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AF 447 Thread No. 10

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AF 447 Thread No. 10

Old 1st Sep 2012, 12:44
  #221 (permalink)  
 
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One more question for the FBW flight control knowledgeable folks here:

With gains fixed for 330 knots, can you get FULL nose down elevator in a developed stall?

How would stall recovery characteristics be affected with fixed gains versus active gains?

Last edited by OK465; 1st Sep 2012 at 12:45. Reason: added 'developed'
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Old 1st Sep 2012, 12:52
  #222 (permalink)  
 
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Can one achieve sufficient ND to escape the STALL in the first place?

Because in a stable aircraft, the Nose drops on its own?

Say PF sees the error of his ways, and wants full ND, instant. Does he get it?

Would not Direct Law be a better Law in this condition?

After erasing the protection and entering Alternate Law, controls hobble a potential recovery? Madness.

Last edited by Lyman; 1st Sep 2012 at 12:58.
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Old 1st Sep 2012, 13:13
  #223 (permalink)  
 
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Can one achieve sufficient ND to escape the STALL in the first place?
Hi Lyman,
Page 90 of the final report shows the elevator at -30 degs (Max) and the stab trim at -13 degs (Max).
Why do you suppose maximum Nose Down Elevator & Stab Trim would not be available if the stick was held fully forward?
If that was not sufficient to lower the nose, then they could have selected idle power.
Neither of the above were attempted because they never diagnosed (believed?) they had stalled.

Last edited by rudderrudderrat; 1st Sep 2012 at 13:14.
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Old 1st Sep 2012, 13:30
  #224 (permalink)  
 
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Rate. It took sixty seconds for the THS to cycle max. It took forty seconds to STALL.

Seems to me had the a/c degraded into Law Direct, the Stall happens earlier, and recovery if any, begins with more energy. Also, the likelihood of tail low entry diminishes?

With three lost ADR, the gains go to 330 knots default? Doesn't that have the effect of acclimating the flying pilot to gentle response in Pitch, and without speeds or AOA, he is lulled into confusion? , his senses are Acclimated to things happening at the airplanes rate, slowed, confident, non emergent?
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Old 1st Sep 2012, 14:06
  #225 (permalink)  
 
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I would think that full nose down elevator at 330 knots would provide for quite a bit more than -1G.

With the load factor protection in effect, and fixed gains, wouldn't this limit the amount of elevator deflection for a full ND SS input at any actual speed?

Last edited by OK465; 1st Sep 2012 at 14:10. Reason: reason
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Old 1st Sep 2012, 14:49
  #226 (permalink)  
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OK465;

In the sim exercises, the SS was held full forward to achieve about 10degND pitch. The THS followed up on the command and returned to about a -3deg position. Recovery took about 40 seconds. (Note for others: I realize the sim cannot replicate full-stall conditions due absence of data but neither is the behaviour completely irrelevant).

For the exercise anyway and from my pov, there was sufficient elevator available to get the nose down, and the job was made easier by the THS following up the SS ND commands.
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Old 1st Sep 2012, 15:27
  #227 (permalink)  
 
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PJ:

Are you sure you had 330 knot fixed gains?

With full forward SS at 330 knots you get about 1/4 ND elevator deflection.

(With full forward stick, you can do an outside loop in some aircraft in 40 seconds. )
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Old 1st Sep 2012, 15:34
  #228 (permalink)  
 
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Endlessly, monotonously, the pilot is trained to fly one gee. It is grail. Nothing can go wrong at one gee, even the pilot not flying was disturbed by "excessive" lateral inputs....

So any number less or more is to be avoided. Add a nagging suspicion of overspeed, and voila, don't ferchrissakes "lower the nose..."
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Old 1st Sep 2012, 15:37
  #229 (permalink)  
 
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Question Stewpid Newbie Question

May I ask a stewpid newbie question? OK, thanks, I will.

I have always heard/read that airliners travel "at about 600 MPH." Actual speeds are sometimes referred-to (by badly informed authors?) as 500-something MPH.

330 KTS apparently translates to ~380 MPH. What gives? I do realize that actual airspeed is different from ground speed. What am I missing here?

