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AF 447 Thread No. 12

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AF 447 Thread No. 12

Old 26th Jul 2014, 00:42
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I was wondering if you could clarify a bit what you meant by "...why does the computer air data program attach quite so much importance to pitot static airspeed?".

What I meant here was as I recall when the airspeed data corrupted, the aircraft went into alternate law mode, which seems to have added to the crew confusion, yes you are right about the stab trimming nose up due to the sidestick input, but I understood this trimming happened automatically under the "alternate law", recovering from a stall with full up stab trim would vastly confuse things.

This is what I meant by "so much importance to IAS", to me it seems there should be more parameters measured, before he system reverts to alternate law? For example shock stall and crit alpha stall cause the same symptoms as such (buffet), and with a falty IAS reading in IMC, the only sure way to tell the difference is AoA? If the crew had had this, would they have realised they were at low speed, not overspeed?

This may have been discussed earlier, but this was a very interesting accident (very tragic too), as it touches on the clash between classic stick and rudder skills, and modern aircraft AP systems.

(By the way, how do you do a quote on a forum post? I looked in FAQ but no luck? I am anashamedly a very pre computer ager, when I joined the air force, the most odern piece of kit on the Sqn was a hand cranked copier!)
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Old 26th Jul 2014, 01:25
  #222 (permalink)  
 
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This has all been covered in the mega-threads, so I'll go over this once only for the sake of my sanity...

Originally Posted by PerAsperaAdAstra View Post
What I meant here was as I recall when the airspeed data corrupted, the aircraft went into alternate law mode, which seems to have added to the crew confusion...
Going by the report, the only reference made on the flight deck was the PNF making the callout from the ECAM. The PF never acknowledged the callout.

Alternate Law should be neither confusing nor a big deal - in a pinch all you need to remember is that the "hard" protections are no longer there - i.e. it's possible to stall or spiral dive the aircraft if you overcontrol (just like a conventional aircraft), and that depending on the mode you're in, the aircraft will be slightly more sensitive in roll. In short, all it means is fly normally but be careful with the controls.

you are right about the stab trimming nose up due to the sidestick input, but I understood this trimming happened automatically under the "alternate law", recovering from a stall with full up stab trim would vastly confuse things.
Autotrim is active in Normal Law as well. I was lucky enough to be able to perform some experiments with a friend in an A320 sim, and what we discovered was that it was possible to roll the trim forward again with sidestick on its own as long as it was caught early enough (link to summary below).

http://www.pprune.org/tech-log/46062...ml#post6793521

This is what I meant by "so much importance to IAS", to me it seems there should be more parameters measured, before he system reverts to alternate law?
The drop to Alternate Law had nothing to do with the buffet or any other stall-related phenomena. As I said above, Alternate Law takes away the hard protections (i.e. limits) because the design rightly assumes that the pilots should have the final say if the data being fed to the computers stops the system from working effectively. The control laws are really just the equivalent of what the Flight Engineer used to do in the event of a technical problem - i.e. reconfigure the aircraft's systems in a way that provides the greatest degree of controllability and safety.

it touches on the clash between classic stick and rudder skills, and modern aircraft AP systems.
AP wasn't really involved here, as it disconnected right at the start of the sequence. FBW is *not* automation in that sense.

(By the way, how do you do a quote on a forum post?)
Check your PMs.
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Old 26th Jul 2014, 01:57
  #223 (permalink)  
 
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Thanks Dozy, you seem anything but! Interesting stuff the 447 case. One last question, the sidestick, is it programmed to give a stick force feel, (I think it is?), and will it show sloppiness at low speed, tightening up at high speed as such? Why the need to autotrim the stab with sidestick input? It seems to me a better option to let the FP choose the option surely?
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Old 26th Jul 2014, 13:30
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Originally Posted by PerAsperaAdAstra View Post
Thanks Dozy, you seem anything but!
Hahahaha - quite a few folks on here would say otherwise!

One last question, the sidestick, is it programmed to give a stick force feel, (I think it is?), and will it show sloppiness at low speed, tightening up at high speed as such?
No - it's passive spring feel. Breakout from centre position and you know you're doing it, it's a chunky piece of kit! Very well balanced though - check out the summary

Why the need to autotrim the stab with sidestick input?
Easier for the PF, bearing in mind there's no force-feedback. The autotrim system works just fine, though it means that the Airbus FBW series is a little different from a conventional setup there. Interestingly in the A320 sim we found that the trim would stop at a certain point, and had to wind it back manually to simulate AF447's condition - this is apparently different from the A330 which will autotrim all the way if the pilot appears to be demanding it.
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Old 27th Jul 2014, 08:16
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"Another AF447"? Not even slightly
This one was similar -

West Caribbean Airways Flight 708 - Wikipedia, the free encyclopedia

Captain flying this time.
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Old 28th Jul 2014, 17:42
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@Oakape: Yup - we covered that on the megathread. What's interesting there is that WCA708 provides an instance of rebuttal to the "connected controls" argument, as that aircraft (an MD-80) had connected yokes, and the PNF still didn't see (or respond to the evidence) that his Captain was in fact pulling harder into the stall. Also that the MD-80's automation was not sophisticated enough to detect that it could not maintain the requested altitude with the engine thrust reduced by the anti-ice system.
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Old 28th Jul 2014, 19:25
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The issue is that we're not given any context as to what that sim session was about. That series was broadcast in 1996.

