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AF447 Thread No. 3

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AF447 Thread No. 3

Old 2nd Jun 2011, 19:38
  #1121 (permalink)  
t54
 
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BOAC
I got that "stall warning triggered by low air-speed indication from -
Initial Air France Flight 447 Black Box Info Raises More Questions Than Answers: Pilot | NYCAviation

I was a bit surprised because I had thought stall was purely a function of AOA, and no doubt air density as well. But I can see that air speed might come into it to, but I would have to have the mechanism of that explained to me
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Old 2nd Jun 2011, 19:47
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LW50:
Referring to AoA as a crosscheck if Airspeed becomes unreliable has been suggested (to confirm "what is my wing doing?") as a suitable improvement to the pilot's tool kit. (A lesson learned, if you wish).
Before adding AoA crosscheck to the "pilots tool kit" it needs to be added to a display somewhere a pilot can access it.

Unless I misread something UAS blanks out the one "proxy" AoA display available to the pilots.
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Old 2nd Jun 2011, 19:47
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Dozy:
In that case, all the ADIs (powered by AHRS) were functioning perfectly, and the relief F/O repeatedly called out "ADI!" on the way down, pointing out that the aircraft was in an extreme nose-high attitude (the other F/O, who had a working ASI repeatedly called out that they were stalling) - all the way down to the ocean.

The pilot in that case, as I've said before - no low-hour newbie, but an experienced ex-Air Force jockey appeared to be so overwhelmed that he failed to check the ADI in front of him that was telling him he was nose-high, even as his F/O's were emphatically telling him what to look at.
The human factors element of this mishap seems an apt parallel to what AF 447's PF might have been experiencing.

@Murphy:

Yes, of course, sorry I left that out. As I noted earlier, choosing what piece of "real estate" in the display area would house an AoA gage requires careful consideration, and some ergonomic and "scan efficiency" study before a decision is taken.
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Old 2nd Jun 2011, 19:50
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Originally posted by tk54
I read somewhere that the initial two stall warnings were a valid response to the invalid 60kt pitot reading.
Maybe the stall warning comes first in the relevant part of the computer program , before the validation of the data upon which the stall warning is based?
Originally posted by Lonewolf_50

Stall warnings, and stall α, are shown in HazelNuts39's graph
They are based on AoA, the magnitude of which is influenced by airspeed/mach number.

I don't understand how you reach the idea of a stall warning based on an invalid (or even a valid) airspeed reading, since stall warnings are (should be?) based upon AoA which is not the same as airspeed. It is a different parameter, influenced by airspeed, attitude, gross weight, angle of bank, G, air density ... etc

From earlier posts: what the 60 knots threshold seems to have triggered is a disabling of a stall warning, which is itself enabled by an AoA reading as shown in HazelNuts39's graph.
From the report:

Note: When the measured speeds are below 60 kt, the measured angle of attack values are considered invalid and are not taken into account by the systems. When they are below 30 kt, the speed values themselves are considered invalid.
The question is has the system rejected a/s (60 kt) by this point? If so, why is it still being honored by AoA logic?

"When they are below 30 kt, the speed values themselves are considered invalid."

Considered invalid by which sub-system? AoA logic? BEA doesn't say.
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Old 2nd Jun 2011, 19:51
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Welsh Wingman

"possible weather deviation limitations for this plane on this particular route (unless the flight crew want to land in Bordeaux and endear themselves to flight ops...)"

Weather deviations should cost peanuts in fuel quantity (ask your math teacher) so can we put this one to bed please...
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Old 2nd Jun 2011, 19:52
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Originally Posted by Lonewolf_50
The human factors element of this mishap seems an apt parallel to what AF 447's PF might have been experiencing.
LW_50 :

Many thanks. I've been saying this since the tail-end of the previous thread, when interest picked up over the BEA's "note", but very few people seem to have noticed. In addition, I was also clued in to the BEA's report on the Moscow A310 incident, which referred back to data that says that without proper training, 80% of pilots instinctively pull back on the yoke or stick when confronted with an unexpected stall warning.
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Old 2nd Jun 2011, 19:54
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Originally Posted by Bearfoil
I am weary of folks accepting counterintuitive and plain wrong assumptions, including me.