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Old 1st Sep 2012, 16:01
  #230 (permalink)  
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OK465;

No, I don't know whether we had 330kt gains. I'm probably not providing sufficient information but the exercise began at cruise speed, (273kts), I believe the sim/check pilot failed 3 ADRs for the exercise, the pitch was increased, the sim stalled and the recovery was started at various points in the descent. I realize control systems are capable of outside loops etc but of course there is no indication of the nature of few gains for the pilots and the FCOM isn't sufficiently detailed so I can't offer an accurate assessment of what we had. I believe we had full down deflection of the elevator as indicated on the Lower ECAM Flight Control page but I'd have to review the videos to confirm. One question I would have regarding "high gains" (or 1/4 elevator only available) is, would the THS still follow through? I expect it would as this is still a trimable force exerted by the elevators which would be trimmed out, but these are esoteric areas for a non-engineer pilot of these airplanes!

Last edited by PJ2; 1st Sep 2012 at 16:15.
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Old 1st Sep 2012, 16:07
  #231 (permalink)  
 
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Originally Posted by Organfreak
What gives? I do realize that actual airspeed is different from ground speed. What am I missing here?
Simply the difference between True Airspeed and Indicated Airspeed.
Indicated airspeed - Wikipedia, the free encyclopedia
True airspeed - Wikipedia, the free encyclopedia

The first airspeed controls many important aerodynamic control parameters such as g available and angle of attack.
The second airspeed (TAS) indicates how fast you are booking through the molecules of the atmosphere.
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Old 1st Sep 2012, 16:37
  #232 (permalink)  
 
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I believe the sim/check pilot failed 3 ADRs for the exercise...
Recalling the picture of the PFD with the FPV visible, one would assume only 2 ADR's were failed. You would not have an FPV with 3 ADR's failed.

Variable Airspeed Drift malfunction may be a better way to do this than constrained ADR failure. The ECAM messages are ADR disagree ones (like 447) and not ADR fault.

Double ADR fail is indeed ALT2 and good for stall demos with the FPV, but it is not ALT2(B).

...but these are esoteric areas for a non-engineer pilot of these airplanes!
Agreed, but they do fall into the category of nuts and bolts. Hard to keep an old Flight Engineer trained guy like me from pursuing this kind of 'trivia'. Remember the old Burtek Boards?
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Old 1st Sep 2012, 17:27
  #233 (permalink)  
 
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@OK 465

With gains fixed for 330 knots, can you get FULL nose down elevator in a developed stall?
Sure - why not? The gains, as I tried to explain, relate to the transient response characteristics; that is to say to the first 3 or 4 seconds of any manoeuvre. If the elevator deflection is not enough to satisfy the pilot's demand the system goes on winding in elevator until the demand is met. The 'gains' are irrelevant in that context. At the speeds involved in a developed stall there would be no hinge moment restriction on control deflection or rate of movement.

How would stall recovery characteristics be affected with fixed gains versus active gains?
Not at all - it is the steady state conditions that will dictate stall recovery not the entry transient.

I would think that full nose down elevator at 330 knots would provide for quite a bit more than -1G.
So it would, but (a) the aircraft was not at 330 kts - more like 155 kts and (b) you have to allow for the deflected THS opposing the down elevator.

With the load factor protection in effect, and fixed gains, wouldn't this limit the amount of elevator deflection for a full ND SS input at any actual speed?
No, it is the other way around. One can look at it several ways, but if you go back to the graph of AoA vs equivalent elevator angle that I posted way back you will find that with 13 deg NU THS (-19.5 deg equivalent elevator) and full ND elevator (+15 deg) giving an equivalent -4.5 (NU) elevator deflection, the aircraft would settle out at about 4 or 5 deg AoA. At the speed then in existence the 1g AoA would have been about 10~12 deg I think, so the best one can hope for until the THS starts to move back down is around 0.5g (ish). Difficult to say when negative load factor protection would have kicked in because the THS movement and general speed increase go together.

With full forward SS at 330 knots you get about 1/4 ND elevator deflection.
Where does this come from please?
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Old 1st Sep 2012, 18:09
  #234 (permalink)  
 
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Previous quote:

I think this would lead the system to apply less elevator (to drive the pitch acceleration) than it would normally use at these low speeds,
Then you mean this only as a transient with the eventual elevator deflection for a sustained full ND SS input at 'slow' speed being the same in both Normal & ALT2(B)?

Where does this come from please?
SD Flight Control page with sustained full ND SS input at both 330K & 200K in ALT2(B). It is constant, and the same in Normal at 330K. I don't even see full elevator deflection in Normal at 200K with a full ND SS input.

So it would, but (a) the aircraft was not at 330 kts...
But the FCC's were. I'm still trying to reconcile this with what I've encountered. I'm just not seeing what you're describing.

In any airplane I ever flew, I certainly never claimed to have all the answers, but I always strived to have all the questions.
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Old 1st Sep 2012, 18:16
  #235 (permalink)  
 
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Originally Posted by OG
Difficult to say when negative load factor protection would have kicked in because the THS movement and general speed increase go together.
Does negative load factor require negative AoA (zero lift at about -2)?