In general, Black Box was an excellent series, and I still have it kicking around on VHS somewhere - however in this episode it does make the mistake of assuming the technology of the time was a first step in removing the pilot, which was never true.
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Old 5th Aug 2014, 21:58
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Originally Posted by DozyWannabe View Post
@Oakape: Yup - we covered that on the megathread. What's interesting there is that WCA708 provides an instance of rebuttal to the "connected controls" argument, as that aircraft (an MD-80) had connected yokes, and the PNF still didn't see (or respond to the evidence) that his Captain was in fact pulling harder into the stall. Also that the MD-80's automation was not sophisticated enough to detect that it could not maintain the requested altitude with the engine thrust reduced by the anti-ice system.
Incidents like the one cited argue for better training on stall, incipient stall, stall recognition, and recovery regardless of make and model.

Not a technical point, this being Tech Log, other than the matter of technical training associated with operating complex machines.
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Old 6th Aug 2014, 09:40
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Winnerhofer:

Stall warning is set to occur before natural (aerodynamic) buffet starts. The stall warning threshold is approximately 1 deg less than the buffet onset AoA. At 7 - 8 degrees the airplane would be well into buffet:



P.S.
Although the EICAS is for system failures rather than pilot errors, perhaps a message like: "STALL: RELEASE STICK OR PUSH" would help a disoriented pilot?

Last edited by Gysbreght; 6th Aug 2014 at 10:13. Reason: P.S.
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Old 6th Aug 2014, 18:24
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Originally Posted by Winnerhofer View Post
Is the SW programmed to kick in @ 5.8 AOA too early?
Where are you getting this info from? Unless you have a triple ADR failure, Stall Warning is calculated differently depending on the current Mach value. Are you thinking of the FL250 limit with the BUSS due to fixed SW value (not relevant to AF447)?

Originally Posted by Lonewolf_50 View Post
Incidents like the one cited argue for better training on stall, incipient stall, stall recognition, and recovery regardless of make and model.

Not a technical point, this being Tech Log, other than the matter of technical training associated with operating complex machines.
Hullo sir - as far as the latter point goes, I think it's fairly wide-ranging. I agree totally with your first point, however I also find that incidents like WCA708 and Birgen301 provide a useful counterpoint to those who reflexively claimed that AF447 would not have happened on a Boeing/MD with linked yokes. To be frank, I'd rather such commentary didn't come up - as it is a distraction from the matter at hand - but alas it has done fairly frequently.
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Old 7th Aug 2014, 00:40
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incidents like WCA708 and Birgen301 provide a useful counterpoint to those who reflexively claimed that AF447 would not have happened on a Boeing/MD with linked yokes
3 accidents with many other contributory factors is too smaller sample base to draw any definate conclusions

No one is seriously saying AF447 would not have happened in a Boeing. However, many experienced pilots here have explained how a yoke may have helped the pilots achieve a better outcome. Let them have their opinion jeez
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Old 7th Aug 2014, 07:56
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Sorry for a non-pilot making a statement here. The father in law of a good friend of mine was an airline pilot with 25+ years of experience. He started originally on Tu-154 then transferred to B-737 for many years. Then he was supposed to be retrained to Airbus due to changes in the fleet of the carrier he worked for.

He expressed many times his disappointment with FBW environment with sidestick without feedback from control surfaces and said that this is not old school flying anymore and the joy of flying is gone.

Making the long story short the pilot ended his life by suicide. It would be very inappropriate to claim that the old guy decided to leave the world just because of necessity to change to new type of a/c with SS but as the family says he was very bitter with the fact. It is known that suicide cases have (like accidents) more contributing factors. Also this is not an attempt to make a flame war regarding Boeing and Airbus proponents.

What I can say as a consultant which is partly involved with safety in various industries - if the operator on any machine or vehicle doesn't like the controls of it then he is more prone to accidents.

Now back to pro pilots
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Old 7th Aug 2014, 17:04
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Originally Posted by Cool Guys View Post
No one is seriously saying AF447 would not have happened in a Boeing.
You say that, but in the "dark" days of the old AF447 mega-thread, I remember seeing that opinion advanced - usually in complete seriousness - at least once every two or three pages.