If the a/s was 60 knots, and the a/p was in, we need to talk. If the Stall warning was on, and the a/c was not in ABNORMAL LAW, likewise. Monsieur takata swears that a/p drop was UAS driven, in spite of a time line provided by BEA.

Screw ACARS, all right? Old habits, etc. If the a/p was in, and the PF had a Stalling a/c in his mitts, he had a correct response.

Cruise flight is not easy to parse into BITS, it is a dynamic Dance, and these guys had an "Unfolding" (Unwinding) Flight Path. May we start there? Back to read only.
What a load of twaddle!

This persistent habit of inventing stuff out of the blue, and presenting it as it is somehow fact, while ignoring or distorting what is known fact, to allude or insinuate that the aircraft suffers from some unspecified flaw and that the BEA is in cahoots with the establishment to somehow pin it on the crew is really getting old.

Give it a rest, for everybody's sake.
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Old 2nd Jun 2011, 19:56
  #1128 (permalink)  
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Doze

"Bear, you've got to be careful how you phrase this stuff. A newcomer to this thread would think that what you're presenting is a known fact rather than a theory posited by a single poster that is based on no evidence whatsoever."

There is no evidence as yet. Evidence is entered under penalty of perjury, and as "held and factual".

Here, we have ACARS, some few motes of "data", and miles of "O'Briennese" (no offense, mate). If a/p dropped, any flight vectors that had been under control, would have reverted to untrimmed, or been patent under a/p commands. Thus, "Trending". Not obvious? Fine.

Any one who is misled by reading any of this, needs a therapy. Informed opinion, at best. You give at least this old pilot too much credit.

happy day, Sir.
bear

ec, one outsider........


"This persistent habit of inventing stuff out of the blue, and presenting it as it is somehow fact, while ignoring or distorting what is known fact*, to allude or insinuate that the aircraft suffers from some unspecified flaw and that the BEA is in cahoots with the establishment to somehow pin it on the crew is really getting old."

*"known fact"........Sorry?
 
Old 2nd Jun 2011, 19:57
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jcgeant
"Sorry if I'm dumb .. but .. someone can make a technical comment (explanation)
about the text in bold .. ?

From the 27 May BEA report:
Quote: Around fifteen seconds later, the speed displayed on the ISIS increased sharply towards 185 kt;
it was then consistent with the other recorded speed. The PF continued to make nose-up inputs. The airplane’s altitude reached its maximum of about 38,000 ft, its pitch attitude and angle of attack being 16 degrees."

That is puzzling. If the wing chord angle is zero it makes sense - the plane was moving horizontally through still air. I can't see how it can ever happen if the chord angle is not zero with a horizontally moving plane (which is implied by "reached maximum altitude").
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Old 2nd Jun 2011, 20:01
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Bear, don't be obtuse. I meant evidence in general terms (as in a fact or collection of facts), not legal.

[EDIT : I note that you're flying your true colours more obviously on the sister thread in R&N:

Originally Posted by bearfoil
Suggest a

"LAWS of TOULOUSE" become a sticky.

( Laws of "too loose"? )
]

Last edited by DozyWannabe; 2nd Jun 2011 at 20:36.
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Old 2nd Jun 2011, 20:11
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t54 its pitch attitude and angle of attack being 16 degrees
its geometrie: in still air if the flightpath = chord angle (mayby + 3 deg....) than pitch attitude= AoA
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Old 2nd Jun 2011, 20:25
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@Hazelnuts39:
I'm amazed. What prevented you from doing the appropriate thing and drop the nose half a degree or so?
I assume, you know the A330 and its related non-normal procedures: It says in QRH 2.21 "Respect Stall warning and disregard "Risk of undue stall warning" status message". In so far, you would be right. However, if you follow the whole procedure, crosscheck your Attitude/power-setting according to the table provided in 2.23, crosscheck with the GPS-derived speed and get the stall-warning nevertheless (BTW: with changing pitch angles) you consider the stall-warning invalid. Even the TRE explained that the simulator does not behave according to the manuals.

All I wanted to hint to is that sometimes the (aural) warnings created by the aircraft are adding up to the pressure you already have and should be able to be cancelled (e.g. by use of the emergency cancel-button, which did not work in this case...)
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Old 2nd Jun 2011, 20:29
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Doze.

Quote:

I know that the increased presence of computers in the flight deck is an emotive issue for pilots, but I can assure you that from the perspective of this software engineer, and, I'd be willing to wager even more - every software engineer who worked on the FBW aircraft that are flying today - we are on your side.

