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AF447 Thread No. 3

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AF447 Thread No. 3

Old 2nd Jun 2011, 21:01
  #1141 (permalink)  
 
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The autotrim in Alt2 will only set the THS to "chasing" inputs if that input is held past the limits of elevator authority, otherwise it simply stays in the last position set when the FMC/autopilot kicked out. Letting the stick go in that situation will result in what you describe - i.e. the aircraft trimming itself back to it's stable position.

The only reason the autotrim moves to match pilot input is to give maximum control authority to a pilot in a significant upset without having to have his or her hands on both the sidestick and the trim-wheel. It follows that to have autotrim responding to sidestick inputs requires an emphatic stick movement held for some time.
Ok, I get it, he told the airplane to climb and it did. In this particular case Direct law would have been better than Alternate law since it wouldn't have trimmed him nose up, but if he hadn't held aft stick to the moon he wouldn't have been changing the trim either...

Somebody once said "it's hard to make anything foolproof because fools are so ingenious"......
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Old 2nd Jun 2011, 21:02
  #1142 (permalink)  
 
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@rudderrudderrat

I didn't say it was. You need to check the difference between Alt 1 and Alt 2. In this case they had Alt 2 + airspeed instrument failure, which pretty much nullifies stall protection. In any Alternate Law mode, the protections can be overridden if the stick input is firm enough and held for long enough. In short - in anything other than Normal Law, do not hold the stick back expecting the protections to have your back.

@engine-eer

Hi - cheers, but that's not what I'm saying. I don't know what he did (outside of the points in time mentioned in the BEA "note"), and neither does anyone else on this thread that I know of. I'm deliberately keeping any speculation to myself as much as I am able, because then I'd be no more useful to the thread than people who are saying that it must have been the pilots, the computer or even the weather. All I am saying is that the systems behave in such-and-such a manner if they are operating properly (i.e. as specified and designed).
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Old 2nd Jun 2011, 21:08
  #1143 (permalink)  
 
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I am a bit puzzled about the concern about AoA indication. I presume maintaining what you guys call 'pitch and power' would probably have saved the day. If the climb was intentional then that's a step in the dark and the inability to adjust thrust to keep air speed up was the issue (not easy without airspeed reports). If the climb wasn't intentional but went unnoticed, seems unlikely that additional information on AoA would have been recognised. In the stall, if it wasn't recognised as a stal,l then what chance an AoA readout would have been noticed/acted upon. If it was recognised as a stall the -40 degree or so readout would have been irrelevant.

On another point, if the systems had some remaining degree of automation, given a pilot input of nose down, would it be so difficult for the system to recognise the intent, recognise the THS setting, and help him out by unfixing and adjusting it ? Seems a bit perverse to freeze the setting and not revisit it in the face of sustained pilot command.
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Old 2nd Jun 2011, 21:18
  #1144 (permalink)  
 
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The total velocity unfortunately mostly downward was enough to fly and create lift, if they only where able to get the nose down in the flight pass direction.
Either the information provided by the BEA is inaccurate, or I have forgotten my trig, or there is a persistent misunderstanding on this thread about the attitude and flight path. The BEA report says pitch is 16 degrees nose up, 35-40 degrees angle of attack, and 10,000 fpm down. To get the flight path angle, you subtract the angle of attack from the positive pitch angle, giving around -20 to -25 degrees. The true airspeed is 10,000 fpm, or around 100 knots, divided by the sine of the angle, which gives around 250 knots. The airplane was in a stalled, mushing descent, not a vertical plunge or falling-leaf type of maneuver. At least this is what the BEA numbers require.

