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AF 447 Search to resume (part2)

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AF 447 Search to resume (part2)

Old 26th May 2011, 01:38
  #2401 (permalink)  
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the changes coming from this crash will be evolutionary (as in "bugfixes") instead of revolutionary.

Of course this will be the case. It has been so ever since the early days of regulated aviation.

No one is suggesting that we will ever go back to steam driven aeroplanes as representing the state of the art - in spite of the deluded and fond views held by many of us dinosaurs ...

From that sort of viewpoint, this mishap is no different to any other - an aspect of the investigation will be to look at what changes might be appropriate within the design and operating paradigms.

The interesting aspect here probably will/might be that the evolution may be more significant than what we might have seen in most previous mishaps ?
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Old 26th May 2011, 01:48
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That is a good point though... (MM43)

Why is there not at least one reversionary A/P mode, when it's either turbulence or UAS that kick it out. As a short /temporary pre-handover mode, a sensible Pitch / Power should not be beyond its grasp, accepting that it's basically a heavily fedback airspeed-centric loop (which is far too inflexible, surely ?)

"Here you are, things are getting difficult for me, but we seem to be flying a stable AoA at a sensible thrust setting - Handing over now "
[.. in a fairly calm but determined fashion, without more than one bell or whistle]
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Old 26th May 2011, 01:51
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bearfoil

There was an intermittent post from bearfoil that seems to have disappeared. There were some important points about how Anomalous Design works. I just wanted to add that in the scenario I envisioned apropos discrepant pitot heat, the probes would still be triple redundant in non-icing environment. In an icing environment, however, they would "switch" to the so called anomalous design mode. I call it switch, but this would of course be completely passive on account of the fact that the heat is applied uniformly through out the entire flight.

This is way outside my areas of expertise, and I may be completely misunderstanding the concept of redundancy, so this post is purely for my own edification.
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Old 26th May 2011, 01:56
  #2404 (permalink)  
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Bearfoil removed his post (although it is still there in the aether, so to speak). You might like to ask him to reinstate it. I thought it a good post
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Old 26th May 2011, 02:02
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bearfoil: request to reinstate

Dear bearfoil,

I'm writing to officially request you to reinstate your post re Anomalous Design.

Sincerely yours,
Cog
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Old 26th May 2011, 02:43
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FLY THE AIRCRAFT.
I agree with this point.

Still, whenever I hear this, it makes me squirm. As the pilots among us know, sometimes its easier said than done. It's important to keep that in mind come Friday.

Peace.
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Old 26th May 2011, 03:02
  #2407 (permalink)  
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cogsim

As the design consideration is approached in real time, redundant systems simply fail together. If one system has sensors that have different designs, the chance of double or triple fail becomes remote.

Identical fuel systems in BA038 failed their TRENT 700's only seven seconds apart. The failure tracked identically as each engine spooled up, then starved, and settled, with very close EPR's.

And that is a complex system. A third engine, or two more, would the failure have been the same? Very likely. A redesigned Heat Exchanger, and the problem is solved. Pitots are extremely simple, yet they have remarkably vulnerable features. A narrow aperture surrounded with a sharp leading edge that ices faster than a blunt one.

If ICE is the consideration, three separate design approaches would likely produce three anomalous solutions to the goal. Redundancy works well if in a "fallback" position, so I like your anomalous heat solution. Exposing three identical devices to the exact same conditions will produce three failed devices. If three concurrent readings are desired, better each device has different characteristics.

This is why I never understood the need to "sample" and then ask for a vote.
Likely the results of the election will be the same. If not, they soon will be.
So this is why anomalous design works well when Redundancy is simply a siren song.

This is from memory, so apologies if it isn't what you saw. I think I inserted a snarky comment about "Backup systems". So tell Air France to cancel their Goodrich deliverables, purchase the BUSS from Airbus, and happy skies......


