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AF 447 Thread No. 5

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AF 447 Thread No. 5

Old 14th Jul 2011, 21:53
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RetiredF4 Do we know, how the lift generating capabilities of those airfoils changed in this high AOA regions?
with very high AoA the effect of the profile with his flow is becoming smaler, the center of lift and Cd and Cl approaches the forces of a flat panel with the same AoA.... with an AoA of 90 deg every different foil ends with an center of lift near 50% and an Cd near 1....
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Old 14th Jul 2011, 22:03
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PJ2, #281

Thanks for the reply and the reference. What was interesting was the
comment about access to tech detail information and it's scarcity. At
the risk of repetition, if I were flying these machines, I would want to
study the detail. A deeper understanding of the machine can only improve
pilot skills, in the same way that knowing what goes on under the hood
of a car helps to appreciate the limitations and to get the best from
it.

Perhaps the (secrecy ?) is a european thing and wonder if boeing provide
more free access to such information.

Anyway, this is probably a bit off topic for tech log ...
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Old 14th Jul 2011, 22:35
  #323 (permalink)  
 
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takata;

I think I basically agree with what you wrote. I just wasn't aware of it and thanks for patiently clarifying this point. However, I'm left with a few grey areas which may be relevant for what the pilots of AF447 saw.

The text you copied from the Air Caraibes memo appears on page 5 in the context of the protection named "CAS MONITORING" which in the BEA reports is the initiating event at 2:10:05 of the whole 2:10 sequence and results in the PROBE PITOT fault message after closing the one-minute correlation window.

Page 7 of the ACA memo shows pictures of the PFD speed scale in Normal and Alternate law in the context of an ADR DISAGREE message with Vsw in the Alternate Law 2 case.

While it may be unimportant whether or not a calculated Vsw is indicated on the speed scale when IAS has dropped to 60 kt or less, the intriguing question for me is whether it would be shown in the intermediate period when one or more of the indicated speeds had become valid again. EDIT:: Why should Vsw not be shown along the speed scale showing a 'valid' speed, first on PFD1, later on ISIS, and at an unknown time on PFD2?

Last edited by HazelNuts39; 14th Jul 2011 at 22:48. Reason: EDIT
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Old 14th Jul 2011, 23:00
  #324 (permalink)  
 
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Originally Posted by syseng68k
Perhaps the (secrecy ?) is a european thing and wonder if boeing provide more free access to such information.
Nope, it's a proprietary-systems-and-software-design thing. I don't think they've even publicly released the tools they use, let alone open-sourced the design and the software, and I'm almost certain that no, B don't do it either.

Bear in mind that in Redmond - a few miles away from Boeing HQ - lives the second-biggest PITA for proprietary software there has ever been!
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Old 15th Jul 2011, 01:17
  #325 (permalink)  
 
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Originally Posted by Lonewolf
Harry, we aren't at a high Mach number if we are stalled, are we?
Can't see where I suggested we were at high mach no

Yes, we can easily be stalled at High Mach, just as in incompressible flow (that's what the lower cruise buffet boundary and coffin corner is about, no?)

What do you mean by high Mach ? > M 0.5? transonic? or supersonic?

Last edited by HarryMann; 15th Jul 2011 at 01:27.
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Old 15th Jul 2011, 02:12
  #326 (permalink)  
 
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Originally Posted by HazelNuts39
The text you copied from the Air Caraibes memo appears on page 5 in the context of the protection named "CAS MONITORING" which in the BEA reports is the initiating event at 2:10:05 of the whole 2:10 sequence and results in the PROBE PITOT fault message after closing the one-minute correlation window.
Right. There is two independant functions for monitoring the speed for essential systems which should be the first to detect probe errors (beside plenty of other systems self monitoring their own recieved imputs, just in case).

a) ADR monitoring at Auto Flight System (AFS) level (-> FMGC). This should also be the first one to react as the threshold is set at 0.45 seconds for a variation of 20 KT.

b) ADR monitoring at Electronic Flight Control System (EFCS) level (-> FCPC/PRIM) which is "CAS Monitoring" you refered to. Here, the threshold is a variation of 30 KT during 1.0 second. It is also responsible for changing immediately the Flight LAW to ALT2 when a variation is detected. Hence, it takes 1 second to change the flight law, but it's at first temporary. After this point, the last valid value (before change) is retained and a comparison is made with the current value 10 seconds later: if the difference is > 50 KT, ALTERNATE 2 is confirmed, whithout the possibility to revert to NORMAL until post flight reset.