It appears to me -- with zero experience of flying this type of aircraft -- that recovery would not have required a great deal of height, once the wing was unstalled by trimming the THS and getting the pitch angle into the vicinity of the flight path angle, ie 20 degrees or so below the horizon.
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Old 2nd Jun 2011, 21:25
  #1145 (permalink)  
 
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jcgeant, see:
http://www.pprune.org/rumours-news/3...yday-call.html

see particularly mm43's posts in that thread
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Old 2nd Jun 2011, 21:33
  #1146 (permalink)  
 
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If the climb wasn't intentional but went unnoticed, seems unlikely that additional information on AoA would have been recognised. In the stall, if it wasn't recognised as a stal,l then what chance an AoA readout would have been noticed/acted upon. If it was recognised as a stall the -40 degree or so readout would have been irrelevant.
Mr Optimistic, I respectfully disagree. Once you've used, or gotten used to, AoA as a gage, or been trained how to use it, it becomes another instrument you can go to for information ... like your engine instruments. Since the information it holds, stall or not stall, is somewhat important to flying ... "AoA high, lower the nose" isn't too hard of an association for a pilot to make. Hearing a stall warning tone is not the same as seeing your AoA. (For that matter, rudder shakers or stick shakers are another place where a sense other than hearing is used to cue a pilot that he is stalled or nearly stalled ... )
Originally Posted by Garrison
Either the information provided by the BEA is inaccurate or there is a persistent misunderstanding on this thread about the attitude and flight path.

The BEA report says pitch is 16 degrees nose up, 35-40 degrees angle of attack, and 10,000 fpm down.

To get the flight path angle, you subtract the angle of attack from the positive pitch angle, giving around -20 to -25 degrees.

This is the flight path angle, and it is far from vertical.

The true airspeed is 10,000 fpm, or around 100 knots, divided by the sine of the angle, which gives around 250 knots.

The airplane was in a stalled, mushing descent, not a vertical plunge or falling-leaf type of maneuver. At least this is what the BEA numbers require.
Presuming AoA is measured in degrees, not units, I follow what you are saying. The velocity seems to have been enough to return to flying if the nose could have been aligned with the flight path angle (presuming enough elevator/THS authority is available to get the nose pitching down).

Is that part of the point you are making?
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Old 2nd Jun 2011, 21:35
  #1147 (permalink)  
 
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Yup. (+ six additional characters)
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Old 2nd Jun 2011, 21:36
  #1148 (permalink)  
 
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No THS Position Indicator??

Indication of the Angle of the THS: I doubt it would have helped.
Do I understand correctly that there is no THS position indicator?

If not, we have a situation where the "system" trimmed the aircraft into a stall, something we learned early on not to do, and yet the pilots had no indication of that.
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Old 2nd Jun 2011, 21:40
  #1149 (permalink)  
 
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Sir, it seems that there is a THS indicator (visual), if one looks down at the trim wheels next to the throttle quadrants. My diagram (old) does not have enough fidelity to tell me how the wheels are marked.

http://avionique.free.fr/IMG/jpg/Cockpit_A330.jpg

That picture (if it is a faithful rendering of an A330 cockpit) shows me what looks to be an index, and large white blob (??) and what may be a scale in green.

Also recall in numerous threads comments about AB pilots needing to set the trim wheel to a particular value for takeoff (probably varies with gross weight??) So they'd need to be able to read the trim wheel's position to confirm their nose trim is at the value they calculated and require for takeoff.

Last edited by Lonewolf_50; 2nd Jun 2011 at 21:52.
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Old 2nd Jun 2011, 21:41
  #1150 (permalink)  
 
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lonewolf: thanks. I am not a pilot so I am in no position to argue ! But if pitch, power and altitude rate were apparent and not taken due accord of.....?

Last edited by Mr Optimistic; 2nd Jun 2011 at 21:42. Reason: possible misinterpretation
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Old 2nd Jun 2011, 21:43
  #1151 (permalink)  
 
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Stall warning

The maximum angle of attack (i.e. where the plane stalls) must depend on a number of factors - pressure altitude, airspeed, load factor, aircraft weight, config. Lots of points of failure for a measurement system.

I'm not sure if actually displaying a raw AoA it to the pilot would be so useful - otherwise every C152 would have such an instrument. How would the pilot know what is the current alpha-max?

