Mach and THS attitude
 

Salute!

Somewhere in the cobwebs of this old brain it seems that temperature is a big player to get true mach, especially up high. An SR-71 buddy of mine reminded me of this awhile back, but what do WE know? And ask a few Concorde pilots about this. So if the Airbus wants to use only dynamic and total pressures to calculate mach, I am wondering a bit.

Secondly, manually trimming the THS for nose down might not help with a neutral stick. That's leading edge of the THS "up". OTOH, it could be the plane has the same characteristic as the Viper, in which we could command a brief nose up attitude and higher AoA, then the nose would fall down and we could "rock" the thing outta the deep stall. In our case the equivalent of the THS was leading edge up to get the nose down, but we pilots had cleverly managed to get real slow at an extreme pitch attitude and the thing settled into a comfortable deep stall with little wing rock and good directional stability.

jez a thot.

This is what occured for the Air Caraïbe case.

In the last moments of the flight what forces were acting on the aircraft...

The tail was stalled so down elevator cannot lower the nose.
The CoG is behind the "center of lift" so that's contributing a nose up pitch.

Seems to be no way to reduce the AoA as per the revised proceedure. Perhaps lowering the gear?

PFD with a 16° pitch
 

Hi there,
Does a PFD with a 16° pitch look like this ?

http://img11.hostingpics.net/pics/358403pitch16.jpg

In the affirmative, so much blue on the artificial horizon should be perceived as a big problem in cruise phase ?
PS) the apogee seems not with negative Gs, not even 0-G: the passengers may have been unloaded by the 3/4 of their weight.

Re bear
 

Hi bear,


I´m not in the know there, but have an oppinion. It was a valid warning, caused by reduced mach to .8, the roll to the right and following correction and the nose up travel of the nose ( note, it is not understandable for me how a crew would have done that amount of NU intentionally, but sure PF had his share in it), mixed with some turbulence. Another reason i think it was valid, the crew honored it with the appropriate ND action.

It was not negative, but it was less than 1 g.

As mentioned, maybe .5-.8 gee, just an unload, and i dont think it would have an immidiate effect. In my F4 it would have to be negative gee to get the nose down, with just an unload to .5 or even 0 gee it would continue on the trajectory until out of speed.

The NU input during the ascent until the apogee at FL 380 could have another reason. The ND was not working, a method to get the nose down is to roll the AC to the nearest horizon. Dont know the bus, we used (if not stalled yet) to pull the nose to the horizon, otherwise the unloaded ship just floated with the bank. In Alt 2 there is no rudder input together with a roll input, am i correct there? If so, the roll may not have worked as planned with the feet on the floor, our ship would be prone for roll reversal.

Not at the first stall, but when the ship went into the stall regime in an unloaded attitude with the nose up, the chance to recover got minimized. I think it just quit flying at FL 380 when the speed was gone and was picking up descent rate in the same attitude NU as it had gotten to that height. Our F4 would have pitched down due to the heavy nose (i had been there, just let go of all controls and wait till its heading down).

An expierience, most nowadays pilots are not granted anymore.


As mentioned, just my thinking out of a lot of practicing of stalls and falls (on the stick and observing / instructing), might well be totally wrong.

So lets wait and see.

Black Sheds, take a look at post #1, it has the links to the latest report. From memory, one of the pilots warns that they will soon be at 10000 feet, so he believed the altimeter and it was falling.

Early warning of Air Speed sensors "failure"
 

Considering the importance of Air Speed (critical to "advanced planes" Systems) why not to implement an "EW AS resource" to alert the crew, before the System actions.

Pitot´s failure (e.g. at cruise) are not sudden ("digital"). They start to be transformed, in altimeters for example, gradually. Thiells 727 (NW N274US) showed this gradual deterioration. I think that an encounter of a dangerous icing condition when cruising, is also gradual, technically allowing a warning (of this crucial info) before "law switching" by the Systems.

And this perhaps could be a "band aid" before we have better AS sensors.

ND using an special type of APU
 

How many pounds of thrust an "augmented" special type APU would need for the task for this class of a/c? With a 45° degrees tilted nozzle.:8

Why AOA is disabled <60 kts?
 

@MB.

Valid and logic too.

MCDU System Tests mode option is disabled after you entered your flightnumber. But hey.....because its a computerized aircraft it can be fooled even in flight! Just have to know how to set the proper bit.

Depends on the type of test: Stall Warning system is tested through MCDU, drives AOA to predetermined test angle by means of the -dual purpose -dampingmotor and set a false speedsignal.