Originally Posted by HazelNuts39
Page 7 of the ACA memo shows pictures of the PFD speed scale in Normal and Alternate law in the context of an ADR DISAGREE message with Vsw in the Alternate Law 2 case.
This illustration is taken from the FCOM and is generic for ALTERNATE if airspeed is not unreliable. In case of UAS, What you will have instead is the red flag "SPD LIM" (see report previous page) without Vmax, VSL nor VSW and... It will last for the remainder of the flight like ALT2! (read me well: even if the airspeed come back to normal after a while, this is lost as it is displayed only when ALTERNATE low/high speed protections could work, which would not be the case after an UAS. Even if coherent, speed will always be considered spurious by the system). That is also the reason why some long haul flights turned back and landed after an UAS event which was not transient (lasted more than 10 seconds).

Originally Posted by HazelNuts39
While it may be unimportant whether or not a calculated Vsw is indicated on the speed scale when IAS has dropped to 60 kt or less, the intriguing question for me is whether it would be shown in the intermediate period when one or more of the indicated speeds had become valid again. EDIT:: Why should Vsw not be shown along the speed scale showing a 'valid' speed, first on PFD1, later on ISIS, and at an unknown time on PFD2?
Quite simply, it is an EFCS function that compute all the characteristic speeds from data comming from the FE part of FMGC and ALL ADRs. This function become inoperative as soon as more than 1 ADR is rejected by the FCPC. After this point, it will consider both FMGC (1 & 2) as inop regardless if the data will be later coherent again. Hence, the display of probe data (speed) on each PFD or ISIS is directly available for the crew but it is unrelated with the function computing the characteristic speeds which is lost. Those data displayed are not what the system could use as it will need two "valid" sources, at any time, for safe operation.

Last edited by takata; 15th Jul 2011 at 02:25.
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Old 15th Jul 2011, 03:46
  #327 (permalink)  
 
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Viper stabilators

For DJ77, et al:

- The stabilators, or horizontal stabilizers/elevators, moved +/- 21 degrees.
- They had a symmetrical airfoil, and...
- They were interchangeable from one side to the other
- They moved independently in order to help with roll, especially when supersonic
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Old 15th Jul 2011, 11:52
  #328 (permalink)  
 
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Harry:
You were talking about a highly separated flow regime. That's (in the context of this case) the low speed regime stall. The case under discussion isn't, as far as I understand, a high speed (and thus high mach for this airplane) stall example.
Yes and when in a highly separated flow regime (fully stalled foils), extreme AoA, we may typically dispense with the assumption we are flying, per se, and often revert to just summing the Resultant Force vectors rather than thinking in terms of Cl, Cd and Cm, associated with the fairly linear ranges of the lift curve slope and pitching moment slope (Cd just about remains a mashup of a parabola and drag dropout buckets at high Mach)
I wasn't sure what to make of your reference to high mach, honestly.
That's why I asked. I think, on reading your post again, you were comparing shapes of Cd curves at different ends of the spectrum ... as a general reference?
What do you mean by high Mach ? > M 0.5? transonic? or supersonic?
In the context of this class of airfoil (large transport) let's say transonic or approaching supersonic (limits of airfoil before the high speed buffet, coffin corner, etc.) would seem to me "high mach" as the airfoil isn't shaped for operations in supersonic flight. (I confess, it's been over thirty years since I studied compressible flow, and things like Prandtl Meyer flow, so it doesn't recall well from memory.)

I don't think we'd get anywhere discussing this airfoil under conditions at Mach 2, for example.
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Old 15th Jul 2011, 11:59
  #329 (permalink)  
 
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Ah yes, thanks
(Cd just about remains a mashup of a parabola and drag dropout buckets at high Mach)
Just covering as you say either end of the spectrum, though drag buckets due to a cruise Cl laminar regime also can have a Mach element (be a function of).
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Old 15th Jul 2011, 12:05
  #330 (permalink)  
 
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Just covering as you say either end of the spectrum, though drag buckets due to a cruise Cl laminar regime also can have a Mach element (be a function of).
Just out of curiosity, is this a boundary layer phenomenon? (BL is an area I wasn't all that strong in when studying fluid mechanics. )
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Old 15th Jul 2011, 12:21
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Yes, very much so...