The Airbus FCOM states that in case of a simultaneous OVERSPEED and STALL WARNING the stall is to be actioned - failsafe: if a pitot fault gives us an excessive speed the alpha-max will decrease, so if we follow the stall warning we basically end up flying a lower AoA then would otherwise be possible. But the 'under-speed' scenario does not seem to be addressed too well.

Now the types I am more familiar with - the C152 and DA20 - have a stall warning system which is basically an orifice on the wing connected by means of a tube to a little buzzer - it detects when the air pressure on the wing suggests an oncoming stall. This system is far superior to what the A330 has in one respect - it is completely independent of the airspeed system.

Now - why can't the Airbus/Boeing engineers devise such a system for the big jets? I do not suggests orifices, tubes and buzzers - rather - pressure transducers (or perhaps strain gauges?), a data transmission network and a clever computer to work out what is actually happening with the wing. A system which tells us if the wing is generating lift or not, how much lift, which part of the wing is stalled.
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Old 2nd Jun 2011, 21:52
  #1152 (permalink)  
 
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A330 Manual Pitch Trim

The manual pitch trim is mechanically connected to the THS. That THS is pretty big.

Just out of curiosity - approx. how many revolutions of the manual pitch trim would it take to bring the THS from the max nose-up angle (12 degrees) to an angle where elevator authority is sufficient to command pitch down (8 degrees I recall). What about 0 degrees?

What force is required to move the pitch trim wheel? It says: "Mechanical connection", but is there some kind of power assist?

If the THS jammed, would the wheel also jam? Can icing cause the THS to jam?

What is the relation between manual trim and autotrim? If I manually trim does the autotrim disengage? When does it re-engage? What if autotrim is commanding nose up and I trim nose down - who has the last word?
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Old 2nd Jun 2011, 21:53
  #1153 (permalink)  
 
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Originally Posted by bearfoil
I think Flight Law at the time of Stall/descent was not auto trim, and would have required manual input on the two wheels bracketing the Throttle pedestal?

(See MartinM's great pic)
At the point of stall Flight law Auto- Trim was active for all that we know.
The only point where it might have reverted to manual was when Alpha = 30 was exceeded at around 2:11:35.
If the BEA 'leak' is correct Auto Trim was even active throughout the whole descent.
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Old 2nd Jun 2011, 21:55
  #1154 (permalink)  
 
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Golf Sierra

The Navy aircraft I referred to earlier get around that with units ... not sure if that is applicable for transport aircraft.

Good points, also the understanding that AoA and stall AoA are not fixed values, but vary as you point out.

I am not sure I understand this.
The Airbus FCOM states that in case of a simultaneous OVERSPEED and STALL WARNING the stall is to be actioned - failsafe: if a pitot fault gives us an excessive speed the alpha-max will decrease, so if we follow the stall warning we basically end up flying a lower AoA then would otherwise be possible. But the 'under-speed' scenario does not seem to be addressed too well.
By "otherwise possible" do you mean "allowed by the protections" or physically possible? Is alpha max a computed value, or is it the raw AoA measured by the AoA probe?

My point is, if you have a false airspeed indication, the AoA probe/detector doesn't give a rat's behind. It is measuring something other than dynamic pressure, and if I may say, it is measuring something simpler: angle of airflow relative to the aircraft. (Or should I say airfoil?) Of course, if the AoA probe gets stuck/frozen rendered moot someway, then ... well, that's another dog's breakfast served to the flight deck crew. (Gee, it goes back to pitch and power again. )

EDIT: yes, there are two different hydraulic boost systems that help the mechanical linkage move the THS.

This picture purports to show A330 AoA probes, which look a lot like AoA probes I've seen on other aircraft. They do not measure what the pitot probes measure. What appears to happen inside the computer in the A330 is that airspeed input is compared to AoA probe reading is compared to other data and, once the calculations are done at the speed of electricity, visual cues are presented on the strips (vertical) next to the pilots attitude indicator on his flying display.

