Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

AF 447 Thread No. 5

Tech Log The very best in practical technical discussion on the web

AF 447 Thread No. 5

Old 9th Aug 2011, 00:10
  #1781 (permalink)  
 
Join Date: Jun 2009
Location: somewhere
Posts: 451
@mm43 & HN39:

Good observation!

Thx for the effort to make this clear.

@Diagnostic & sensor_validation:

so AOA#1 could be effected, however didn't had any influence.

FCPC using median AOA (or outvoted AOA#1), AoAsw using highest AOA value.
A33Zab is offline  
Old 9th Aug 2011, 00:33
  #1782 (permalink)  
 
Join Date: Aug 2011
Location: Near LHR
Age: 54
Posts: 37
Thales AoA sensor EASA ED

@sensor_validation (re post#6628318)

Very interesting AD, thanks for the link! I haven't seen any mention of the specific fitted AoA sensors, but that AD would certainly explain AoA "stickiness" wouldn't it.

It's a relatively recent AD (originally issued Jan 2010), but they are not limiting the list of potentially affected Thales p/n with any specific manufacturing date range - so perhaps that AD could apply to the AoA #1 sensor on AF447, if those Thales AoA sensors were fitted.

(IMHO it also goes to show that any AoA display should display the measured values for all 3 sensors, to allow the crew to take a decision abut which are most believable, in case of discrepancies - it must not display a single computed mean / median, as that calculation could be wrong, as the AD points out.)

@A33Zab - Many thanks for confirming that this erroneous AoA #1 value wouldn't have been used by the FCPC. We seem to be lucky that only 1 (and not 2) AoA sensors were producing erroneous values here - otherwise there's obviously a potential risk of another Perpignan-type situation, where the one "correct" AoA value gets voted out
Diagnostic is offline  
Old 9th Aug 2011, 04:30
  #1783 (permalink)  
 
Join Date: Jan 2008
Location: Atlanta, GA, USA
Posts: 349
Case closed

Smilin_Ed said
Trimming is not difficult and, for a pilot worthy of being called a pilot, it is a natural activity which is so routine that it is done almost unconsciously. I have flown aircraft with the pitch trim on the yoke, on the stick, and on the console. I never found it a burden to activate any of them. I prefer it on the stick or yoke since that does free up the other hand, but the difference is really not material. What I don't want is having the system changing the trim when I want to remain trimmed for a particular speed or AoA. In the case of AF447, the system gave up and turned it over to the people seated in the cockpit, except they didn't turn over everything and I believe that situation makes it more difficult to hand-fly. Autotrim has its place, but not in my cockpit when I am trying to hand-fly.


This is just conclusive. There is nothing else to argue about. I don't know why the apologists for automation cannot simply admit that they are wrong, or at least to admit defeat and claim that they are right anyway, as a face-saving maneuver.
deSitter is offline  
Old 9th Aug 2011, 05:11
  #1784 (permalink)  
 
Join Date: May 2011
Location: Australia
Age: 59
Posts: 200
great diagram mm43.

The thing that really gets to me about the graphic is the intensity of the initial zoom climb and the associated g forces....you cant really call that an appropriate hand flying response to autopilot drop out and speed indication failure by any conception..

I just wonder if the poor sod simply freaked out at all the alarms...tragic if the warnings of irrelevant failures ( irrelevant that is to the important task of of maintaining constant pitch and attitude) ended precipitating a bigger failure.

I suppose you could have some nice relaxing ladies voice saying " pitot failure...then a softly spoken expletive....its your aircraft now..we cant help you anymore...maintain constant attitude and thrust...please check the auto trim etc etc..."
to keep the poor man calm (if Any bright spark wants to automate the emergency response )
Mimpe is offline  
Old 9th Aug 2011, 06:47
  #1785 (permalink)  
 
Join Date: Oct 2009
Location: UK
Posts: 1,271
Hi mm3,

Thanks for the graphic. I'm confused why the stab trim does not move significantly (up to your yellow band) - whilst the CAS has dropped from 275 to about 225 kts. I would have thought the auto stab trim would have followed the real reduction in airspeed. I realise it appears that PF seems to have pulled 1.4g then reduced to 0.5g during this period.

