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AF 447 Thread no. 4

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AF 447 Thread no. 4

Old 21st Jun 2011, 14:53
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PRIM 2 to NORMAL?

SVARIN:
This means Normal law is preferred over any inferior laws. This is the very core of this flight controls design.
Quote:
If PRIM 2, in your view, is in control and able to deliver NORMAL LAW this means that PRIM 1 was not able to compute NORMAL LAW protections


Quite right, except PRIM1 is not faulted before 02:13. Which PRIM becomes master if PRIM1 is ALT2 and PRIM2 is Nz ?
Quote:
Both NORMAL LAW as ALTERNATE LAW are Nz laws, to keep it ‘simple’ you mean PRIM 1 is NOT able to compute NORMAL while PRIM 2 is capable.
Then I agree with you PRIM 2 will be in control.

And actually this happens once in a while in normal flight, our crew reports in such a case: 'Alternate Law message was transient'.
F/CTL ALTERNATE LAW did appear on ECAM and after a while (when system reverted to NORMAL again) the message disappeared from ECAM.

Technically speaking ‘ALTERNATE LAW was triggered at the start of the monitoring window and at the end the monitoring window the system reverted to NORMAL again’
A correlated failure, which triggered the ALTERNATE LAW may be present in CMC, maintenance action however is not required when crew reported ‘transient’.

When the system later on triggers an ALTERNATE LAW again the F/CTL ALTERNATE LAW warning will be set again with the correlated fault message at that time.

Back to AF447,
If PRIM 2 reverted to NORMAL the NAV ADR DISAGREE would set ALTERNATE LAW a 2nd time (at the start of the monitoring window) and that did never occur because system was already in ALT 2 and PRIM 2 NOT in NORMAL!

What we know is that F/CTL ALTERNATE LAW was triggered after (PITOT ICING) and latched to ALT 2 (due missing speed information) at the time of F/CTL RUD TRV LIM FAULT and confirmed by frozen position of Rudder Travel Limiter.
This mode is latched – for the remainder of the flight - to prevent cycling between the modes.

From the BEA report:

Like the FMGECs, the PRIMs consolidate the parameters that they use by
means of monitoring mechanisms.
Concerning the airspeed, it is the voted value that is used.
In normal operation, this is the median value.
When one of the three speeds deviates too much from the other two, it is automatically rejected by the PRIMs and the polled value then becomes the average of the two remaining values.
But if the difference between these two remaining values becomes too great the PRIMs reject them and the control law switches to alternate 2.
Furthermore, another monitoring procedure is applied to the value of the voted
airspeed and triggers switching to alternate 2 law when it falls by more than 30 kt in one second.


In my view, at that time PRIM 1 was in control, PRIM 2 and 3 couldn’t compute NORMAL LAW either.
Due to the PITOT problem they had all the same ADR information.
With all due respect to your extreme technical knowledge, may I introduce this disagreeing parameter ?
WRG:ADIRU1 BUS ADR1-2 TO FCPC2

This means PRIM2 does not have the same ADR set available to it than the other two PRIMs have.
Agree, but doesn’t necessarily mean this set gives a more reliable outcome to the median speed calculation. On the contrary 3 ADRs is more reliable, as long as you can speak of reliable in this case, than 2 sets of signals.

I can’t find it in the documents but it could be that the logics take the amount of available ADR signals in account.

Then for this Maintenance message:

The problem area:




As preface on my argumentation keep in mind, EFCS is the complete Electronic Flight Control System and consist of FCPC1, 2 & 3, FCSC1 & 2 and FCDC 1 & 2.
When a failure is detected, PCPCs and FCSCs take operational reconfigurations and sends failure information to FCDCs, FCDCs analyzes the failure and after confirmation generates a maintenance message. The message source will be EFCSx(FCDCx) as will be the identifier EFCSy(FCDCy).
If both FCDCs are available, FCDC1 sends external and internal (ATA 27 ex S/F) failures while FCDC2 sends all failures as external to avoid double storage within CMC.
ADIRU failures are considered as external failures, however because only FCPC2 reported this failure it is rated (ATA27) internal.
Furthermore FCPC 1, 2 & 3 are interchangeable units (as long as they have the same P/N and OBRM*) only their position in the racks determines if they act like FCPC 1,2 or 3.
*OBRM = On Board Replaceable Module

There are extensive BITE* tests available in the system, the logic however is not detailed.
(Wrap Around, SSM, parity and refresh rate tests may be expected within BITE)
*BITE = Build In Test Equipment

A is not B,
In A. a wiring failure is suspected only, if BITE analysis has revealed its fault. AMM states: “As soon as there is more than one component (more than one part) in the message, A/C wiring can be suspected but it is not indicated in the messages except if it is clearly identified as being the origin of the fault.”