(VLS/V alpah prot V alpha Max)

Last edited by Lonewolf_50; 2nd Jun 2011 at 22:18.
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Old 2nd Jun 2011, 22:04
  #1155 (permalink)  
 
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Golf-Sierra -- you may find this a good place to start:

http://www.smartcockpit.com/data/pdf...t_Controls.pdf
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Old 2nd Jun 2011, 22:10
  #1156 (permalink)  
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CogSim
I don't understand how you reach the idea of a stall warning based on an invalid (or even a valid) airspeed reading, since stall warnings are (should be?) based upon AoA
Stall is due to too an AoA which is outside the boundaries, but stall warnings are in addition due to your AoA being in danger of going outside the boundaries.
If your horizontal velocity is very low you have little lift (even if not stalled) and your downward component of velocity will increase so much that the AoA will go into the stall region.

Last edited by t54; 2nd Jun 2011 at 22:11. Reason: Make it more precise
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Old 2nd Jun 2011, 22:16
  #1157 (permalink)  
 
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Loanwolf your #1149

There was a reference [url] from JT to an earlier thread post from PJ2 detailing the 330 trim wheels on the pedestal the integrated read out of THS trim angle along with the follow up information available on the centre display, can't remember the actual display name 100% so will not say.
all the best
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Old 2nd Jun 2011, 22:23
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t54, that was me, not cog sim, on that AoA tidbit.

FWIW, I see no reason to divorce stall warning from your AoA, but if you are working in raw angles versus units, I can see the issue becoming more complicated as the critical AoA is related to so many other factors. Thanks.

One way or another, be it at or beyond stall value, or "close to stall value, do something mate!" a suitable warning ought to be made.

If your horizontal velocity is very low you have little lift (even if not stalled) and your downward component of velocity will increase so much that the AoA will go into the stall region.
Uh, not quite, I am not sure I understand you there. You can go slow (low horizontal velocity) and level if you are not stalled, and go merrily without falling, or stalling.

That said, once you begin to descend, the second part of your sentence begins to make sense.

If what you are saying is that you are already falling, and have a low horizontal vector, then sure, the vector sums (depending upon rate of descent) change your AoA, and not necessarily for the better.

What has this to do with AoA indication or warning? AoA probe will measure how the air is flowing. (Direction)

Last edited by Lonewolf_50; 2nd Jun 2011 at 22:34.
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Old 2nd Jun 2011, 22:26
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Originally Posted by t54
Stall is due to too an AoA which is outside the boundaries, but stall warnings are in addition due to your AoA being in danger of going outside the boundaries.
If your horizontal velocity is very low you have little lift (even if not stalled) and your downward component of velocity will increase so much that the AoA will go into the stall region.
True.

I think we all have to be very careful to differentiate between the terms "Stall Warning", "Stall Protection" and "Stall".

The first can work to an extent without airspeed data but the second absolutely cannot. So if you get into the first or third in Alternate Law with no speed indication you are going to have to think very hard and work very methodically as quickly as you can.
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Old 2nd Jun 2011, 22:33
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Mr Optimistic:
I am a bit puzzled about the concern about AoA indication.
While true that pitch and power would likely have worked something else -did- go wrong and the pilots ended up in a confusing situation.

At that poin a raw AoA indicator would have likely helped them to sort out
the counterintuitive return of stall warning as they were applying nose down.

In fact the stall warning in this case was a tragically misleading on/off AoA indicator that had 3 states:

Normal : silent
close to stall or stalled : STALL STALL STALL
severe stall : silent

Severe stall results in <60kt sensed speed and inhibition of warning.

This is not related to pitot ice/failure, it is a combination of true speed plus AoA effects on pitot as others have pointed out.

Had the raw data been avaiable they likely would have seen the AoA improving as the warning returned and pressed on with the correct recovery.
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