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AF 447 Thread No. 7

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AF 447 Thread No. 7

Old 8th Dec 2011, 18:50
  #581 (permalink)  
 
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Originally Posted by Lyman
I think PF did fly attitude
time ........ attitude
(hh:mm:ss) ... (deg)
02:10:05 ....... 0
02:10:18 ...... 11
02:10:25 ...... 12
02:10:49 ....... 6
02:11:07 ...... 17.9


Which attitude?
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Old 8th Dec 2011, 19:21
  #582 (permalink)  
 
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Lyman claims that the aircraft was climbing, nose down. I have two questions:
1.Is that supported by the reported data?
2. How is that even possible?
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Old 8th Dec 2011, 19:30
  #583 (permalink)  
 
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1.Is that supported by the reported data?
No, the V/S was going through zero, decreasing
2. How is that even possible?
It is possible, if the airmass is rising fast enough.

Wings were level, rolling right, 8 deg 2.5 sec later.
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Old 8th Dec 2011, 20:34
  #584 (permalink)  
 
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Hazelnuts39 Pls. take note of the VS and the Attitude, two seconds prior to a/p loss.

Note that in two seconds the actual VS had changed from 500 fpm up to a like number down. That is remarkable, and the a/c was at what weight?

The Nose was in transit as the VS reversed, and they were not in tune.

I think the a/p was attempting to rate the UPDRAFT with elevators, and it eventually required too much ND (v/v AoA) to keep up (level). The Screen reported ND at handoff. Not Level.

Have you a reference for the deflection of Vanes due airmass? That geometry is beyond my current take. Something was causing the Vanes to read crazy high, and it was not 'cruise Nose'.
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Old 8th Dec 2011, 21:24
  #585 (permalink)  
 
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Note that in two seconds the actual VS had changed from 500 fpm up to a like number down. That is remarkable, ...
The change corresponds to a normal acceleration of 0.85 g, well within the ICAO “light” turbulence criterion of 0.5 g peak to peak. (3.5 seconds of normal acceleration of 0.85 g changes V/S downwards by 1000 fpm.)

Have you a reference for the deflection of Vanes due airmass
The vanes are shown on the same page 42 as AoA IRS1,2,3.

Last edited by HazelNuts39; 9th Dec 2011 at 08:33. Reason: Comment on V/S added
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Old 8th Dec 2011, 22:00
  #586 (permalink)  
 
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Relevant graphics from the BEA's IR#3 that I posted on page #27 of this thread are available as a "cross-hair version". Enlarged with additional scaling and timing marks, the chances of getting the actual up/down rates etc. will be improved.
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Old 8th Dec 2011, 22:22
  #587 (permalink)  
 
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Really nice one, mm43,
Unfortunately, can't save it in an off-line format, and the cross-hair pointer doesn't show any data.

Give us a clue on how to use it?

CJ
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Old 8th Dec 2011, 23:11
  #588 (permalink)  
 
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@Christiaanj;

With the enlarging of the graphic, additional scale marks have been added, along with an overlaid time scale in each section. The adding of the cross-hairs enables a closer examination of any points of interest with the x:y co-ordinates now easily readable.

To save for offline use, right click outside the graphic (pale blue background) and Save As (webpage complete). The javascript file that generates the cross-hairs will also be saved.
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Old 9th Dec 2011, 00:28
  #589 (permalink)  
 
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Hmm - if work lets up I might have a play with that JavaScript, see if I can't tart it up a little.

So - based on mm43's web graphic we have:
  • The pitch attitude *bang on zero* at disconnect
  • Importantly however, trending nose-up
  • Followed by the last A/P command bringing the pitch attitude to about +1.5 degrees NU
  • A wobble in V/S around the time of disconnect, but *well within the tolerances for "MODERATE" turbulence*
  • A very clear difference in elevator response from 02:10:08 onwards, possibly indicative of manual control
  • A/P elevator control has a max NU of 0.6 (correcting for the bump beginning at 02:10:00) and a max ND of 0.5 - compare this with a max NU of 2.1 and a max ND of 0.6 under manual handling

I still see an aircraft that was relatively stable for the turbulence they were in until the PF starts overcontrolling.
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Old 9th Dec 2011, 05:53
  #590 (permalink)  
 
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How stable is this Airbus? What would happen if the PF wouldn't touch the controls for a while until all the instruments would be functioning and a/p could be switched on again?
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Old 9th Dec 2011, 08:49
  #591 (permalink)  
 
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What would happen if the PF wouldn't touch the controls for a while ...
In the two seconds between A/P disconnect and the PF's first control input, the FCS commanded elevator nose-up which returned pitch to almost normal and normal acceleration to approx. 1 g.
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Old 9th Dec 2011, 12:31
  #592 (permalink)  
 
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@ mm43
Excellent work, i hope we see more of it, especially the times when the stall warnings happened.