The failure messages in EFCS are always declared as HARD, even when it was INTERMITTENT (t<=2.5 s) or HARD due to several INTERMITTENTS during flight.

Based on this it should be considered as wiring fault i.s.o software flaw(OBRM) but can't conclude if it was of any influence (HARD) or not (INTERMITTENT) in law reconfiguration.
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Old 21st Jun 2011, 15:36
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electrical ghost faults

SVARIN:

Lonewolf_50 again :

electrical ghost faults


Going further into detail, I am of a mind that the WRG message is not a physical wiring fault, but a communication problem, software-related, between PRIM2 and ADR1. This would therefore not be a ghost fault. It would be reproductible.

One would need to mount the whole identical system (all computers, soft versions, P/N, etc...) on a simulator and introduce the specific 10 seconds monitoring process, and see what happens. This 10 seconds process being likely the only case for which communication breakdown would occur between these two computers.

If not already done in the last 2 years,
'IRON BIRD' facility, keep left on leaving the Toulouse training centre.

A33Zab.

Last edited by A33Zab; 21st Jun 2011 at 19:24. Reason: Quote the poster not the quoted.
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Old 21st Jun 2011, 16:03
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TO/GA input

From Wikipedia I find: "On Airbus planes pushing throttles to TOGA detent does all regarding flight path and speed", and in Thread 3, post # 16, I find from jcjeant the English translation from French: "At 2H10.51 this is again a stall alarm.the power levers are put on the position TO/GA and the PF keep the command to climb. Incidence angle of 6° when sounded the stall alarm continue to rise.
The adjustable horizontal stabilizer go from 3° to 13° (climb position) in about 1 minute.
He will stay in this position to the end of the flight."


I assume the reference to "incidence angle" is what Americans call "Flight Path Angle"; so my question is: "did the TO/GA selection command a FPA of 6 degrees and drive the elevator trim to full nose up to attempt to accomplish that?
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Old 21st Jun 2011, 16:31
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Originally Posted by wmelvin
I assume the reference to "incidence angle" is what Americans call "Flight Path Angle"...
I think you're a victim of Google translatese.... it almost certainly refers to AoA, and not FPA.
My apologies, but I really can no longer be bothered to spend twenty minutes to go through the French original to find the text in question AND the misleading translation, every time one of these issues crops up, unless somebody pays me.
Most professional translators will give you the same answer.....


No insult to Google translation meant... they have become a lot better the last couple of years!
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Old 21st Jun 2011, 16:50
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ChristiaanJ:

As a fluent speaker of German I can state that Google translations, although somewhat better in recent years as you wrote, still leaves an awful lot to be desired. There are just certain nuances and ideas in other languages that suffer in translation to English.
I'm sure you know that as well.
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Old 21st Jun 2011, 17:10
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Originally Posted by ChristiaanJ
I've seen no mention of the QAR having been recovered. It might have provided even more info.
I think I remember an avionic bay was recovered (coded name, on the wreckage map), and the QAR was (hoped to be?) part of it (or did I assume that? Anyone who know better, please correct me if I'm wrong with this assumption).
Didn't hear about the QAR since, though...
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Old 21st Jun 2011, 18:05
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@BOAC
This is so irrelevant as to be unbelievable! Please continue on the Flying Instructors' thread? What ON EARTH has this to do with AF447?
Based on two years of reading PPRuNe threads on the mishap ... pitch and power. But I get your point on maintaining focus. So I deleted it.

A33Zab
Lonewolf_50 again :
If not already done in the last 2 years,
'IRON BIRD' facility, keep left on leaving the Toulouse training centre.

The post you qouted for that reply was not written by me. Perhaps Svarin?

Last edited by Lonewolf_50; 21st Jun 2011 at 18:23.
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Old 21st Jun 2011, 19:12
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TO/GA

Although the Wright brothers used angle of incidence for what we now call AOA, my question still stands; Does the GA mode also command a flight path? If so, can it use elevator trim to accomplish this? The original French is in Thread 3, post 16.
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Old 21st Jun 2011, 19:47
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wmelvin;
Does the GA mode also command a flight path? If so, can it use elevator trim to accomplish this?
Be careful when reading material which is not in context. The control law being used by AF447 at the time you refer to was not Normal Law. It was Alternate Law No.2 and pitch protections available in Normal Law had been lost.

The elevator NU command was initiated and maintained by the PF and the Trimmable Horizontal Stabilizer moved accordingly to preserve the elevators commanded effectiveness. Thrust was also PF controlled.