Does the delta g prevent the movement of the stab trim during those manoeuvres?
rudderrudderrat is offline  
Old 9th Aug 2011, 07:42
  #1786 (permalink)  
 
Join Date: Oct 2007
Location: Here
Posts: 771
Airbus pitch trim

I get the idea that many real pilots here (I am not one) do not understand the Airbus pitch trim. As far as I can see it's not in any sense Trim as we know it Jim.

As I understand it:-

With autopilot off and hands-off the stick in Normal and Alternate Laws the aircraft maintains constant g (hence absent other inputs - pitch attitude). As I understand it the crew of AF447 (and indeed any AB not in direct law) could have wound the pitch trim wheel as much as they liked and as long as the elevator did not run out of travel no aircraft pitch changes would have occured. The computer would have matched the manual THS movements with elevator movements to exactly compensate so that aircraft g (pitch) was maintained.

The Airbus auto-trim merely seeks to keep an adequate range of elevator authority available (zero degrees of elevator deflection?) - however with a long time constant (delay if you like) so that it does not try to follow every little twitch of the stick/elevator. In Normal and Alternate Laws it has *no* influence on aircraft pitch while there remains elevator movement in the required direction.

So:- the THS and auto trim had NO BEARING AT ALL on this incident. Nil, null, zip, nada.

I will allow that perhaps when in the deep stall at extreme AoA (60 deg was it?), certain THS/elevator combinations may have been advantageous but that is strictly in Chuck Yeager territory and in any case no serious attempt was made to get the nose down so artful trim twiddling was never going to be considered by this or probably any other crew.

Of course all is not sunshine and roses. In direct law the pilot must ensure that the THS is positioned manually so as to allow adequate pitch authority. The exact same issue however occurs on Boeings too and Boeings too have crashed as a result. Airbus knockers can just therefore put their toys back in the pram forthwith:-) The issue of extreme THS "trim" and subsequent lack of needed pitch authority in rare circumstances appears to me to be an industry wide problem. It can occur in Boeings on autopilot disconnection and in AB on reversion to Direct Law. The burden of being in an unusual attitude and needing to use manual trim to get adequate pitch authority to recover is surely pushing at the boundaries of The Average Airline Pilot?
jimjim1 is offline  
Old 9th Aug 2011, 08:29
  #1787 (permalink)  
 
Join Date: Oct 2009
Location: UK
Posts: 1,271
Thanks jimjim1 for that lucid explanation.

So even in ALT2 Law, it would have been useless to hold the stab trim stationary by hand - since the computers would simply displace the elevators to a new "null position" and "trim the new attitude". Longitudinal speed stability is still not returned, hence no feed back of feeling slow and hence heavy in pitch

@DozzyWannabe.
Twitchy my foot. Control at altitude is about very small, corrective movements until you get a feel for how the aircraft is responding (even this "know-all", "office-chair" non-pilot knows that much - and no, that is not directed at you... ) - he was mashing the stick halfway to the stops from the get-go, so much so that the PNF repeatedly chastised him for it.
I wish I was as good as you. You make it sound so easy. Please see:
http://www.pprune.org/tech-log/376433-af447-132.html post 2632.
I have flown an A330 in ALT2 due to a twin ADR disagree situation (one blocked pitot + a different failed ADR) and I was quite surprised at how rapidly the aircraft banked versus the normal roll rate. It also exibited a strong natural tendency to return to wings level with the stick neutral. It is very easy to overbank in ALT2 law.
rudderrudderrat is offline  
Old 9th Aug 2011, 08:34
  #1788 (permalink)  
 
