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AF447 Thread No. 3

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AF447 Thread No. 3

Old 13th Jun 2011, 23:29
  #1961 (permalink)  
 
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A33Zab, coincidences make me nervous. Why did this not manifest on take off roll or even when the plane was "turned on?" The take off roll has the high frequency vibration that would work a connector loose or break a wire from excessive flexing. The turbulence I've felt even flying into Orlando from Dallas in rough weather as SLF doesn't feel right for breaking a wire loose or a connector loose. It's all lower frequency "stuff" which low mass items simply follow rather than amplify or lag significantly. It's these factors that break wires even in bundles more than high frequencies.
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Old 13th Jun 2011, 23:32
  #1962 (permalink)  
 
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Smilin_Ed,

OK, firstly I was not involved in the design of the FBW systems, but as I've said earlier in the thread, my Software Engineering/Reliability professor was heavily involved in the debate over FBW in airliners back in the late '80s and early '90s. I posted a write-up of his regarding a visit to Toulouse in 1993 a short time ago. I should add that he was very much a sceptic, but it appears he was won over to some degree by that visit - he never stopped asking questions though. On a sadder note, as a result of private communications over this thread I've learned that he passed away a year and a half ago.

Now, regarding what you're saying - the autotrim will only move the THS in Alternate Law if the limit of elevator travel is reached and the pilot continues to demand pitch-up via the sidestick. It is highly unlikely - if not impossible - for an "inadvertent" sidestick deflection to move the THS. It needs to be aggressive and maintained for a period of time. I did confirm this via PM with PJ2 before I put my oar in, so please don't think I haven't done my homework here!

[EDIT - Also, a pilot can disable autotrim no matter what law the aircraft is in simply by moving the manual trim wheel. There seems to be some debate as to how this knowledge is disseminated in line training, but IMO a good pilot should know their aircraft as near inside-out as possible.]

Last edited by DozyWannabe; 13th Jun 2011 at 23:55.
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Old 13th Jun 2011, 23:39
  #1963 (permalink)  
 
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Quote from Smilin_Ed:
When practicing stalls, student pilots are warned NOT to trim into the stall. That is so when they let go of the stick, the airplane will return to a trimmed speed which is BELOW the stall speed. Further, if you trim into the stall, the trim might not move fast enough to get you out when you need it to.

In case DW and others get confused: think you meant to say "ABOVE the stall speed"? (That is, nose below it...)

Now, can I go back to sleep?
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Old 13th Jun 2011, 23:43
  #1964 (permalink)  
 
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@Chris Scott - thanks, though I got the gist of what he was saying. I'm not a professional pilot, but I am a former Air Cadet with some Chippie hours under my belt and a well-thumbed copy of HTBJ.

Basic aerodynamics not a problem, but some of the AoA trig goes over my head...
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Old 13th Jun 2011, 23:44
  #1965 (permalink)  
 
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bearfoil, may I suggest that while "did everything it was supposed to do" may be true "did everything it was told to do" is more accurate to what the data shows?
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Old 13th Jun 2011, 23:45
  #1966 (permalink)  
 
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Wiring CAN Fail

JD, no wiring installation is perfect. A minor imperfection hidden for years, when subjected to normal airframe vibrations, could break down without warning. We used to use soldered connections but manufacturers prefer crimped connections because they are cheaper to implement. Crimped connections are not sealed by solder and are subject to dissimilar-metal corrosion. In the Navy, we found this to be a serious problem because we operated in a salt-air environment. Connections can, and do, break down. I am not saying that this happened here or that there was any wiring fault at all. All I'm saying is that it could.
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Old 13th Jun 2011, 23:46
  #1967 (permalink)  
 
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Thanks Chris!

Yes, I did mean ABOVE the stall speed. I'm showing my age.
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Old 14th Jun 2011, 00:00
  #1968 (permalink)  
 
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Smilin - indeed wires and connectors can fail. I've had it happen to me. It's usually when accompanied by "sharp" high frequency vibration. (The extreme case was in a vibration test environment when the tech pulled a wire from the table's drive while it was still powered up. 100G shocks do things to even small electronics.)

I'm troubled by the coincidence when BEA says it is also triggered by good data transfer of bad data as their working point. It may be wiring. I don't see that as a sure thing. It seems to push coincidence too hard. And I don't see anything that makes it a sure thing rather than as a path for conjecture. It's surely a valid point for conjecture. I'd not take it as the only path for conjecture.

Edit: I think we agree. I am in disagreement with A33Zab's assurance this is the problem.
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Old 14th Jun 2011, 00:02
  #1969 (permalink)  
 
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Unhappy

Originally Posted by bearfoil View Post

IMAO,
The one basic and glaring error in the "Philosophy" of fbw as practiced by the folks @ Airbus, is the following:

NORMAL is Rules based, and these rules are explicit. Flying is situational, and the computer doesn't have the capability to program for all of them.
To me, the one difference in FBW "philosophy" between A and B is hard vs soft limits / protections. "Rules" is not the difference - the rules are there in both cases.