Mr.O

"Black Sheds, take a look at post #1, it has the links to the latest report. From memory, one of the pilots warns that they will soon be at 10000 feet, so he believed the altimeter and it was falling."

I propose that this statement was Made by PNF. It is an 'observation', and the PF was.......flying. F/O (RHS), was PF, he had duff (and unrecorded) data, this made him the one flying with displays that will not be known. Later, at dual ss input, the PF gives the ss to LHS (relief pilot). This is an intuitive notion, a "maybe you should try something" action?

RR_NDB

"Pitot´s failure (e.g. at cruise) are not sudden ("digital"). They start to be transformed, in altimeters for example, gradually. Thiells 727 (NW N274US) showed this gradual deterioration. I think that an encounter of a dangerous icing condition when cruising, is also gradual, technically allowing a warning (of this crucial info) before "law switching" by the Systems."

With no liquid water available, the assumption is entry of ice crystals? If so, the likelihood is they collected at the drain, and slowly plugged it. This increases the pressure sensing and increases ias.; the aperture then plugged, and stabilized the pressure, then the plugs melted, and the actual pressure became available? This happens over a period of time, as you say, and since in solid state, the water has slower Thermodynamic behaviour, lengthening the event?

Hyper-V.... That would be pretty close. But the stall was pretty mushy up at 37,000 feet so the passengers probably felt little, as did the flight crew on a turbulent night. The big clue should have been that display you depict ALONG WITH the altimeter dropping. Even without reliable airspeed, it should have indicated that the nose needs to go down to get the airplane flying again. The cockpit voice recorder transcripts will be telling. It will be interesting to see if the vernacular involved is about flying the airplane or system observations.

The passengers would have felt the 40 degree rolls, but I do agree with this "The cockpit voice recorder transcripts will be telling. It will be interesting to see if the vernacular involved is about flying the airplane or system observations."

Dealing with Pitot data, sometimes unreliable
 

Hi,

I am assuming that most of Pitot discrepancies (among the 3) takes some time to stabilize in a condition capable to trigger a "switch law".

I am just imagining as "technically feasible" an EW resource that could warn the crew, before a major System change, any discrepancy and it´s rate of unstabilities, i.e. characterizing the differences between the (odd) number of AS sensors.

I consider an almost "useless" redundancy (for cruise level) the 3 identical Pitot´s and capable to trigger a "law switch". Doing that, (scanning and processing the 3) the System is almost like "amplifying" the Pitot problem.

Certainly Airbus SAS is working in better processing the info presented by current available Pitot, data. And also doing R&D on this issue. The Laser AS sensor filed patent by them shows it.

Yes, I saw that but I was not convinced that it was a factual statement, nor that it fitted the current (at the time) situation.

POB "feelings" after sudden climb
 

What plane "attitudes" was probably mostly felt by PAX?

1) Sudden climb at 7,000 fpm
2) perigee deccel together high pitch
3) perigee zero (vertical speed) to > 10,000 fpm, speed variation
4) Rolls
5) Increasingly aerodynamic noise (and buffeting) of stalled a/c at > 10,000 fpm
6) TO/GA to idle noise engine variations
7) Turbulence during perigee to SL travel

I guess most "realized" the "deep trouble" after the above combined points. Even the pax sleeping before the sudden "climb", at high rate.

BlackSheds: trouble is the released transcript extracts have been selected on unknown criteria, however the crew weren't slow to say when they had what they took to be no valid indications. Without any spoken caveats the phrasing sounds like it was information being given by PNF as Bear deduces, with the intent of relaying information taken as fact at the time. However it is like reading a story with two out of every three lines blanked out so it's filling in the dots.......

Why AOA is disabled <60 kts?
 

Could it be a mod post 346 in the wall in Toulouse ...
http://www.pprune.org/rumours-news/3...ml#post4594759

Mr.O

"However it is like reading a story with two out of every three lines blanked out so it's filling in the dots......."

It is more challenging, and more interesting, to have too little, than 'enough.'

Not even Perpignan is "Safe". Cold Cases are the best cases.

RR_NDB

Redundancy is a fossil left over from the days when engines could last 2000 hours on the wing, and radios were "crystal".

A back step from that old worn out bromide is necessary to save lives, and modernize design. Better a redundant system but anomalous design than the false security of "numbers". Hard to believe that "More is better" is still around....

Bear, true enough. Can't understand why the whole transcipt minus anything upsetting or not relevant wasn't released but there will be a reason (maybe give the lawyers a head start ?). Since the whole story will be known at some stage the extracts and commentary must be factual, as far as they go. Can't release something now and have to contradict it later.