Join Date: Aug 2005
Location: London
Posts: 78
Posted by jimjim1
So:- the THS and auto trim had NO BEARING AT ALL on this incident. Nil, null, zip, nada.
Exactly, at last someone has noticed the absolute irrelevence of much that has been posted about this accident, particularly concerns about autotrim and alleged skittishness of 'manual' control at high altitude. All these would be appropriate to a conventional aircraft controlled in a conventional way, but the Airbuses after the A320 are NOT conventional in their control systems, and as jimjim says the sidestick commands g. And it is a confessed non-pilot who has to point this out. One minor quibble
maintains constant g (hence absent other inputs - pitch attitude)
is only true if the AT is maintaining speed correctly, if the thrust is insufficient for the drag, the speed will reduce and the controls will maintain g by increasing pitch attitude until... stall, which is what happened here. You can argue whether this is a good way to control the aircraft, but the first duty of anyone flying one is to understand this crucial point. Clearly the AF447 crew, and many others posting here, don't.
gonebutnotforgotten is offline  
Old 9th Aug 2011, 08:47
  #1789 (permalink)  
 
Join Date: Jun 2009
Location: Paris
Posts: 691
Hi deSitter,
Originally Posted by deSitter
I don't know why the apologists for automation cannot simply admit that they are wrong, or at least to admit defeat and claim that they are right anyway, as a face-saving maneuver
A very long time ago, this case was closed for any people able to understand how autotrim works and that manual trim is available, at any time, if one doesn't like what autotrim is doing. One, which is able to understand it, obviously doesn't need to save his face either.

AF447 case is all about pilot's imputs which are not coherent with the situation, not about an automatism which stupidly followed orders given; it is like asking that the elevators should not follow nose up orders when stall warnings were sounding. In fact, it is a case calling for more automation as some would have liked that this aircraft could be able to determine all by itself that pilot's orders were stupid and should not be followed by any aircraft stupid control surfaces.
takata is offline  
Old 9th Aug 2011, 09:02
  #1790 (permalink)  
 
Join Date: Jun 2009
Location: Paris
Posts: 691
Hi rudderrudderrat,
Originally Posted by rudderrudderrat
So even in ALT2 Law, it would have been useless to hold the stab trim stationary by hand - since the computers would simply displace the elevators to a new "null position" and "trim the new attitude". Longitudinal speed stability is still not returned, hence no feed back of feeling slow and hence heavy in pitch
Sorry but it doesn't make sense. How would the computer "simply displace the elevators to a new "null position" and "trim the new attitude" without changing the THS setting? When trim is activated manually, autotrim is frozen, can't displace anything anymore. If one is trimming manually, he's overiding autotrimming and computers would resynchronise with manual imputs after this point.
takata is offline  
Old 9th Aug 2011, 09:15
  #1791 (permalink)  
 
Join Date: Oct 2009
Location: UK
Posts: 1,271
Hi takata,
When trim is activated manually, autotrim is frozen, can't displace anything anymore.
I can't find any reference to that - it would appear to be in error.

How would the computer "simply displace the elevators to a new "null position" and "trim the new attitude" without changing the THS setting?
In the same way the system copes if the stab is "frozen" in a position with the loss of the relevant hydraulic systems.
Similarly I'm saying that if the stab trim is held stationary, it makes little difference to Normal or ALT LAW in pitch, because the elevators will be repositioned to compensate - until they run out of authority.

edit.
In fact, it is a case calling for more automation as some would have liked that this aircraft could be able to determine all by itself that pilot's orders were stupid and should not be followed by any aircraft stupid control surfaces.
The design of Pitch stable Law without Alpha Max protection, on an aircraft which is inherently naturally speed stable (by design concept), during UAS is not logical. Why would you want to do that?

Last edited by rudderrudderrat; 9th Aug 2011 at 09:29.
rudderrudderrat is offline  
Old 9th Aug 2011, 10:09
  #1792 (permalink)  
 
Join Date: Jun 2009
Location: Paris
Posts: 691
Hi jimjim1,
Originally Posted by jimjim1
The computer would have matched the manual THS movements with elevator movements to exactly compensate so that aircraft g (pitch) was maintained.
It is not correct. THS do not replace elevators control and elevators are not used to replace THS trim.
(In flight phase) The C* law is activated. It combines the elevator control and the THS controls (AUTOTRIM function). It generates a load factor demand as a function of the position of the side sticks and inertial feedbacks. The gains depend on the speed (Vc) and center of gravity.
Consequently, any large change of pitch is not compensated by THS change and longitudinal stability is maintained. Moreover, in alternate law, C* law gain is reduced, hence 1g is not maintained "at all cost", and certainly not when the sidestick is moved back and forth.