Check this summary of A330/A340 Flight control laws for further info.
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Old 21st Jun 2011, 20:10
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Originally Posted by wmelvin
Although the Wright brothers used angle of incidence for what we now call AOA, my question still stands;
Dassaults Lexique bilingue of aeronautical terms gives these translations:
angle d'incidence = angle of attack/incidence
angle d'incidence critique = stall angle
angle de pente = flight path angle

At 2:10:51 AoA was increasing, FPA was passing through a minimum of about 1.5 degrees up.

For what it's worth, wmelvin's question reminds me that at that point the stall warning computer (the computer exercising that function) must consider its polled airspeed to be valid, to calculate a stall warning threshold of 6 degrees. Iow the ADR DISAGREE condition occurred later.
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Old 21st Jun 2011, 20:31
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HazelNuts39
At 2:10:51 AoA was increasing, FPA was passing through a minimum of about 1.5 degrees up ... wmelvin's question reminds me that at that point the stall warning computer (the computer exercising that function) must consider its polled airspeed to be valid, to calculate a stall warning threshold of 6 degrees. Iow the ADR DISAGREE condition occurred later.
Does this mean that all three computer must agree (closely enough) at the threshold value to get a stall warning ...
or
If two of three are polled and agree at the threshold value, you get the warning
or
If any one of the three is polled, and the value hits the threshold value, you get the stall warning?

The last seems least likely from a system's design perspective, in terms of avoiding spurious warnings.

I am trying to understand clearly the point you are making.
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Old 21st Jun 2011, 20:42
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HazelNuts39;

I seem to remember that A33Zab had determined that it was the mean of the polled values - be it from whatever number of sources.

EDIT :: Not correct ... see posts below.

Last edited by mm43; 21st Jun 2011 at 21:34.
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Old 21st Jun 2011, 21:06
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Mean & Median

mm43 wrote regarding redundant ADR airspeed data used by PRIMs :

the mean of the polled values - be it from whatever number of sources
Respectfully disagreeing here, the information I have says :

When THREE sources are available, median value is used. That is, for 3 values, it is always possible to order them in such a fashion : lowest <= median <= highest

When TWO sources are available, mean value is used. That is mean = (valueA + valueB) / 2.

An expression seems to be used that covers both cases, and that is voted value, which could be either mean or median, depending on the number of available sources.
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Old 21st Jun 2011, 21:10
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Lonewolf and mm43;

From BEA's no.2, para. 1.6.11.4 (again):
In alternate or direct law, the angle-of-attack protections are no longer available but a stall warning is triggered when the greatest of the valid angle-of-attack values exceeds a certain threshold. In clean confi guration, this threshold depends, in particular, on the Mach value in such a way that it decreases when the Mach increases. It is the highest of the valid Mach values that is used to determine the threshold. If none of the three Mach values is valid, a Mach value close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8.
Here the stall warning was 6°, not 10° or 4°, corresponding to the actual Mach number of about 0.68. So one of the Mach values was valid. The 'polled' speed is what the PRIM's select to use. If all three values are valid, it is the middle (median) of the three values. If one ADR has been rejected, it is the average (mean) of the two remaining values. If the difference between the two remaining values exceeds a tolerance for a certain time, you get ADR DISAGREE. That's about the limit of my knowledge.

Furthermore it is noteworthy, since stall warning and stall do not occur in normal law, that the master PRIM was functioning in Alternate (2) law.

Last edited by Jetdriver; 22nd Jun 2011 at 14:16.
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Old 21st Jun 2011, 21:12
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Svarin;

You are right. I realized after I posted "mean" that "median" also came into the equation".
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Old 21st Jun 2011, 21:24
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mm43,

no worries, I thought a clear summary of that particular bit might help all readers. Hope it was clear enough. Best regards.
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Old 21st Jun 2011, 22:53
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Svarin:

Why are you pushing so hard for a software failure above all else in the face of the evidence we have so far?
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Old 22nd Jun 2011, 04:52
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For pitot redesign folks:

Shaved bat wings show sensory hairs help manage flight

Edit: I have'nt gotten hold of the paper yet, but from the abstract: "This finding suggests that the hairs act as an array of sensors to monitor flight speed and/or airflow conditions that indicate stall."

Last edited by CogSim; 22nd Jun 2011 at 05:57.
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Old 22nd Jun 2011, 05:15
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Interesting! But although that would tell the bat it was flying near stall, it wouldn't give it airspeed. A similar technique (wool tufts taped to the upper surface) is sometimes used full scale to check the airflow over the wing in stall tests to see at which bit the airflow first breaks down.
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Old 22nd Jun 2011, 05:18
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Those who cannot remember the past ........

syseng68k
Easy to say sitting at home in an armchair, but who can say what the
reaction would be on a cold dark night, with no visual cues and lost faith
in the machine's integrity...

Last edited by PickyPerkins; 24th Jun 2011 at 04:52.
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