Amen. And thank you...
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Old 2nd Jun 2011, 20:29
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Airbus control logic and alternate mode control

One problem that I see here with the Airbus approach to degraded mode operation is that the aircraft can be put into a flight mode that is not natural. In normal law the aircraft flies to a point in space where the pilot tells it to. In Direct Law the airplane is the same as a conventional aircraft, but in Alternate law the aircraft doesn’t have enough information to control the aircraft in a stable manner and it relies on the pilot for pitch stability. That is, when flying in normal mode the aircraft is controlled to point in the sky, and the autopilot system controls flight path by adjusting power and trim to match the control inputs of the pilot and the speed selected.

That’s all fine and good until something goes wrong with the inputs. When the control system goes into Alternate Law things get much more difficult. What I understand happens in that instance is that the pilot becomes the one who is controlling the aircraft in pitch and, the system is not stable in pitch. With a conventional trim system you set the trim angle and if you release stick pressure the aircraft will return to the speed required to match that pitch trim setting. This is the definition of a fundamentally speed stable system, remove the inputs and the aircraft goes back to a trimmed airspeed. With such a system, if the pilot made an incorrect input or responded to turbulence, all you have to do is relax stick force and the aircraft returns to a stable condition. With the Airbus the pilot must maintain pitch control with reference to the HSI and the trim will move around. Some of the Airbus pilots here say that isn’t excessively difficult, but it is surely a much higher workload to constantly keep the required pitch in the HSI while you are trying to fly through turbulence.

The Airbus system, in Alternate Law, will adjust the THS to “chase” the pilot inputs by changing the trim position. This is fundamentally unstable and consequently, unless you make absolutely no forward and aft inputs, the aircraft will climb or descend and the speed will increase and decrease. If the inputs are large enough and long enough, the trim will change and the aircraft will remain in a climbing or descending mode until the pilot applies input. It seems to me that Alternate Law without airspeed is really kind of a bastard system in that the control really doesn’t have enough information to fly the aircraft, so the pilot has to step in and provide stability.

In my opinion, the proper control system response to loss of airspeed information would be for the autotrim to be disabled at the same time as the autothrottle and set to the same condition as it was just before the autopilot dropped out. So I guess that in the case of loss of speed sensors I am thinking the system would be much better off in direct law.

Then the aircraft will be remain at a known pitch condition and therefore be speed stable. You don’t know what the speed is, but it was fine where it was and it isn’t going to change. The pilot then only has to control altitude with the throttle and keep the wings level and he is in a much safer place. If the pilot decides he wants more or less speed he can adjust the trim wheel to get it.

Somebody please correct me if I am wrong.
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Old 2nd Jun 2011, 20:41
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@ engin-eer:

The autotrim in Alt2 will only set the THS to "chasing" inputs if that input is held past the limits of elevator authority, otherwise it simply stays in the last position set when the FMC/autopilot kicked out. Letting the stick go in that situation will result in what you describe - i.e. the aircraft trimming itself back to it's stable position.

The only reason the autotrim moves to match pilot input is to give maximum control authority to a pilot in a significant upset without having to have his or her hands on both the sidestick and the trim-wheel. It follows that to have autotrim responding to sidestick inputs requires an emphatic stick movement held for some time.
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Old 2nd Jun 2011, 20:47
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Quote:
t54 its pitch attitude and angle of attack being 16 degrees
its geometrie: in still air if the flightpath = chord angle (mayby + 3 deg....) than pitch attitude= AoA
Yeah, I got that; but the flight path is horizontal at the top of the path.
You could only get pitch = AoA in horizontal flight if there is an upward wind.
Maybe there was and that's what helped to get the plane up there in the first place.
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Old 2nd Jun 2011, 20:48
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Old China

Cross purposes, I think.

"
Welsh Wingman
"possible weather deviation limitations for this plane on this particular route (unless the flight crew want to land in Bordeaux and endear themselves to flight ops...)"