However, many experienced pilots here have explained how a yoke may have helped the pilots achieve a better outcome. Let them have their opinion – jeez
I'm not stopping anyone from having an opinion! Though I would question how many of the experienced pilots advancing that opinion have actually flown a FBW Airbus. As you state, three or four incidents aren't really enough to draw a scientifically significant conclusion from. What is noteworthy in an anecdotal sense is that in the time since the Airbus FBW types were introduced and became widespread, there have been more LOC accidents of this type on airliners with yokes than those with sidesticks.

In theory, the connected yokes should provide an extra cue in the visual and tactile channels, but in practice it doesn't seem to make a great deal of difference.

What this means is that no matter what anyone's personal feelings are on the matter, both designs are - as near as we can tell - pretty damned safe, and that the various pros and cons of each are immaterial in real terms.

Last edited by DozyWannabe; 7th Aug 2014 at 17:25.
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Old 8th Aug 2014, 06:54
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yeah, Ive read the "mega" thread. There are many interesting and valid opinions from some smart and experienced people.

Because the sample base is very small, a comparison based on statistical evidence has minimal value.

Last edited by Cool Guys; 8th Aug 2014 at 06:56. Reason: correct spelling
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Old 11th Aug 2014, 00:46
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@Cool Guys - Right - and I remember your posts on the subject and others. If I recall correctly, you're an engineer with an interest, like me.

Experience and intelligence are definitely good things - however the best kind of experience also comes with a continued desire to learn and evaluate changing circumstances. An experienced person who reflexively dismisses a different paradigm as inferior without properly evaluating it is as potentially dangerous as a less experienced person who blindly accepts each change as inherently better.

@Winnerhofer - The MD-80's pitot-static system does not automatically cross-check for data validity in the same way as the FBW Airbus system does. I don't think the author of your linked article is aware of that fact.
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Old 13th Aug 2014, 11:57
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I haven't got all the facts at my fingertips as I sit here now, will go over the info I have when I get the chance but 447 seems to tie in with the A320 LOC when carrying out an acceptance test flight for the return of the aircraft to service with Air to New Zealand. As I recall, an unplanned approach to the stall was carried out, as IAS was reduced, the aircaft systems trimmed the tailplane full nose up, just before the stall, the engines were powered up, but due to the up trim and the nose up pitch caused by the engine thrust lines on pod engines, the aircraft did not respond to recovery nose down side stick input as was expected. This caused confusion resulting in a relaxing of the nose down input. The aircraft subsequenty stalled and crashed into the ocean.

It seems to me the synthetic feel of the sidestick was an issue, can't help but feel a good old Boeing style yolk would have removed all doubt of what control input and to what extent, was being made? Would like to have a go at a sidestick if I got the chance, as I do regard them with some suspicion...along the lines of, it's all good as long as things are ops normal ...but when things are going wrong...
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Old 13th Aug 2014, 12:20
  #237 (permalink)  
 
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Hi Dozy,
In industries that produce equipment that can kill people if it goes wrong, those who are careful about adopting new non validated technologies pose far fewer risks than those who accept new technologies without proper evaluation.

Last edited by Cool Guys; 13th Aug 2014 at 12:20. Reason: spelling
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Old 13th Aug 2014, 15:36
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@Cool Guys - Correct, but I'd say a decade of development and testing (from the Concorde "minimanche" experiments in the mid-'70s onwards) constitutes a very rigorous evaluation process, even by aviation standards.

26 years since the A320 went into service and the number of hull-losses attributable to the FBW system and the flight deck design on all Airbus FBW types remains at zero. The Airbus FBW types have a safety record that compares very respectably with other types and thousands of the things fly daily.

So I'd say that the worst fears of the more reactionary "experienced" pilots back when the A320 was launched haven't come to pass, and on this occasion they were wrong about a lot of things.

Last edited by DozyWannabe; 13th Aug 2014 at 16:36.
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Old 13th Aug 2014, 16:27
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PerAAA #275


" aircraft trimmed nose up..."
" thrust lines on pod engines..."


How often does one include a visual check of the current state of the trim whilst in normal flight, when it is all done automatically for you ?


How often do you have the opportunity to alter the power from cruising power to flight idle and then back to climb power - whilst in level flight, to see what happens ?


One of the many performance graphs for AF447 appeared to show that power was reduced to Flight Idle - and the nose dropped for a few seconds (as it must). Then TOGA was restored. IIRC this was a few second prior to the final stall. ( I think that nothing was said at the time on the CVR by PF or PNF. The Captain was called at about this time.)
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Old 13th Aug 2014, 16:59
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Originally Posted by DozyWanabee
safety record
We could hope much better records than AF447 and such wonderful flights.

Last edited by roulishollandais; 13th Aug 2014 at 22:01. Reason: than
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