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AF 447 Thread No. 12

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AF 447 Thread No. 12

Old 16th Oct 2014, 08:26
  #681 (permalink)  
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Originally Posted by OG
That being said, it seems to me that such a function would fill what BOAC is looking for rather well, and I can't see any particular reason or difficulty to prevent it.
- agreed, but if I may make a plea - while this thread is about 447, it should really be expanded to cover all instances of autotrim which have caused numerous LOC incidents. It appears that we can no longer expect pilots to monitor airspeed (where available) nor to monitor aircraft attitude, so something surely needs to stop HAL or whatever we choose to call the system from trimming to absurdity. Again, a 'stop' at a sensible limit with a 'conscious' over-ride option for when the horses in the hold break loose would do it, would it not? The bonus is that we don't need any 'sophisticated' software with its inherent traps to do it - a simple position transducer triggering a switch with a 're-close' button would be fine.

WRT Airbus in particular, rrat at post #818 with an 'AND' switch is the answer plus inhibition of autotrim in ALL modes following the subsequent stall warning, and this should go a long way to helping those are not skilled pilots.

Not forgetting, of course, for all types, a seismic shift in training policies.
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Old 16th Oct 2014, 08:46
  #682 (permalink)  
 
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posted by BOAC


Again, a 'stop' at a sensible limit with a 'conscious' over-ride option for when the horses in the hold break loose would do it, would it not?

Maybe, but a sensible limit would be "configuration conscious". 2 or 3 ANU might be fine for cruise, but full flap, extreme fwd CG and Vref with a modest allowance for a steady turn might need 12 deg or so.
Possibly one simple solution might be to have THS deflections limited according to the slat/flap selection (with no over-ride - anything beyond that limit would have to be held by elevator deflection)
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Old 16th Oct 2014, 08:53
  #683 (permalink)  
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with no over-ride - anything beyond that limit would have to be held by elevator deflection)
- not easy with that - I would like the option to trim for a one-off really extreme condition, but in your latter case there should indeed be no problem having to hold back stick for the duration of the turn, although how that would fit with the computer operators....................
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Old 16th Oct 2014, 09:20
  #684 (permalink)  
 
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OK, but remember that the pilot won't be the one to hold the elevator, the EFCS will do that - you just won't get it bled off by THS movement
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Old 16th Oct 2014, 09:58
  #685 (permalink)  
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I assume that is AB? It certainly would be the 'pilot' in a 737. So, for AB, a limit of less than 12 could be used but it might require a positive action to over-ride in some situations - eg your example? Would that be a problem?

I am having trouble trying to imagine any 'normal' pilot in 447 running out of back stick due to this 'limit' trying to hold 10 degrees nose-up whilst zooming up above max alt and thinking "Oh yes, I need to over-ride this THS limit or the nose will drop". One would hope that that and rrat's continuous stall warning might have some effect?
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Old 16th Oct 2014, 14:58
  #686 (permalink)  
 
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Originally Posted by rrr
A320 FCOM OP-020, Flight Controls, Alternate Law
In low speed (at a speed threshold that is below VLS), the automatic pitch trim stops and natural longitudinal static stability is restored, with a stall warning at 1.03 VS1G.
In these conditions, the automatic pitch trim operation would also stop on the 330 in ALT1, but not in ALT2 with failure of 2 ADRs.
Don't you have that ALT distinction on the 320 ?

Originally Posted by Bpalmer
AF447 did not reach an unusual attitude at any point. The only parameter that could have triggered the unusual attitude law would have been the 45° AOA.
Speed below 60kt in another one.

Originally Posted by BOAC
It appears that we can no longer expect pilots to monitor airspeed (where available) nor to monitor aircraft attitude, so something surely needs to stop HAL or whatever we choose to call the system from trimming to absurdity. Again, a 'stop' at a sensible limit with a 'conscious' over-ride option for when the horses in the hold break loose would do it, would it not? The bonus is that we don't need any 'sophisticated' software with its inherent traps to do it - a simple position transducer triggering a switch with a 're-close' button would be fine.
Has been effectively required for ages, the presence of a simple DIRECT switch to get rid of any sophistication or protection on demand.
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Old 16th Oct 2014, 17:59
  #687 (permalink)  
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the presence of a simple DIRECT switch
- I say! Steady on! You'll be expecting crews to actually fly the aeroplane next................
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Old 16th Oct 2014, 21:32
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Originally Posted by BOAC View Post
- agreed, but if I may make a plea - while this thread is about 447, it should really be expanded to cover all instances of autotrim which have caused numerous LOC incidents.
Such as? I'm pretty sure this is the only one.

There also seems to be a continuing misinterpretation of the B777's bypass switch (aka "Big Red Button") - it needs to exist in the B777 design due to the software-driven force-feedback. If that subsystem goes and provides misleading control response to the crew, things could get ugly very quickly - so the bypass acts as a "failsafe" for that scenario. Airbus's system doesn't use force-feedback, so there's no need for that kind of failsafe requirement.