I find there is always a lot of confusion between separated flow, turbulent flow and laminar flow. The later two are boundary layer states, the former (whilst to some extent the result of boundary layer effects) is a total flow phenomena.
Saying the flow is turbulent does not mean its separated, indeed a turblent boundary layer has a greater facility to resist negative pressure gradients (behind a body) and remain attached.. .nor laminar meaning low darg and attached (indeed again, laminar flows easily separate , the classic being a 'laminar separation bublle' at the l.e. of a thin foil at high alpha; draggy too (pressure drag)... whilst the laminar layer itself has low (frictional) drag.
So it isn't difficult to spot a lay commentator when it comes to aerodynamics, or certainly flow discussions

Last edited by HarryMann; 15th Jul 2011 at 21:24.
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Old 15th Jul 2011, 12:23
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Originally Posted by Lonewolf
Just out of curiosity, is this a boundary layer phenomenon?
Stall always involves the boundary layer. It wouldn't occur in friction-less flow, but there wouldn't be any lift either. To comment on the Mach involved - I noted a while back that alpha-max is defined by low-speed stall at more or less constant AoA up to M=0,275, and is defined by Mach-related buffet above that M. In AF447 stall occurred at about M=0,6 (2:11:06, 185 kCAS at FL375).
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Old 15th Jul 2011, 12:33
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Harry/HN:

Thanks to you both, some of it's coming back to me. I am thinking that I am wrong to posit that the stall wasn't at "high mach," given the numbers HN kindly reviewed. The conversion of CAS to IAS at high altitude reminds me of how fast the plane was traveling at stall entry ~ M0.6. I can see where one can say that it was a stall at a relatively high Mach number ... depends on frame of reference for what "high" or "low" means.

(HN, I am aware that stall involves boundary layer effects, what had me curious was the Harry's pointing out the mach elements within drag ... I suspect that was once taught to me but it wasn't in long term memory).
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Old 15th Jul 2011, 15:32
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Originally Posted by HN39
Low Speed Stability is lost in Alternate (2) Law.
Not always true :




Originally Posted by cwatters
Won't going to Direct Law remove the nose down demand that it just tried to apply? It says in Direct Law all protections are inoperative.
That must be part of the complexity of the system, laws, protections, stability ... In the mean time, can we presume that if under Low Speed Stability in ALT2, autotrim does not operate ?

Originally Posted by henra
Caveat for any one who wants to bash Airbus Autotrim now: An AP mistrimming slowly and quietly and then disconnecting and handing back to the pilots can be equally bad or even worse as you cannot fix it just by applying ND on the Yoke, which would work on an AB as long as you have Autotrim.
If no autotrim never under manual flying, at least a pilot knows trim is under his watch always - No ambiguity.
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Old 15th Jul 2011, 16:58
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henra, CONFiture.

henra's comment ( "a/p mistrimming, disconnecting and handing back", etc.), was one of the very first progenitors of passionate disagreement here on thread. I said, (roughly) that getting the a/c back under these 'unproven as yet conditions' would be like having a wet enchilada dumped in one's lap. Distracting, uncomfortable and in immediate need of a clean up.

THS as TRIM versus THS as CONTROL? How can one be separated from the other? It seems an incestuous combination of duties, and ripe to provide confusion.

As CONFiture has said, "One way or the Other". SA should be natural, intuitive, and rapidly moving. Stopping to think just to get a handle on the moment seems a dangerous waste of time.

Doesn't the Computer (FCS) take as much as TEN SECONDS to validate a LAW? In some situations, one imagines ten seconds may be an eternity, and in danger, delay is the Devil. (Unless it is appropriate, there's the rub).
 
Old 15th Jul 2011, 17:10
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takata,
This illustration is taken from the FCOM and is generic for ALTERNATE if airspeed is not unreliable. In case of UAS, What you will have instead is the red flag "SPD LIM" (see report previous page) without Vmax, VSL nor VSW and... It will last for the remainder of the flight like ALT2!
Hmmm, may be the AB engineers cited in the ACA memo don't understand their aircraft, but they say explicitly on page 6: "Sur les PFD les vitesses VaPROT et VaMAX sont remplacÚs par Vsw".
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Old 15th Jul 2011, 17:12
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@syseng68k:

Thanks again for the illustration. From that, I assume from that pitch
and roll are shown yellow and blue gear train and that primary computer
pots are red, green and blue, while secondaries are red and blue only.