Originally Posted by rudderrudderrat
I can't find any reference to that - it would appear to be in error.
See yourself:

THS control
The elevator orders are progressively transferred to the THS through a low-speed integrator to decrease the drag. This is the AUTOTRIM function. The THS movement is inhibited:
- under 50 ft in manual mode (100 ft in AP mode),
- when the high-speed and Mach protection is active,
- in case of manual action on the hand wheel,
- when the load factor is lower than 0.5 g,
- in case of abnormal condition law.
The THS movement is limited in up direction:
- when the alpha protection is active,
- when the load factor is higher than 1.25g,
- when the bank angle is above 33 deg,
- in case of low speed protection (alternate law).
takata is offline  
Old 9th Aug 2011, 11:09
  #1793 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 81
Posts: 1,689
Angle of attack data processing algorithms

Originally Posted by A33Zab
FCPC using median AOA (or outvoted AOA#1), AoAsw using highest AOA value.
According to the ATSB interim accident report on QF72, the FCPC treats AoA differently from the other parameters (which was an important factor in the QF72 occurrence):
Angle of attack data processing algorithms
There was a potential for the AOA sensors on the right side of the aircraft (AOA 2
and AOA 3) to provide different values to the AOA sensor on the left side of the
aircraft (AOA 1) in some situations due to aircraft sideslip.28 In order to minimise
the potential effect of this difference, the PRIMs used different processes for AOA
compared with other parameters when determining the value to use for calculating
flight control commands. More specifically, the processing of AOA data involved
the following:
• As with the other parameters, the PRIMs would continuously monitor the AOA
values from the three ADIRUs. AOA data was sampled about 20 times per
second.
• To confirm the validity of the AOA data, the PRIMs would compare the median
value from all three ADIRUs with the value from each ADIRU. If the difference
was greater than a set value for more than 1 second continuously, then the PRIM

(Note 28 - Sideslip: a condition in which the oncoming airflow is at a sideways angle to the aircraft’s
centreline.)

would flag the ADR part of the associated ADIRU as faulty and ignore its data
for the remainder of the flight.
• To calculate a value of AOA to use for calculating flight control commands, the
PRIMs would use the average value of AOA 1 and AOA 2. In other words,
(AOA 1 + AOA 2)/2. This value was passed through a rate limiter to prevent
rapid changes in the value of the data due to short-duration anomalies (for
example, as a result of turbulence).
• If the difference between AOA 1 (or AOA 2) and the median value from all
three ADIRUs was higher than a set value, the PRIMs memorised the last valid
average value and used that value for a period of 1.2 seconds. After 1.2 seconds,
the current average value would be used.
In summary, in contrast to other parameters, only two values of AOA were used by
the PRIMs when determining flight control commands. However, several risk
controls were in place to minimise the potential for data inaccuracies to affect the
flight control system.
HazelNuts39 is offline  
Old 9th Aug 2011, 11:47
  #1794 (permalink)  
 
Join Date: Oct 2009
Location: UK
Posts: 1,271
Hi takata,
When trim is activated manually, autotrim is frozen, can't displace anything anymore.
Did you mean for the rest of the flight?

I interpreted the line "in case of manual action on the hand wheel," meaning only whilst the hand holds the trim wheel.

elevators are not used to replace THS trim
How does the system cope with stab frozen then, either due to relevant double hydraulic loss, or if the stab trim wheel is held stationary by hand?
rudderrudderrat is offline  
Old 9th Aug 2011, 12:35
  #1795 (permalink)  
 
Join Date: Jun 2009
Location: VA, USA
Age: 54
Posts: 561
Did you mean for the rest of the flight?

I interpreted the line "in case of manual action on the hand wheel," meaning only whilst the hand holds the trim wheel.
Manual trim overrides autotrim (micro-switches disconnect the auto-drive if I remember correctly) for the duration of manual input... hold the trim wheel or move it and that prevents autotrim - the computers monitor the trim state during manual trim and on manual 'release', the computer reacquires control from where the manual inputs left off.