I remember a statement that the Captain announced (en francais) 'this is a stall' or similar. That must have been in a newspaper 'leak' as I can't now find it. Odd, but the leaks seem to have stopped.:}

jcg and mr. optimistic, we do not know that the passengers were not instructed to be seated and belted. If the PF reduces airspeed to 0.8 for turbulence penetration, would you not expect him to have the seat belt sign on?

On TW800, about 100 bodies were recovered floating on the surface, and its very likely the fasten belt sign was still on when the center tank went.

"Can't release something"

full stop.

SaturnV, thanks, yes I would have thought so too. So the early finds were not indicative of passengers not seated or unbelted and the CC were either seated or unprepared in the rest facility I presume with the impact forces and immediate structural breakup doing the 'selection' ? Don't like to think of any other scenario.

Bear, don't know what the cloudbase was like but at some stage they must have seen the ocean and maybe got an horizon. Bet that prompted an utterance or two.

The radalt and gpws would have given them some new clues as to the impending disaster. They were monitoring the altimeter tape so it also.

On a moonless or a moon obscured night, getting a horizon may not have been possible,

Either way, at 10000+ ft/min descent the number of seconds for the radalt info and the gpws alerts would have been minuscule.

Computers
 

Gentlemen,

computers will always do what their programming tells them to do. Sure.

First caveat is "programming" takes human beings, who make mistakes, be they engineers in a design committee, or pilots on a flight deck.
Second caveat is when physical failures affect the computerized system.
Is it not obvious in our present case that a "physical" fault occured : wiring ?
BEA offhandedly ascribing it to "PRIM2 rejected ADR1" is a feeble excuse for an explanation. No mention of "wiring" ?

I have reliable confirmation from the relevant documents that this failure :

WRG:ADIRU1 BUS ADR1-2 TO FCPC2

is indeed a "wiring" fault, between ADR1 and PRIM2.

This fault appears "simultaneously" (look at ACARS and CMC time stamps) with the acknowledgement by the system that speeds have gone haywire :

PROBE PITOT 1X2,2X3,3X1

These faults are unrelated : wiring + icing. How can this be ?

Has this strangest of dual failures ever been foreseen ?
How can one expect the design to operate as advertised in this likely un-designed for situation ?
Does anyone have reliable information as to the exact state of PRIM2 operation after this combination of faults ?
How can anyone be certain that PRIM2 did indeed latch Alternate 2 Law ?
How can one be sure it did compute identical to PRIM1 & PRIM3 when it is obvious its state is different ?
What happens if PRIMs differ in their assessment of the outlier ADR ?
What happens if, after their 10 seconds ADR cross-checking "window", PRIMs end up in different states, deciding on different flight laws ?
Has anyone considered what could happen to PRIM2 if it lost datalink with ADR1 at the very moment when the PRIMs tried to eliminate the outlier ADR ?
Has anyone studied carefully the first BEA interim report and extracted the PRIMs behaviour at the start of UAS sequence ?

I did. PRIM2 could very well have reverted to Normal Law. With erroneous data fed into it. Definitely not designed for. But quite possible.
Please, do prove me wrong. Do not recite the manufacturer's mantra, I know it and I do not care what the salesmen say. Sh't happens but it does not sell.

So many people posting here assume that the computers reacted "as expected", confusing it with "as programmed". I am deeply disturbed at seeing so many people who work in science and technology embrace blind religious faith into man-made technology. Where has the scientist's healthy doubt and critical judgement gone ?

Any high-tech item is an artifact : man-made. It is never perfect. It fails, even if rarely. It errs. It does unexpected things.

I am of the opinion that in this instance, statistically remote, it did unexpectedly err, compounding a surprising but otherwise manageable situation into something far more dangerous and incredibly difficult to understand in a limited amount of time.

The initial pitch-up, more than 10 degrees nose-up attitude, up to +7000fpm vertical speed, +3000ft altitude deviation, has been discussed quite a lot. Such discussion is always along the lines of :
- deliberate pilot action : then why ?
- unintended pilot action : then how ?

Outraged whys and hows all over. Naive reaction to a very narrow extract of all the available data, delivered to the press at a commercially critical time. Understandable coincidence.

So few are those who suggested it could have been undesired, an unexpected reaction from the aircraft itself, its flight controls computers confused.

