AF447 Thread No. 3
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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.
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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kilomikedelta
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.
I'm just asking why the software would accept the THS being at its limit for so long.

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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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And that it would return as you recovered?

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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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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No time, confused data and three pilots possibly adding to the confusion.
We grieve for all on board.
We grieve for all on board.



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@MurphyWasRight
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
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
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

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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.
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.

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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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.
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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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.
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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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 (can´t imageine that would work)?
It would explain the initial maneuvre.
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 (can´t imageine that would work)?
It would explain the initial maneuvre.

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Chris Scott
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.
Why are the AoA values considered invalid below 60kts IAS when the A/C is not on the ground?

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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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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
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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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?

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Is AOA visible on all flt Control screens as well?


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