If this is correct, there is in fact no redundancy at hardware level in
terms of (ideal?) multiple pots per axis, per computer. For this configuration,
redundancy can only be done in software, between computers, using
messaging. I guess it makes sense. It reduces the overall number of
pots, still provides the required redundancy, but potentially introduces
a far more complex set of failure modes...
In fact there is another (identical) set of xdcrs for the roll channel.

De redundacy is: 2 potmeters for each Flight Computer (2x roll & 2x pitch)
If 1 input rod becomes disconnected, both potmeterclusters are driven by the other crank & geartrain by means of the shearpin.
If 1 input rod/geartrain is jammend, the shearpin will break and allows free operation of the other set of potmeters.
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Old 15th Jul 2011, 17:14
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To J T the Mod - a FBW thread?

I suggest we interested parties crank up a thread in the Tech Forum specifically devoted to FBW systems - philosophy, real engineering crapola, human factors, experience from several folks here when things went awry, and of course lessons-learned from AF447 and other FBW airline types.

I can't moderate, but can help.

I only suggest this as I was around when we saw the first fully-FBW system fielded in significant numbers - a charter member, if you will. My engineering background is so-so, not half as good as some here, but "adequate". I also worked with the high-tech avionics in operational jets earlier than many here due to my military experience, and most of that time I was solo.

I recall a question from a U.S. Senator at a hearing with our new Sec Def ( McNamara) who was fascinated with high-tech and other aspects of military equipment designs. He asked the Secretary, "Just what experience, if any, do you have in these matters?"
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Old 15th Jul 2011, 17:23
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Shuttle Flight Computer

I apologize if this has already been posted.

For safety critical system design folks:

Spaceflight Now | STS-135 Shuttle Report | Astronauts restart computer; Obama to call Atlantis today

From the article: (bolding mine)

"Well, it [flight computer] actually failed last night, I think it was about an hour and a half after we fell asleep the alarm went off," Ferguson told CBS Radio in an orbital interview. "I think we all looked at each other, had that bright-eyed, sort of bushy-tailed look, and raced up to the flight deck. The folks on the ground did a nice job helping us get through that. We brought up another GPC to help out with the functions (GPC-)4 was performing. We got it, hopefully, back up and running this morning so it's hanging in there and we're confident it's going to work for entry for us."
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Old 15th Jul 2011, 17:34
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Originally Posted by CONF iture
That must be part of the complexity of the system, laws, protections, stability ... In the mean time, can we presume that if under Low Speed Stability in ALT2, autotrim does not operate ?
I don't think that is correct - I think autotrim is always active outside of direct law (or manual backup) except in abnormal attitude law. Abnormal attitude law could, I guess, co-incide with low speed stability activation, but would not automatically follow from it.

If no autotrim never under manual flying, at least a pilot knows trim is under his watch always - No ambiguity.
It's a good theory, but there are incidents (more than one 737 I can think of) where it hasn't held up in practice. In situations where the automatics dump the plane back in the pilots' lap unexpectedly (perhaps trimmed up to the limit) it can only add to the workload.

Also, I think autotrim is effecitively requried for C* control law, so without it you are looking at direct law, and no protections/limits whenever A/P is out. Not sure that is going to be an improvement.

You could reduce the number of laws/transitions to simplify things - as others have suggested. Go straight to direct law, and hence manual trim, when things start to go wrong. I'm ambivalent on that - I can see from a design & engineering point of view that the current laws are a nice graceful degradation, which should make things easier.... however, all the different laws, sub-laws, caveats and footnotes add up to a lot of combinations to learn and fit in training on. It's arguably little use having intermediate graceful degradation modes if pilots are never trained on them.

Be careful what you wish for though - there are reasons why non-normal laws get latched, and the A/P doesn't handle direct law, so you could be looking at hand-flying a long way when stuff goes wrong. On the other hand, maybe if the previous UASs had been a bit more of an "event" the **** pitots might have got fixed sooner (like when AB suggested it, or earlier).
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