My apologies - I presumed that Bearfoil could see and reinstate the original post which he had deleted. FYI, mods can see a deleted post and reinstate it. All deletion does is hide it from general view. There is one caveat - delete post #1 and the thread disappears into the never never.In future, if you want a deleted post undeleted - simply ask and your wish will be granted. JT
 
Old 26th May 2011, 03:40
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Exposing three identical devices to the exact same conditions will produce three failed devices. If three concurrent readings are desired, better each device has different characteristics.
If the devices in question are state of the art, then would not the characteristics required of the other two be a step back?
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Old 26th May 2011, 04:16
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BEARFOIL
As the design consideration is approached in real time, redundant systems simply fail together. If one system has sensors that have different designs, the chance of double or triple fail becomes remote.
Insert the word 'identical' in front of 'redundant' and I will fully concur. That was also the point of my earlier post seeking diversity in critical systems.

Yes, additional sensors will increase the logistic tail required to support an aircraft.

I would not advocate wholesale replacement of pitot tubes with a 'new concept' system, that would destroy diversity, not improve it.

Some are pointing to the fact we have documented ~18 instances of Airbus pitot tube freeze up with no adverse result. If AF447 is the one with the adverse result, then how do those statistics look?
Pretty P poor.
Even if we had 500 instances with one bad result, the rate is still unacceptable. The key to good outcomes is avoiding unnecessary risk.

Suppose there is a functional [email protected] airspeed system available already. Suppose the barrier to its use is "we already have a good system with the pitot tube." Don't you think it is possible that by being technological Luddites, we may be holding back a promising new development that only needs a foot in the door to advance? (And to the best of my knowledge, none of my $$ is invested in such a company offering such an airspeed monitoring system.)

In the near term, I like MM43's idea about tiding the aircraft over until things can be stabilized. Give those easily confused computers some realistic data to chew on.
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Old 26th May 2011, 04:27
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Thanks for the post bear.

If ICE is the consideration, three separate design approaches would likely produce three anomalous solutions to the goal. Redundancy works well if in a "fallback" position, so I like your anomalous heat solution.
You also raised a valid point vis-a-vis the statistical credibility of such a design. Placing confidence in the divergence as you put it. (sorry to keep quoting the deleted post.) Here I think the data collected by Northwest can come in handy. Airbus has a baseline to compare against any data that may be generated from experiments, if they so chose. Who knows, even the beans may be convinced.
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Old 26th May 2011, 05:00
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Hate to state the obvious, but how many Boeing/Douglas/Lockheed airliners have suffered multiple pitot clogging from ice?

When the pitot get iced, I have seen no mention of windshield getting iced, which means windshield heaters are more robust than pitot heaters.
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Old 26th May 2011, 05:15
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the egg or the chicken

(Beg your mercy beforehand...)

If the rumour is right and there was an uncommanded "pitch-up":

Which really might have happened:

1 - iced pitots leading tho this pitch-up ?

2 - this pitch-up leading to a reduction higher than 30knots in measured airspeed and (not really sure) consequently to the famous acars message about the probes (and maybe the probes weren't "iced" at all) ?
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Old 26th May 2011, 05:40
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GB
how many Boeing/Douglas/Lockheed airliners have suffered multiple pitot clogging from ice?
Offhand, I can think of a B with a bad result, but that was a switchology problem.

I suspect that the question is better asked from the FBW/ Non-FBW perspective.
There are differing dependencies as mentioned earlier.

Does anyone know of a FBW airliner that does not use airspeed in the control gain calculations?

Isn't comparing windshield and pitot heating like comparing apples and oranges? Very different systems, performing different functions with different operating principles.
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Old 26th May 2011, 05:53
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Thumbs down

guys,

why don't you wait a few days to get the result, instead to come with xxxx thousand of hypothetical answers.?

get a life!
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Old 26th May 2011, 06:23
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Machinbird:
Isn't comparing windshield and pitot heating like comparing apples and oranges? Very different systems, performing different functions with different operating principles.
Very different devices, sure, but they're heated just enough to keep ice from accumulating. They're in the exact same icing, which is reportedly outside of certification requirements, yet no report of windshield icing over?