Approach the limits in A and the system starts ignoring your command, and/or takes other action to bring you back into the envelope - "I'm sorry, Dave. I'm afraid I can't do that".

Approach the limits in B and the system starts to fight you - more like "sorry Dave I don't want to do that, you'll have to pull harder to make me". It's still a rule. Moreover, it still has the potential to go wrong in very interesting ways if the system loses knowledge of how fast it's going or which way is up. Just like A, B will also in some cases take uncommanded action to keep you in the envelope - rules rules rules.

And rules can lead to wrong decisions, eg. after A/P and A/T turned off:
autothrottle activated and automatically advanced the thrust levers when it sensed a low-speed condition as a result of erroneous data being provided by the ADIRU
A or B ? Surely that's the A "rules" philosophy ? [Actually it's not.]

Rules and hard limits/protections have been part of aviation engineering before FBW and extend far beyond it. Want reverse thrust in flight ? No can do. Why ? It's a rule. Why ? It makes big smoking holes in the ground, so we made the rule. What if I need it ? We can't think why you ever would. But you can't think of every eventuality at design time...

And then of course the rule can end up going wrong - put the a/c down ever so lightly on a wet greasy runway, fail to trip to sensors and... no reverse. Oops.

The rules aren't perfect - fbw or other. They won't ever be. The aim is merely to cause fewer big smoking holes in the ground overall - and even that is very difficult to assess in the stats when the accidents are so few. Every rule, every protection or limit, every safety feature will kill someone, in some scenario, sooner or later. Car seatbelts and airbags kill people, reliably, every year - doesn't mean they are a bad idea.

A belief is a conclusion based on data. Faith is a format based based on Intuition, and "feel". When my beliefs are dashed, I rely on my Faith, which is in me, and my F/O.

The Wall of misunderstanding twixt the engineer and the Pilot will keep killing people until both understand the other. Please stop stubborn Pride from putting people at risk.
Faith is also belief in the absence of data/fact or contrary to it.

Interesting paper on A vs B approach (GPWS escape): http://www.rvs.uni-bielefeld.de/publ...9_CFIT_FBW.pdf

Conclusions (as I read it):
  • The A approach, hard limits, stops you getting that last bit of performance that might save you, sometimes
  • But the A approach gives more reliable escape performance most of the time
  • The B approach might be better overall given a perfect pilot
  • But for real human pilots, the A approach will save your arse more often
Yet the test pilots preferred B.

Why ? "Faith" - in their abilities, even when contrary to the data ? Dare I say "stubborn pride" ?


And of course the final irony is that we are discussing this in the context of an accident where those rules and protections you so dislike didn't kick in, but rather packed up and left. The cyber rules engine, deprived of its senses, took itself out of the loop, and handed full unprotected control to the meat...
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Old 14th Jun 2011, 00:04
  #1970 (permalink)  
 
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JD-EE

Originally Posted by JD-EE View Post
Kinetic energy is 1/2 mV^2. Potential energy is mgh. So the mass washes out of the equation. So a 3000' climb would give "gh" = 1/2 v^2 = 96000 ft^2/s^2 equals about a 438 '/s velocity change or about 300 MPH.
I am probably being a bit thick, but does this relate to the aircraft in question and if so, are you saying the aircraft lost 300mph in its climb?
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Old 14th Jun 2011, 00:15
  #1971 (permalink)  
 
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Airbus WRG does not necessarily mean a wiring fault. Simply put, is pitot icing failure a designed in and expected fail? I am sure that many who have worked the A330 have exhausted the troubleshooting manual, where a system is no longer working or failing as it should.
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Old 14th Jun 2011, 00:43
  #1972 (permalink)  
 
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DEEP STALLS some thoughts

DY
Several mentions have been made a 'deep stall' - why? This is a specific stall condition mostly defined as "A condition such that the turbulent wake of a stalled main wing "blankets" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall." Neither the BEA note, or any of the more 'thoughtful' contributors believe this aircraft was in any kind of "deep stall" I think it fair to say.


At the top of the zoom climb with the airspeed dropping to below 60 kts the flight controls are literally useless. What the plane will do then is literally unknown. But this one did not respond to any inputs. It did not change until the end. Thus it was out of control at that point. Like many many other craft in the same situation.
And therefore in a "DEEP STALL"!
PS: I'm thoughtful.
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Old 14th Jun 2011, 01:06
  #1973 (permalink)  
 
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At the top of the zoom climb with the airspeed dropping to below 60 kts the flight controls are literally useless. What the plane will do then is literally unknown. But this one did not respond to any inputs. It did not change until the end. Thus it was out of control at that point. Like many many other craft in the same situation.
And therefore in a "DEEP STALL"!
PS: I'm thoughtful.
I don't think "any inputs" is the correct statement? NU inputs were almost continuously applied according to the BEA, once past the zoom climb? Were those correct? At 13 degrees NU the THS wasn't helping much now was it - how about ND for a sustained period, trim out the THS and see what happens? My interpretation of DEEP STALL is one that is nigh impossible to recover, maybe this was that, but NU doesn't seem the way out?

