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AF 447 Thread No. 6

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AF 447 Thread No. 6

Old 6th Nov 2011, 15:07
  #1681 (permalink)  
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I think the simulator has a small, random amount of air resistance worked in - even when flying in "clear air", there are still small "bumps", just as in the real world. The (simulated) FBW systems therefore add very small corrections to compensate.

Obviously the simulation is not going to be 100% precise, but as long as the general trend follows suit the exercise is valid. You'll note that the real elevator and G traces diverge from the normal significantly more, and more frequently than those on the simulated traces - this appears to indicate the effect of turbulence.

CONF - the final report on this accident is still some time from completion, give them a chance. Having said that, given the number of ambulance-chasing vultures that would attempt to twist the data to suit their commercial ends, I'm not surprised that the release of data is limited.

Last edited by DozyWannabe; 6th Nov 2011 at 15:20.
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Old 6th Nov 2011, 19:12
  #1682 (permalink)  
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Originally Posted by DozyWannabe
I think the simulator has a small, random amount of air resistance worked in
Seems like a very reasonable explanation, but why is it virtually absent until 02:09:50, then growing in amplitude until 02:10:05, when it similar to that encountered by AF447, but without effect in Az and elevator?
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Old 6th Nov 2011, 21:12
  #1683 (permalink)  
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A reaction to autoflight disengagement perhaps? In all honesty I don't know, but the deviations around the normal prior to manual input still seem to be less than those encountered on the actual trace - it's just that because the AoA ended up at such an extreme value, the vertical element of the graph is compressed in it's early stages.
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Old 6th Nov 2011, 22:21
  #1684 (permalink)  
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Vertical Speed = 10,000 fpm, How can one not realise one is stalled?

This goes to the heart of the question that is continually in my mind. With a descent rate of 10,000 fpm for over 2 minutes why did the crew not realise they were stalled ? Even during the last few seconds of the flight the CVR indicates anguished surprise that the aircraft was going to crash. Was this parameter not prominently displayed on the PFD unwinding rapidly downwards ? If not why not ?
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Old 6th Nov 2011, 23:59
  #1685 (permalink)  
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dnr, FWIW, I have recalled tentative conclusions drawn by others in my post no. 1646. Briefly, some ATPL’s thought PF confused the status with overspeed. There are several bits of evidence to support that, though I think it is only a theory, but seems to me the best one at present.

So once fixated upon that, he kept pulling back, presumably trying to pull out of a dive and failing to understand why it wasn’t working.
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Old 7th Nov 2011, 01:09
  #1686 (permalink)  
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So, I was talking about some research this weekend. I got in touch with an old Aero Engineering pal of mine from Uni and he managed to wangle us some spare sim time at his facility in the wee hours inbetween training sessions. What we had was an A320 sim rather than an A330, which comes with some key differences - the most obvious of which is the lack of Alternate 2, the nearest equivalent being Alternate without speed stability, and a different underlying architecture past a certain point.

Due to time constraints we could only run each experiment once, preceded by some familiarisation time handling the sim manually in Normal Law, albeit at low level, following the FDs around basic turns and level changes.

The first experiment involved setting the conditions to night IMC with CBs in the vicinity, having set the autoflight to take us to 35,000ft and hold us there. We had a friend of his who is a TRE sitting in the LHS to provide guidance and monitor what we were doing. He then failed the ADCs, leading to autopilot disconnect and a drop to Alternate (without speed stability) and we tried to follow through and maintain a 15 degree pitch angle. Things we noted:
  • I'd suspected it would involve considerable effort to hold the sidestick there for a significant amount of time, but I was genuinely surprised at just how much.
  • The zoom climb occurred exactly the way we expected
  • The Alternate Law (no speed stability) on the A320 seems to have a hard trim limit of 3 degrees nose up
  • It was definitely possible to hold the aircraft in the stall with 3 degrees of nose-up trim and full back stick, but it required effort
  • The aircraft wanted to nose down and recover itself, and with about 10 degrees of nose-down maintained with the sidestick at the moment we passed about 30,000ft, we managed to effect a recovery with the speed coming back up to a point where we could level out safely at about 20-25,000ft judging by the standby altimeter.

The second experiment was the same as the first, but as my pal had noted, the A320 has a hard limit of 3 degrees NU trim available via autotrim in the secondary Alternate Law. We tried again, this time winding in full nose-up trim manually just prior to the point of stall. This time:
  • The aircraft seemed more willing to hold pitch with the trim at full-up, but to hold it at 15 degrees still required considerable effort
  • We had to add a touch of rudder (on the TRE's advice) to control the roll.
  • Despite full nose-up trim, we elected to start a recovery as we came down through about 35,000ft this time, just to see if it was possible using sidestick only
  • Following the same 10 degree nose-down sidestick demand as before, the trim rolled forward with the sidestick demand, returning to around neutral within about 5-8 seconds, and we came out of the stall as before.

Based on this, as far as the A320 is concerned at least, recovery is possible using autotrim via sidestick only even when the trim has been manually wound fully nose-up. Given more time we'd have liked to see what happened attempting recovery at lower altitudes, but the general take-away seems to be that with sufficient forward sidestick demand it is possible to recover from stall even with trim forced to where it's not supposed to be.

Of course, these were purely technical experiments. Not only was this a sim session with only pride at risk, but we all knew what was coming and had a pretty good idea of how to get out of it. This does not and cannot compare to a situation where you're trying to get out of it for real, especially with the added handicap of limited manual flying experience.

Whether the A330 behaves differently I don't know, but I've called in my favours for now and am eternally grateful to the people who made it possible. Someone else is going to have to take that on.

Last edited by DozyWannabe; 7th Nov 2011 at 01:28.
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Old 7th Nov 2011, 04:43
  #1687 (permalink)  
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Big thanks DWB, well balanced post.
Was it configured heavy, I recall 447 was a tad over loaded on take off?
Also did you find out what max rate of decent you had when stalled?
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Old 7th Nov 2011, 07:52
  #1688 (permalink)  
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Originally Posted by xcitation
I recall 447 was a tad over loaded on take off?
-aha! A new rumour! Goodie.
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Old 7th Nov 2011, 10:41
  #1689 (permalink)  
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Thread #7 starts here
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