Hazelnuts
In the two seconds between A/P disconnect and the PF's first control input, the FCS commanded elevator nose-up which returned pitch to almost normal and normal acceleration to approx. 1 g.
I tend to disagree.

At the very moment, when the autopiloit disconnect occurred, we have the following values:

-vertical speed zero with tendency to decrease
-normal acceleration .85 g`s (@mm43, i think your labeling is off by one, the line you labeled 0 should actually be 1 g)
-pitch attitude 0°
-AOA 2.25°

The moment the AP drops out and stick is in neutral , NZ-Law will maintain flightpath / 1 g with reference to the flightpath, the aircraft was in at AP disconnect. (Whatever position intended by the AP before does not matter any more).

That would be a descent under the above recorded parameters.

franzl
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Old 9th Dec 2011, 13:14
  #593 (permalink)  
 
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RetiredF4 but if you extrapolate the rollrate of 4 deg/sec to the right between 2:10:05 and 2 :10:07 you will only a short time descent peacefully before you dive.....
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Old 9th Dec 2011, 13:18
  #594 (permalink)  
 
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RetiredF4;

I simply read the traces. After Clandestino in post #393 (p.20) I'm not so sure about Nz law maintaining flightpath / '1g' in turbulence.
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Old 9th Dec 2011, 15:10
  #595 (permalink)  
 
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The point I am making is this: Whilst the airframe accelerated upward, the Nose Pitched Down. It is at this exact moment that the a/p quit.

The Nose is "trending" UP, the VS is trending DOWN, and that signifies...

"MANEUVERING"

For five seconds, the airframe is not stable v/v level, speed, or acceleration. Well and good to pontificate re: what PF must do, but this massive airliner is spunky as a little Robin, and that ain't right.

"I have the controls...." Whether OR NOT the PF knows the genesis of disconnect, he is well within his rights to assume the a/p has left because of control demands, not ICE. At this point, he does not know, and after two plus years on, neither do we.

I sense HN has some misgivings about a/p in the stink. So did Smilin Ed, and so do I. The A/P has quit, the a/c is wanting management, and the Bus has reverted to "Other than NORMAL LAW". Free to STALL, you're on your own. The BUS has left the building.

Make NO mistake, what the pilot does now is the precursor, and (I think) the procuring cause of manually induced UPSET and LOC. Is there a 'sit on your hands' command for the AUTOPILOT? Thought not.

Wanting to establish a 'normal' flight path is a natural for any pilot flying an airliner. Perhaps there is a flaw in the training concerning this situation.
NOT "I have the Controls", but instead "monitor the Stick, No Touchee".

Wait, what good is there in 'monitoring' something that does not move?
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Old 9th Dec 2011, 15:20
  #596 (permalink)  
 
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Hazelnuts
RetiredF4;
I simply read the traces. After Clandestino in post #393 (p.20) I'm not so sure about Nz law maintaining flightpath / '1g' in turbulence.
I do the same, but then lets see what Clandestino wrote:

............Sidestick neutral iz not 1G it is 0G.
There is one word missing at the end, it is 0 g change. As the basic and overall topic of Nz-Law is to maintain a flightpath ( = flightpath stable = 1 g flight ) the sidestick in neutral commands no deviation from this 1 g flight, which means no loadfactor change at high speed or no pitch rate change in low speed.

Sidestick command does not order G in absolute terms. It adds G demand to already measured, therefore if hit by updraft giving you 1.3G, pull on the stick that would give you 1.1 absolute from straight and level will now result in 1.4 pitch up. Push giving 0.9 would now be 1.2.
He is wrong there. What he might be is referring to s the fact, that a dampening input is added to the normal LAw command.

LTTM- TURBULENCE DAMPING FUNCTION (bolding by me)

General:
The purpose of the Turbulence Damping Function implemented in the Electrical
Flight Control System is to damp the structural modes induced by atmospheric
turbulence.

Architecture
The Turbulence Damping Function consists of two lanes:

Longitudinal lane. The longitudinal Turbulence Damping command is computed by the FCPC1 (FCPC2 as a redundancy) as a function of the Nz accelerometer information. It is added to the normal law command and transmitted to the associated elevator servo-controls.

Rear lateral lane
The rear lateral Turbulence Damping command is computed by the FCPC1
(FCPC3 as a redundancy) as a function of the informationof a specific Ny
accelerometer located at the rear bulkhead level. It is added to the normal law command and transmitted to the associated yaw damper.
To sum it up, if the SS is in neutral (= 1 g flight) and turbulence is disturbing the flightpath away from this one g flight, an opposing command is added to the existing 1 g flight command or like Clandestino likes to put it to the 0 g- change command).