Originally Posted by OK465
SD Flight Control page with sustained full ND SS input at both 330K & 200K in ALT2(B). It is constant, and the same in Normal at 330K. I don't even see full elevator deflection in Normal at 200K with a full ND SS input.
How about the THS, did it move at all?

Last edited by HazelNuts39; 1st Sep 2012 at 18:45.
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Old 1st Sep 2012, 18:41
  #236 (permalink)  
 
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@OK 465

Then you mean this only as a transient with the eventual elevator deflection for a sustained full ND SS input at 'slow' speed being the same in both Normal & ALT2(B)?
Yes

SD Flight Control page
Sorry, I don't recognise what you mean by 'SD'

But the FCC's were. I'm still trying to reconcile this with what I've encountered. I'm just not seeing what you're describing.
We may be a bit at cross purposes. I was referring to the aircraft response to actual full ND elevator deflection. Were you perchance referring to full ND SS command? My point was that at low speed you will need a lot more deflection for a given delta gee than you will at high speed.

@HN39
Does negative load factor require negative AoA (zero lift at about -2)?
Yes - I agree zero lift at about -2 deg.
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Old 1st Sep 2012, 18:53
  #237 (permalink)  
 
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Sorry, I don't recognise what you mean by 'SD'
Apologies,

Systems Display (normally central lower display unit MFD), shows the position of all flight control surfaces at any time, as well as FCC status.

Were you perchance referring to full ND SS command?
Yes

My point was that at low speed you will need a lot more deflection for a given delta gee than you will at high speed.
Exactly, but can you get it when the FCC's continuously think you're at 330K?

I understand you're saying that you can. I may be misinterpreting what I'm seeing.

Always possible.

edit:
How about the THS, did it move at all?
I think I see what you're getting at, as far as elevator effectiveness, but THS 'specifically' not noted. Another 'question' to ponder over a 3 day weekend.

Last edited by OK465; 1st Sep 2012 at 19:29. Reason: HN39's question
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Old 1st Sep 2012, 19:40
  #238 (permalink)  
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OK465, I can't speak to the method used to bring about the UAS event. The FPV was available during the descent as, (I believe) one ADR was reinstated.

We just had chalk and a board for the DC8 plus some stuff from the maintenance manuals and other sources. The later CBTs were initially pretty dry but they got better. Like you I prefer understanding what I'm doing in my aircraft. I prefer nuts-and-bolts (or bits and bytes) just because it's fun.

If the simulator experience is to be believed then for whatever technical or aerodynamic reasons it demonstrated that the aircraft was recoverable with full ND stick held in until the stall warning ceased. Secondary accelerated stall was not a problem in the pull-out. "Why?" again in technical and aerodynamic terms , is an excellent question which is why contributions on this topic are such excellent reading. O.G. has previously offered varying recovery scenarios and the numbers for the higher-altitude recovery weren't that different from what we observed. We never tried recoveries below about FL300 and when I get a chance I'm going to try them.

Last edited by PJ2; 1st Sep 2012 at 19:49.
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Old 1st Sep 2012, 20:00
  #239 (permalink)  
 
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Excellent discussion.

A question if I may.

If after years and thousands of point counterpoint, an internet group of obviously intelligent/knowledgable people don't understand just how the THS/pitch control behaves in all circumstances, does anyone really expect pilots to know how the aircraft behaves after only a day of flight control systems training?

And a point........this question would never come up with a DC9
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Old 1st Sep 2012, 20:55
  #240 (permalink)  
 
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@OK465

Exactly, but can you get it when the FCC's continuously think you're at 330K?

I understand you're saying that you can. I may be misinterpreting what I'm seeing.
No, I think it is I who may have it wrong
I see what you are getting at now. I had been assuming that only the C* gains defaulted to their 330 kt values, but from what you describe it sounds as if the steady state gain (delta gee/unit SS deflection) may default as well. In that case full ND sidestick would only produce the elevator deflection to give (I suppose) -1g at 330 kts so that at 155 kts the available gee would be much less. [But in that case how did they get 30 deg NU elevator on AF447?]

If the maximum ND elevator were only 5 deg one could only get down to about 15 deg AoA until the THS came off its stop. OTOH, given the insistence of many contributors that professional pilots would be unlikely to push beyond 10 or 15 deg ND pitch, perhaps consideration of -1g is a bit academic?

Last edited by Owain Glyndwr; 1st Sep 2012 at 21:29.
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