineer;

About 11 hours ago, very near the end of the Part2 thread, I noted that PJ2 was in the air, and therefore no comment from him in the short term.

I suspect he is by now absorbing the confusing data. I have slept on it, and am still confused by many things. Not least is the lack of coherency in what has been reported by the BEA, which I believe is possibly a deliberate attempt not to put any party in a bad light, and also to placate while side-stepping any judicial implications.

GarageYears has recently posted the Flight Laws link, but here it is again:-

A340 / A330 Control: flight & laws

.... which I have tidied up during the past couple of days.
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Old 27th May 2011, 22:09
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Unmanned/unwomanned transportation in the pointy end is the way to go for the future. Just think of all the carnage humans have created in past wrecks.
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Old 27th May 2011, 22:09
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Surely their horizions, PFD and standby would not be affected by AD errors? They corrected roll deviations that must have been indicated by those instruments. If they were getting accurate roll info, surely the instrument would be indicating high nose up angles as well?

Low speed / high alpha / rapid descent = Stall.. Not a dive. So why continue to hold a nose up attitude? We will never know unfortunately.
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Old 27th May 2011, 22:10
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Question to THS Trim and pitch rate

As i understand the previus posts concerning the nose up THS trim, this is said to be a normal function of Pilot nose up stick input and the computers.

Can somebody explain, how tis is done in relation to the airspeed? I mean, at high airspeed the change of the trim for a given stick input would be slower and at low airspeed it would be higher?

What kind of speed input does the system use, the wrong one from the iced up pitots? If that is the case, in the beginning the actual speed was still high, but the sensed speed was wrong and already much lower.

PNF recogniozes abnormal speed and sets pitch and power, in this case correcting also for the drop of the right wing while trying to get the nose to 5 pitch. The system generates a much greater pitch imput due to the wrong airspeed, which the PF does not recognize right away due to other distractions.

Or does the system generate the same input regardless of airspeed (cant imageine that would work)?

It would explain the initial maneuvre.
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Old 27th May 2011, 22:11
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Chris Scott
Why are the AoA values considered invalid below 60kts IAS when the A/C is not on the ground?
Because 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.
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Old 27th May 2011, 22:18
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Checkboard; Would the cockpit displays tell the pilot that the THS was at maximum nose-up so that stick aft wasn't helping?
The pitch trim position is shown on the flight control page of one of the displays - but you would have to select the correct page on the screen to see it. It is also constantly shown on the physical trim wheel in the cockpit.
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Old 27th May 2011, 22:21
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Is AOA visible on all flt Control screens as well?
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Old 27th May 2011, 22:21
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Ashling,
I agree with you. Applying the UAS QRH memory items "should" have saved the day.

I was also _not_ aware of the fact re stall indications during the recovery with the airspeed decayed to such low values. This is certainly something to put away "just in case" and to pass on to others.....

Regarding selecting Flaps below 20000ft, "our generic" QRH has some interesting indentation wrt the "If in clean configuration and below 20 000ft":
The way I read this, literally, is: Only if you are out of the stall you select Flap 1. Clearly that is wrong, but if I was a lawyer.......

Incidentally, it is the same in the QRH "on board"!

Cheers

Last edited by Peter Fox; 27th May 2011 at 23:29.
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Old 27th May 2011, 22:22
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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?
I can't remember if I did as I'm not current on type, but I do remember that in the event of unreliable airspeed you'll could get a whole range of varied and contradictory warning signals and I'd place no faith in any of them. Pitch and power are the only things you can rely on until normality is resumed and getting a further stall warning in response to doing the right thing, whilst damn confusing, shouldn't be unexpected.
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Old 27th May 2011, 22:26
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Is AOA visible on all flt Control screens as well?
AoA isn't displayed directly anywhere - however if you select the flight path vector (FPV) on the Primary Flight Display (PFD), then the gap between the FPV and the pitch reference is the AoA.



The image on the right is displaying an Angle of Attack of around 5 (Normal cruise is about 3.8 - 4, stall about 16 )
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