Why is this possibility unacceptable for so many involved people ? All aircraft types, from all manufacturers, have at times suffered serious design-induced malfunctions which caused crashes. Such problems are rare. All modern jet airliners are statistically safe. Why the reluctance to acknowledge a very specific flight controls problem ? Why the brand of blasphemy when such problem is invoked ? Why the need to invoke "pilot error" when questions point towards the manufacturer and its design ? This blame-shifting attitude is contrary to the necessary openness and honesty that could improve flight safety. I find it very much akin to religious intolerance.

If pilot error is to be found in this case, it is twofold :
- calling for the Captain to come back to the flight deck used up resources which should have been entirely allocated to flying and cross-checking instruments, control inputs and results between PF and PM, especially in the early stages of the accident sequence. This could have allowed identification and isolation of a flight controls issue.
- failure to quickly and decisively use manual pitch trim, against training, in a flight law that usually excludes its use, to effect the desired attitude and trajectory if sidestick was found insufficient. This could have maintained a safe path against the undesired actions of confused computerized flight controls enough for extended troubleshooting.

These are real things that can really be corrected both in the attitude and flying skills of modern age airline pilots. These can improve flight safety. But the initial 3000 feet climb, 10 degrees and more pitch-up is unlikely to be "pilot error", even though some powers that be, and hordes of blindly faithful technology worshippers would prefer it to be so.

Fault Tolerant Systems
 

In respect to FBW, Airbus SAS philosophy and computers with it´s intrinsic complexity (software testability issues, etc.) the redundancy implemented is mandatory for it´s advanced planes.

The use of the 3 Pitot´s (identical) subject to the same problem (recorded in many other similar cases) being scanned by the System and "capable to trigger" a major a/c change makes me concerned.

I think this is a point (before better available Pitot´s) to be reviewed.

Sounds like System Designers used the idea of Redundancy in an almost "wrong sense".

In summary: To scan and process an information of this magnitude of importance and "switch laws" based on multiple "non reliable sensors" does not seem to me a safe approach. Better perhaps to "warn only" and present the separate info to the 3 crew (LH, RH and ISIS).

During "short period Pitot failures" this could be adequate.

And i insist, Pitot´s problems for this new advanced planes deserves immediate attention and this not seems to me a technological problem. IMO it´s just a product (specs, cert. etc) issue. Not critical in other less "advanced" a/c.

Svarin, yes I recall that you raised the 'wrg' report some time ago without getting much response. Your opinion on the apparent NU inputs ?

Probabilities are just that but the CVR would make it clear:

Option 1: one rare failure plus maybe large sub-optimal response (not proven)
Option 2: two independant rare failure plus maybe small sub-optimal response (not proven).

Time will tell.

Perhaps significant attention was being paid to trying to get the autopilot and or computers back on line (you have already suggested this some while back). I noticed in the N Atlantic upset, the report seemed to indicate that hand flying was started only after attempts to re-engage the autopilot failed. Seemed the wrong way round to me. Note I am SLF with perhaps too much to say, so hope no offence.

Edit: Didn't BEA also say something along the lines that the a/c systems were not at fault (excluding pitots one assumes).

Svarin,
Masked faults in aircraft are nothing new. We realize you may be on to something, but we all have to realize that BEA is investigating this accident, and if they have integrity, then they will address this question in their final report. Let us hope they do.

With the highly reliable systems of today's aircraft, you can fly thousands of hours until a random event unmasks a hidden fault. Perhaps they ought to swap out which PRIM has the lead position just to unmask such potential faults. Of course, you have to give the crew the ability to quickly switch back to a stable configuration should a fault show teeth.:eek:

Actually the opposite, as reported by AVweb:
"At no point" on the cockpit voice recorder "is the word stall ever mentioned," Chief Investigator Alain Bouillard said in an interview.

So one can reasonably conclude that the crew did not know that they were in a stall.:(

Fault Tolerance and complex Systems behavior
 

Hi,

Mac

Svarin, your dense post requires extra commenting, to be made later.

Post Stall Aerodynamics
 

When you look at a typical Coefficient of lift curve in a publication, they always stop just past the Clmax point. See example below:
http://www.pilotoutlook.com/images/a...igure15_13.jpg

Did you ever wonder why they don't carry on further to the right? It is because past that point, it gets a lot more complicated, varying with aspect ratio, thickness ratio, and Reynolds Number.
A typical extended curve looks more like this:
http://home.comcast.net/~shademaker/...tStall4412.jpg
The double humped curve shown above is rather typical actually. Now we have a clue where the notch in the Viper pitching moment curve comes from.