For now I'm gonna stick to conventional stall.
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Old 14th Jun 2011, 01:14
  #1974 (permalink)  
 
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Deep stall recovery

How would you recover from a deep stall in a twin-turbofan conventional tailplane aircraft? Do you create a lot of asymmetrical thrust to either create a big side-slip and possibly large bank angle, or even roll over if absolutely necessary? If there's nothing for the aero-surfaces to bite on what do you do?
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Old 14th Jun 2011, 01:19
  #1975 (permalink)  
 
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...... Flaps 1
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Old 14th Jun 2011, 01:26
  #1976 (permalink)  
 
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Several mentions have been made a 'deep stall' - why?
Wallybird, you need to do some reading. D.P. Davies, Handling the Big Jets" is a good starting point.

Just because it isn't a T tail doesn't mean that it can't get in a deep stall. Look at Gum's past postings regarding the F-16 deep stall. I just posted something yesterday on post stall lift coefficient curves. I'd define a stall that results in a flight path angle of 45 degrees to the vertical as pretty deep, particularly with an AOA of 61 degrees.

I know you are a Johnny come lately to the AF447 tech threads, but you need to get out of the rut of past habits and impressions. When you get into this stuff, it is pretty interesting and very educational.
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Old 14th Jun 2011, 01:49
  #1977 (permalink)  
 
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Machinbird: The F-16 deep stall was/is a consequence of it's intentionally unstable aerodynamics - the FBW allowed this to work for the aircraft, but those wily pilots found ways to get the aircraft into attitudes that were beyond the design curve. As "gums" can better recount certain flight techniques and changes to the systems were made to assist recovery if you did get the aircraft in a deep stall. I don't think a Viper deep stall has much to do with AF44, except to illustrate that FBW laws and protections can be exceeded - however, again I do not subscribe that was the case here.
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Old 14th Jun 2011, 02:24
  #1978 (permalink)  
 
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We used to use soldered connections but manufacturers prefer crimped connections because they are cheaper to implement. Crimped connections are not sealed by solder and are subject to dissimilar-metal corrosion.
With all due respect, I'd like to add that crimped connections, when done properly, are just as good (if not superior) as soldered connections in all respects, including resistance to corrosion. What's more important, crimped connections - again, using the right tool - are far more consistent, even when done by underpaid and ignorant personnel, which is often the case (hope that does not apply to Airbus).

Some years ago I was involved in a project which included installation of two different systems in a subway trains. One of the systems, done by "old school guys", was using soldered connectors throughout; the other one used crimping. Each system had a few hundred connectors per train, exposed to elements and quite a lot of vibration for many hours a day. Not a single crimped connector ever came loose, while soldered ones were keeping maintenance department busy for a long time after the systems were installed. But then, this is anecdotal evidence which (hopefully) does not apply to aircraft industry.

Connections can, and do, break down.
So very true.
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Old 14th Jun 2011, 02:53
  #1979 (permalink)  
 
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GY, The F-16 deep stall was a result of its pitching moment characteristics in the stall, nothing more, nothing less. Not necessarily relevant to its unstalled characteristics which happened to be unstable by design while subsonic (and stable while supersonic). All different parts of the aircraft's flight regime.

To my mind once you get your airliner well away from the immediate area of the of the CLmax point on the high angle of attack side, you are beginning to get into an area of untested aircraft performance where anything can happen. You may as well define this as deep stall, because at that point, you are the test pilot. It is too risky for the airframe manufacturers to map this area. AF447 may or may not have been 'locked' in its stall but at 61 degrees AOA that is a deep stall in anyone's book. Even with positive pitching moment, it would take some time to rotate the aircraft to align with the airflow, and it would be very easy of overshoot into a negative angle of attack stall.

I went to some effort to obtain the PDF file I attached to this post http://www.pprune.org/6509546-post1873.html . Do give it a read. The post stall characteristics of the airfoils tested were rather surprising to me.


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Old 14th Jun 2011, 03:14
  #1980 (permalink)  
 
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Connections, wiring and cabling

Hi, Iceheart

crimped connections, when done properly, are just as good
I agree. I started to use soldering irons in 1965. Despite "being an old guy" i consider crimped connections a very good approach.

I would like to known what kind of wiring its being used by Airbus SAS. I hope has not Kapton.

And also i would like to known if Aluminum cable is used in 330/340 (being used in the 380).

Connections can, and do, break down
In FBW most of connections are for "signal". With high quality certified hardware they are "state of the art".

This is not the case in high current connections like windshield heater connectors. There was a number of cockpit "fires" due "ohmic" losses in the connector (aging effects under high current).

In new planes like F-GZCP the chances of a wiring fault (in computer systems interconnections) "simultaneous" to the Pitot problem and under a/c moderate turbulence i would risk to say being practically zero.

I am very curious on the nature of "wiring" fault showed in ACARS.

In naval aviation the issue is much more complex. And Kapton use was dangerous.
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