I don't think FBW by itself is such a big deal, except it would seem easier to incorporate a reversionary pitch/power mode.
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Old 26th May 2011, 07:05
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Very different devices, sure, but they're heated just enough to keep ice from accumulating. They're in the exact same icing, which is reportedly outside of certification requirements, yet no report of windshield icing over?
From this NASA tutorial:



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Old 26th May 2011, 07:09
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@GB

When the pitot get iced, I have seen no mention of windshield getting iced, which means windshield heaters are more robust than pitot heaters.



or possibly just that the windshield is less prone to icing than the pitots ? My (non-expert) understanding was that small items protruding ( such as pitots) ice up more easily than large surfaces (such as windshields) .
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Old 26th May 2011, 07:41
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John Thanks, I'll remember that.

********


If a particular type is prone to eccentric behaviour it can be banal or it can be lethal. Boeing Rudder Actuator, TRENT FOHE, MD screwjack Mx demands, etc. I take particular note of NASA's warning re: Probe Icing. Rather than wait for UAS as a predictor of Probe Icing, how about a lit probe just out the F/O's side glass?

Kidding aside, Pitots are simply not the problem, not the proximal problem, anyway. There is a "procuring cause" but that remains to be seen.

OK465 "If the devices in question are state of the art, then would not the characteristics required of the other two be a step back? " See below? Anomalous design? The bold after "most definitely".

Machinbird "I would not advocate wholesale replacement of pitot tubes with a 'new concept' system, that would destroy diversity, not improve it."

Most definitely. "The original code for this system was written in isolation by three separate teams." Different is good, but not in sensors?

The AD's spend all their time assuring the three probes are not discrepant, then go all wonky when they are? Good mission, no follow through? This just smells like over reliance on development, and over confidence in outcome. Otherwise UAS would be a yawner.

kiwiandrew You and cogsim crossed.

If the leading edge of the Probe aperture needs to be razor sharp for less drag, why not polish the radome instead and make the business end of Thales a doughnut?

It isn't the pitots; with a boring flip (FMS) to the PF, who needs state of the art pitots?

The bottom line for me is that the system looks superb, but someone went to lunch instead of the meeting where the Pilots and the FMS were introduced.

Someone gas up the forklift. I think Friday, Truth be told, will be about humans, not pitots?


Machinbird: Pitch rate/g-rate transparency, Perpignan?

Last edited by bearfoil; 26th May 2011 at 07:56.
 
Old 26th May 2011, 07:44
  #2419 (permalink)  
 
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I don't think FBW by itself is such a big deal, except it would seem easier to incorporate a reversionary pitch/power mode.
Maybe we will learn something Friday regarding how different FBW is.
Here is some "homework" for those who wish to get their minds ready to understand a bit more about what makes FBW different.
Fly-By-Wire A Primer for Aviation Accident Investigators
The quiz question is: How does the pitch feedback in the A320 at 180 knots differ from that at 230 knots?
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Old 26th May 2011, 08:08
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Anyone here that can comment on ultrasonic airspeed measurement ?

I did a little googling of this potential alternative (??) to pitot sensing and found out that there are commercial "high grade" products available for wind sensors as well as for helicopters. But the limits of its accuracy seems to be around 150 knots top speed.
Any hope of developing this for airliners? (Wikipedia does note that "Another disadvantage is lower accuracy due to precipitation, where rain drops may vary the speed of sound.")


Another potentially usable technology would be the hot wire mass flow sensor, where the air flowing past cools down an electrically heated wire and the airspeed can be calculated from the wire's electrical resistance which changes according to temperature. But this is not seen on airliners either. Anyone knowing if this is due to inherent drawbacks compared to pitot tubes, or just an aversion to try new technology?

Obviously these technologies must have some drawbacks compared to pitot tubes, otherwise they would already be in widespread use. It would be interesting to know where the problem areas are.


******
Bearfoil

how about a lit probe just out the F/O's side glass?
ATR's used to have exactly such a device. IIRC it was deployed after the Roselawn crash, but nowadays it's not not so widely used (optional equipment?) as the knowledge of the plane's icing behaviour and how to manage icing conditions is improved. Today they handle icing conditions just fine.

Last edited by snowfalcon2; 26th May 2011 at 09:12. Reason: added ice detection probe text
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