Weather deviations should cost peanuts in fuel quantity (ask your math teacher) so can we put this one to bed please... "

I didn't mean the particular de minimis deviation cost in this instance, but rather in the wider systemic issues that I was discussing. I was under the impression that this plane was close to MTOW at GIG and this route is close to its maximum range (never flown an A330-200 or any other Airbus before retiring in 1992, so not claiming any expertise), so merely hypothesising whether the BEA are hypothesising about the wisdom/relevance (if any) of even sending pilots into the ITCZ in the knowledge that a "major" weather diversion could mean a refuelling stop short of destination. I recall an old SAA friend telling me a few years back that some SAA pilots were frustrated at having to fly the A340-600 on the Joburg/New York route, when a longer range A340-500 would be perfect for the route if SAA had properly invested in the right equipment for the route.

Feel free to shoot this down, now that I have made clear. Not arguing that this is likely to be one of the major factors in the hull loss, but wouldn't yet discount it altogether from the overall mix in the context of not always giving equatorial CBs the healthy respect they deserve (unless I am wrong about my understanding of the range of the A330-200, which I accept that I might well be).

Not wanting to send anyone off on a wild goose chase.

I was thinking more about my old history teacher rather than my math teacher - divide-up the understanding of historical events into long-term causes (aviation industry focus?), short-term causes (aircraft manufacturer and/or aircraft operator focus?) and triggers (aircraft operator SOPs and flight deck focus?)....
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Old 2nd Jun 2011, 20:55
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Cool

Hi,

jcgeant
"Sorry if I'm dumb .. but .. someone can make a technical comment (explanation)
about the text in bold .. ?

From the 27 May BEA report:
Quote: Around fifteen seconds later, the speed displayed on the ISIS increased sharply towards 185 kt;
it was then consistent with the other recorded speed. The PF continued to make nose-up inputs. The airplane’s altitude reached its maximum of about 38,000 ft, its pitch attitude and angle of attack being 16 degrees."

That is puzzling. If the wing chord angle is zero it makes sense - the plane was moving horizontally through still air. I can't see how it can ever happen if the chord angle is not zero with a horizontally moving plane (which is implied by "reached maximum altitude").
Well I asked a explanation from pilots as I was puzzled by "Level 380" ... because this other A330 case (AF445) same sector .......

An Air France Airbus A330-200, registration F-GZCK performing flight AF-445 from Rio de Janeiro Galeao,RJ (Brazil) to Paris Charles de Gaulle (France), was enroute at FL380 overhead the Atlantic on airway UN741 just before waypoint DEKON about 680nm northeast of Fortaleza,CE (Brazil) and 750nm southwest of Praia (Portugal, Cape Verde), when the crew called Mayday on the international emergency frequency indicating, they encountered severe turbulence and were descending to a lower altitude. The airplane was seen enroute at FL280 overhead France and landed safely at Paris Charles de Gaulle 6:40 hours after the emergency call.

The Mayday call was relayed by the crew of a TAM Airbus A330-200 registration PT-MVG performing flight JJ-8055 from Paris CDG to Rio de Janeiro,RJ (Brazil) at around 03:50Z (Nov 30th).
They were descending .. or they were stalling ?
Note that all recorded datas of this flight were never examined by any regulator or the BEA (AF was not able to produce those datas)
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Old 2nd Jun 2011, 20:59
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@ engine-eer, I agree. In direct law I'd have a conventional Boeing with natural speed stability - perfect.

@ dozywannabe. Alt Law in pitch = "In flight, the alternate law pitch mode follows a load-factor demand law much as the normal law pitch mode does, but it has less built-in protection".
It doesn't sound much like direct law to me.
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Old 2nd Jun 2011, 21:00
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Dozy:
I understand the advantages of autotrim, but I have reservations about the reason autotrim matches pilot input as explained here.
The only reason the autotrim moves to match pilot input is to give maximum control authority to a pilot in a significant upset without having to have his or her hands on both the sidestick and the trim-wheel.
Pilots have two hands. There are two pilots on the flight deck. Four hands. Autotrim available in an upset seem to me a very handy tool, particularly as the pilot (as I understand it) can override it via side stick commands.

Would it make sense that if one is in a Law or condition that calls for manual trim, that procedures would call for the PNF to be on the throttles as required/requested by the PF? That leaves PF with one hand on side stick and one hand on trim wheel.

I am in a CRM zone here.

That strikes me as how to avoid worrying about a two handed pilot having a three handed job, which you seem to allude to above.

Does the approach to this known possible condition vary between companies, or is there a generally agreed standard on how to deal with this in Direct law, upset or no?
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