Interestingly, the B777 bypass mode has never been used, even though there have been a couple of incidents where it could have come in handy (e.g. Malaysian 124 out of Perth [IIRC] springs to mind). The crews in the incidents didn't take advantage of it.
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Old 16th Oct 2014, 21:48
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Winnhofer, I understand your point on flying is suboptimal conditions. It can raise the pucker factor, to be sure.

One of the things that you rely on it situations that begin to go pear shaped is training. I had a few brown flight suit nights over the dark, no horizon oceans that all worked out in the end ... not because I was some sky god or some super pilot, not hardly. It worked out because we were trained, had procedures for dealing with things going wrong, and the crew (both pilots and the crew in the back) worked together to get us back to the ship in one piece.

As I commented to Clandestino, I get the impression from the transcripts we have available that the flight deck crew were in a problem solving mode (though it seems that they were behind the aircraft), and grew frustrated as well. I don't get from that limited representation of the event sequence the symptoms of fear.

But it may have played a part. We don't know.

If it did, that's another area of concern for Air France, in terms of training, and possibly at other airlines.

What do you have to fall back on when things begin to go wrong?
Your training and proficiency are key resources at such times.
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Old 16th Oct 2014, 22:37
  #690 (permalink)  
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Originally Posted by DW
Such as?
- try post #792. Your focus is too narrow. In each autotrim caused major problems.
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Old 16th Oct 2014, 22:50
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Those two (TC-JGE and G-THOF) are autoflight-related issues, both involving the Boeing 737. Autotrim in the Airbus sense is not a part of the autoflight system, but a component in the EFCS setup.
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Old 17th Oct 2014, 08:10
  #692 (permalink)  
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As I said, narrow focus. My suggestion was to limit autotrim, full stop.
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Old 17th Oct 2014, 09:01
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I agree with Lonewolf50.

Winnerhofer,you are writing some fantasy
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Old 17th Oct 2014, 16:55
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Fear

Then they did stupid pilot stuff and rode a perfectly good airplane 35000 ft in a stall with the stall horn going
Is that distracting enough to forget there is GPS groundspeed and an iron gyro AI available for cross reference before (inexplicably) yanking and banking on the joystick?
GPS Groundspeed...Really?
Try this on your next flight. Tell your fellow airman to close his eyes and tell you what the GPS groundspeed was sometime in the last minute. Try to mentally correlate that to airspeed and make appropriate adjustments to keep your aircraft within the narrow indicated airspeed window—all when your struggling to keep things right side up. In the fear mode we don't think well.

The far better answer, one that is the standard official technique when encountering severe turbulence, is to try to hold a normal ATTITUDE. Don't chase the altitude or the airspeed (if it's working). That is much easier to grab on to, and requires no mental math. Do you really want someone jockying the thrust levers to chase a GPS groundspeed? Compound that with the fact that if you are in this situation you're probably in an area where the wind is changing and what was a good GPS groundspeed 10 minutes ago is now 50 knots too fast or too slow—either one of which critically dangerous.



If the AF447 crew had done that, had Bonin resumed the normal pitch attitude when Robert was telling him to "go back down," we never would have heard of them.
The question is did Bonin even know what the normal attitude was without the aid of the flight director? That's where a bit of regular "turn it all off" pays dividends. If you're hand flying with the flight director on it's like typing a dictated story thinking you're coming up with all the good ideas.
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Old 17th Oct 2014, 17:13
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Originally Posted by BOAC View Post
As I said, narrow focus. My suggestion was to limit autotrim, full stop.
Why? What good would it do?

I'd dispute the "narrow focus", the scenarios are completely different and completely unrelated - other than the point that the crews involved didn't keep an eye on their trim status when they should have done so.

In fact the two B737 incidents began with failing to monitor airspeed and thrust settings on approach - the trim situation was an afterthought. I'm pretty sure I remember reading that SOP in a B737 go-around scenario from autoflight involves adjusting the trim to a setting commeasurate with go-around, because approach trim configuration isn't going to work there.

AF447 wasn't caused by the automatic trim, and unlike the B737 incidents, the trim was effectively set by the pilot's commands - not autoflight.

Those are pretty different scenarios no matter which way you slice it.
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Old 17th Oct 2014, 17:27
  #696 (permalink)  
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other than the point that the crews involved didn't keep an eye on their trim status when they should have done so.
- starting to widen? In terms of the 'big picture' it is a lot worse when autotrim takes you out of 'acceptable' areas when flying manually as with Airbus, but still bad when it does it in autopilot in any type.
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Old 17th Oct 2014, 17:48
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Originally Posted by BOAC View Post
it is a lot worse when autotrim takes you out of 'acceptable' areas when flying manually as with Airbus...
Well, it's interesting you should say that. Let's not forget that when the A320 was launched, there was a very vocal group of pilots who were up in arms about letting the technology decide for them what was "acceptable" and what wasn't.