Fortunately the wind turbine industry has need of post stall airfoil performance and someone has studied the subject a bit. The following pdf file is on a slow server, so give it some time to load.
http://home.comcast.net/~shademaker/...tStall44XX.pdf

I researched this topic a bit because I was interested whether or not AF447 was likely to get stuck in a pitching moment notch of its own. Maybe so, but the THS lower aspect ratio than the wing indicates decreased likelihood of having as much of a notch as the wing might have, i.e. more nearly linear performance.

Svarin


Svarin

"So many people posting here assume that the computers reacted "as expected", confusing it with "as programmed". I am deeply disturbed at seeing so many people who work in science and technology embrace blind religious faith into man-made technology. Where has the scientist's healthy doubt and critical judgement gone ?"

The answer is to pose 'silly' questions, and plant the seed. Engineers and Pilots both have to have the answer, and be first. It takes patience, but in an endeavour where Time is cheap, it is crucial.

First input, then, was in Normal Law?

just sayin'

RR

"Sounds like System Designers used the idea of Redundancy in an almost "wrong sense"..."

They were on the right track when they RFP'ed the software to isolated teams, encouraging anomalous, (but equitable), performance.

Think about it. Three Thales Probes. Let's eliminate two, and retain one. Gone is the need for constant "voting" and the very real possibility of fatally confusing the systems and pilots on the "Altar" of "guess we got that figured out".

With one Thales, one has binary reliability, if it packs up for any reason, we have BUSS. BUSS came after THREE THALES, so it was an acknowledged improvement in the name of critical safety.

Complex is not always the best. If one starts to incorporate some real analysis, the need for complexity disappears. Wait, doesn't that make _____ look bad?

boy howdy. So here comes Pride, Apathy, and Money.

I say NORMAL LAW or Manual flight. Making up stories is starting to get expensive, in machinery, and human souls.

HF, thanks. Difficult to believe the word wasn't mentioned when the stall warner triggered.

Machinbird: there was a change of course at some (late) stage as shown in the BEA graphic, so something was able to effect change.

Svarin: could 'wrg' refer to a failure in message protocol rather than anything physical ?

With a full readout of the DFDR AND CVR recorders, I hope BEA is still trying to get info out of the unprotected quick access recorders. It may not be trivial at all, but will probably be well worth the effort. May even open up new areas for the investigation. (Of course that can be a bit inconvenient when the report is almost ready to publish.:hmm:)

Comparable to seeing things with an earth bound telescope compared to the Hubble telescope.

Machinbird : thanks for the expanded curves, greatly interesting and informative.

Mr Optimistic : Yes, indeed. I tend to incline towards software versions that could communicate very well under all normal circumstances but failed to do so under this very specific ADR-checking 10 seconds process. To be verified and pursued further, but I lack the relevant documents. Of course, software incompatibility is not part of the design assumptions, thus it could only appear as a "wiring" fault.

Svarin, thanks. That is not a 10^-9 probability, I won't bet against you.

From LazerDog, #1729:

From DozyWannabe, #1730
There seems to be some suggestion of a possible problem with altitude data from ACARS Message 8 (WRN/WN0906010210 344300506NAV TCAS FAULT). According to BEA Interim report No.2, section 1.16.2.4.1, analysis of message 8 , the most likely reason for the generation of this message seems to be, “...is a monitoring process internal to the TCAS which applies to the standard altitude parameter. The latter is received from the active transponder (it can thus be the altitude elaborated from ADR 1 or 2) and is submitted to a “credibility” test. In actual fact the TCAS elaborates an altitude prediction that it compares permanently with the altitude received.”According to section 1.6.11.6 a compensation factor is applied to standard altitude dependant on Mach but in the example given, for a difference between Mach of 0.8 and 0.3, the compensation to standard altitude is only 0.7%, whereas an 'incredible' gap between a measured and a predicted altitude would IMO be of the order of 2 to 5%, say around 1000' or more.
Another possible reason for the PF NU input?

Khashoggi,

From first interim report on the experience of AF459, flying about 35 minutes behind AF447. (Bolding mine.)

As AF447 did a loop in the last several minutes, the cockpit view would have become oriented back toward the core of the meso convective complex.

The Lufthansa 744, flying 20 minutes ahead of AF447, also observed no lightning.

Actually, both are "true" in that both "this is a stall" and "at no point is the word..." have been reported in different media "leaks" from teh investigation.

So, one can resonably conclude that at least half the journalists reporting "leaks" from the investigation and actually just making it up.