If not hands off, but with a SS command of 1.2 g an updraft and thus increase of commanded g change by .4 g (which would give us 1.6 g) would stimulate the system to counter this increase by adding up to the commanded 1.2 g, thus maintaining the ordered 1.2 g of the SS as close as possible.

Any deviation from the flightpath (1 g with hands off, ordered g change with SS or AP inputs) will be opposed by the dampers by an respective counter action command within the capabilities of the system.

Let´s face the fact, anything else would be not helpful at all.

That the SS command adds to the already existing g, thus the SS command not giving a specific amount of flightpath change (load factor change at high speed, pitch rate change at low speed) is against any basic principle the C* law is funded on. But anybody feel free to point me to some reference here. Imho such an aircraft would not be controllable.

In the discussion there were also comments concerning load factor demand in high speed and pitch rate in low speed. Some here seem to believe, that in low speed the aircraft keeps the last pitch, if the stick is in neutral.
The aircraft will maintain a given flightpath, if we intend to change that flightpath with a SS input the gain will be load factor change at high speed and pitch rate change in low speed, but it still will maintain flightpath and not pitch in low speed with SS neutral (assumed that no protections are involved).

Again feel free to correct my research here.

When discussing the crew actions, their motivation of those actions and the resulting aircraft behaviour, which again constitutes an feedback channel to the crew, it is of immense importance to know how the systems work.

franzl

Last edited by RetiredF4; 9th Dec 2011 at 15:53.
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Old 9th Dec 2011, 16:02
  #597 (permalink)  
 
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Some basics

A319/320 description Nz Law

Manoeuvre demand law as basic flight mode
- neutral speed stability with full flight envelope protection
-Vertical load factor control proportional to stick deflection : C* law
- independent of speed, weight, center of gravity ;

stick displacement : Nz = n Nz = n + 1g
stick neutral : Nz = O Nz = 1g


C* = q + K_C* x delta_Nz
q = pitch rate
delta_Nz = incremental load factor
K_C* = C* crossover gain - chosen to balance q and delta_Nz contributions

Flight path stability instead of speed stability
- control inputs are made to alter the flight path, not to hold it.

Medium-term flight path stability :
- maintenance of parallel trajectory 1g in pitch even after atmosphere disturbance.


C* (pronounced "C Star") is the popular name for a control law in which Nz (g) and pitch-rate feedback are blended. (In the late 60s and early 70s, Nz feedback was called the C law. NASA space shuttle approach studies added pitch-rate feedback, which was called C*.) At low speed in a C* airplane, pitch rate is primary; at higher speeds, g is primary. The changeover is transparent and occurs at about 210 knots in the A320 ("Fly-By-Wire for Commercial Aircraft: The Airbus Experience," C. Favre, 1991).


4.3.4. Longitudinal static stability. The A320's C* pitch control law is a manoeuvre demand law: the pilot's control inputs are interpreted as a demand for a given level of manoeuvre rate and the control system provides the surface deflection needed to generate this rate. Releasing the side-stick commands flight path stability.

. C* (pronounced ‘‘C-Star”) is a term that is used to describe the blending of the airplane pitch rate and the load factor (the amount of acceleration felt by an occupant of the airplane during a maneuver). At low airspeeds, the pitch rate is the controlling factor. That is, a specific push or pull of the column by the pilot will result in some given pitch rate of the airplane. The harder the pilot pushes or pulls on the column, the faster the airplane will pitch nose up or nose down. At high airspeeds, the load factor dominates. This means that, at high airspeeds, a specific push or pull of the column by the pilot will result in some given load factor.


And who likes to dig deeper and compare it to the 777, can read here.

Going back to my armchair again

franzl
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Old 9th Dec 2011, 17:02
  #598 (permalink)  
 
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Originally posted by RetiredF4 ...

.... i think your labeling is off by one, the line you labeled 0 should actually be 1 g
Thanks for pointing that out. The graphic label now reads 1.0 (1,0)
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Old 9th Dec 2011, 17:08
  #599 (permalink)  
 
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Evidence. Three seconds prior to a/p disconnect, the airframe was subject to a robust Updraft (from DFDR). The FPC ordered Nose Down, to arrest the ensuing ascent (climb). This is of record.

After this event, the a/p quit, and PF took control.

Still looking for direct evidence of ICE, not unsubstantiated opinion.
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Old 9th Dec 2011, 17:30
  #600 (permalink)  
 
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Some here seem to believe, that in low speed the aircraft keeps the last pitch, if the stick is in neutral.
SS neutral at low speed = zero pitch rate demand

What you see at lower speeds (ALT2) with speed bleeding off and with a 'hands off' neutral SS is essentially pitch attitude maintenance while the FPV (if selected) gradually drifts lower (not FP maintenance) as AOA increases.

(No turbulence damping in ALT2.)

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