So for whatever reason, the A330/340 implementation of autotrim always defers to the pilot as to what is "acceptable" when the systems are in a non-normal configuration. Again - intentionally or not, the THS was *commanded* to that position by the pilot flying.

@Bpalmer - re: pitch/power/attitude - absolutely right, and the correct pitch/power settings were in that supplementary FCTM sheet that AF apparently didn't deign to pass on to the pilots.
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Old 17th Oct 2014, 18:11
  #698 (permalink)  
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the A330/340 implementation of autotrim always defers to the pilot as to what is "acceptable" when the systems are in a non-normal configuration
- you are still 'missing it'! Not in the case of 447, it didn't. The pilots had no idea what they were doing, coupled with a design fault in the stall warning logic, so the system just carried on willy nilly as programmed - and to call that 'deferring' is sad. It is like holding up 4 fingers to a blind man and asking him how many.

Anyway, Dozy, we are entering Oozlum bird land, so I'll sign off with you.
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Old 17th Oct 2014, 19:31
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Originally Posted by BOAC View Post
- you are still 'missing it'! Not in the case of 447, it didn't. The pilots had no idea what they were doing, coupled with a design fault in the stall warning logic, so the system just carried on willy nilly as programmed - and to call that 'deferring' is sad. It is like holding up 4 fingers to a blind man and asking him how many.
With all due respect, I don't accept that. SS input affecting both elevator position *and* THS position has been "baked in" to the Airbus EFCS design since day one. It's one of the aspects of flying a FBW Airbus that is somewhat different from the traditional setup, and as such should be one of the fundamental things understood as part of conversion/type rating training. There should be no excuse for crews not to have that in the back of their mind when manipulating those controls.

The technology can't know whether the pilots know what they're doing. It isn't psychic - hell, it isn't even that "intelligent". The point is that you can either design a system which constrains the pilots on that basis, or you can design it on the basis that assumes the crews will know what they're doing and have it follow suit. The A330 autotrim design seems to have erred more toward the latter than the former. I don't know why - it just seems to have been that way.

Furthermore, calling the SW behaviour a "fault in logic" is a bit harsh in my book. For one thing the problem is quite complex in nature (as we hashed out in the older threads) - SW relies on data from the AoA vanes, so how do you deal with a situation where the AoA data may be suspect? What makes it extra complex is that finding a solution to one scenario may potentially cause problems elsewhere - for example if you take rrr's suggestion:

Originally Posted by rudderrudderrat View Post
You simply make the stall warning invalid below 60 kts AND on the ground (to prevent false nuisance stall warnings during the early take off roll).

However, when you are airborne, if the angle of AoA sensors think you are stalled and even if you have less than 60 kts indicated forward airspeed - then you are definitely stalled. Therefore remove the 60kt logic once airborne.
There's still a potential "false negative" scenario where the AoA vanes are frozen in a "non-stall" condition.

Then there's XL888 and MH124 both showing the potential consequences of allowing "bad" AoA data through to the EFCS - another thing that must be borne in mind.

Anyway, Dozy, we are entering Oozlum bird land, so I'll sign off with you.
That's a shame - I think it's perfectly reasonable for two people to have differing opinions and accept that to be the case.

I do, however, suspect that there's a bit of a double standard at work here. I was just having a look at the R&N thread about the Thomas Cook B757 incident at Newcastle in which it appears the PIC messed up a go-around by pushing the wrong button and following the (incorrectly configured) FDs immediately afterwards. There seems to be no shortage of posts giving the pilot a shellacking - but I can't help but think that had it been an A321, there would have been a much more concerted attempt to blame the technology.

As long as the "Airbus FBW was designed to patronise pilots" meme holds some belief (even though it's bobbins), that''ll be the case - and I think it's a bit sad that we're in this position after 26 years of the things flying safely.
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Old 18th Oct 2014, 01:18
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Originally Posted by Dozy
Interestingly, the B777 bypass mode has never been used, even though there have been a couple of incidents where it could have come in handy (e.g. Malaysian 124 out of Perth [IIRC] springs to mind). The crews in the incidents didn't take advantage of it.
You still did not get what happened to that 777 ...
http://www.pprune.org/7022213-post1332.html

Airbus's system doesn't use force-feedback, so there's no need for that kind of failsafe requirement.
And you still did not get what would be the purpose of a DIRECT switch for the Airbus.
Some crews would have loved to have one :
http://www.pprune.org/tech-log/50207...oa-probes.html

More on the new Gulfstream :
GULFSTREAM INTRODUCES NEW AIRCRAFT FAMILY | Gulfstream Newsroom
The industry-first ACSs (active control sidesticks) offer enhanced safety and situational awareness over passive sticks through tactile feedback. With electronic linking of the ACSs, the pilot and co-pilot can see and feel each other’s control inputs, which helps improve pilot coordination in the cockpit.
The active control sidesticks enable both pilots to be consistently on the same page, enhancing safety
Common sense, but Airbus didn't think so ...
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