So let's assume PF realizes he has inadvertently driven the trim to +13NU due to his excessive NU elevator demands - all he has to do is twiddle the trim wheel ND to something he likes (let's say +3NU) and let go. The trim will remain at or about +3NU unless he starts waving the SS in a nose up position for a long enough to cause the lagged THS autotrim to decide that the PF really wants more NU. To be honest it doesn't seem at all evil or that hard to understand, so long as you are half-aware of the systems of your aircraft - you know the one you are qualified to fly.
GarageYears is offline  
Old 9th Aug 2011, 13:42
  #1796 (permalink)  
 
Join Date: Nov 2006
Location: SoCalif
Posts: 898
an aircraft which is inherently naturally speed stable (by design concept)
Doesn't speed stable also mean pitch stable? When I studied aerodynamics a hundred years ago, the instructor said all conventional airplanes are unstable in pitch. Their sphugoid may be long in period, but still there. Nobody challenged him.

From what I read, the A330 was not designed to be pitch/speed stable, but pitch/speed neutral.
Graybeard is offline  
Old 9th Aug 2011, 14:01
  #1797 (permalink)  
 
Join Date: Jan 2005
Location: W of 30W
Posts: 1,939
Originally Posted by GY
To be honest it doesn't seem at all evil or that hard to understand, so long as you are half-aware of the systems of your aircraft - you know the one you are qualified to fly.
It seems nice the way you tell things, but now if you have a look at the Airbus documentation, where is the quote, where is the "BE AWARE" that in some cases you would be better use MANUAL TRIM despite the fact AUTOTRIM is active or at least operative ?

In other words, which simulator exercice has ever encouraged such way of thinking ... ?
CONF iture is offline  
Old 9th Aug 2011, 14:14
  #1798 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
To be simple, this aircraft is not meant to be hands fly. To keep some stabilities, a pilot must be known to fly with gentle move. No offense to this Airbus, hand fly is a definition to Pilot. To know this aitcraft is to know in his Law, the rolling is by direct to controls. This why the aircraft is worming through the air, sometines great, but always for the Pilkot mto be anxiuous, because he is causoing these rolls.

Trim is not an emergency to control for recover stall. It is to reduce drag, and too much small moving the elevators. It is slow for rewasons as above, the elevators are the prefer mechanism.

These Blue Angels fly their plane close to the ground, loudly, and slowly, with the mush,. it is as ballet.

But when these planes go 'depaert' in the altitude, the pilots must let go for the RBW to get flying back normal? Recover by wire. Red face rfor pilots who let go. Wht not some help automatic for these pilots of the 447.

Who wants trim to be silent, large, and possible to kill in this Stall. Automatic trim. This is for nonsense. See if you like, this pilot was not gentle. He was not confident, but he was stubborn. These make a pilot the choice not to be flying, but others. Where is this Captain?

Self confidence is good, but not bravado. For rolling, possible this right wing is trimmed down, for some reason, but pilot can control, but he plays back and forth, instead of holding some gentle trim? This trace for roll is too accurate to be some weather, too. consistent.

For Mr. takata Does this THS not start moving UP during the first Pitching UP by pilot? This because he is accedlerating above 1.2 g and the THS inhibits in this region? Would it have started to move soonerr, if he was more gentle in the pitch? Thank you sir.
Lyman is offline  
Old 9th Aug 2011, 17:10
  #1799 (permalink)  
 
Join Date: Jun 2009
Location: I am where I am and that's all where I am.
Posts: 660
3holelover re my comment about the plane image control surfaces clue, "Airbus will also display that on the ECAM FLT CTL page. (but you all knew that, right?... Is the suggestion for a separate screen always displaying this?"

Yup. It's there when you need it. Otherwise ignore it. If it also included an arrow to show AoA that would kill two birds on one separate display that is ignored most of the time. "Mousing" through a list of display menus when in the situation the plane fell into is not feasible - even if it is one push button beside the FD display.
JD-EE is offline  
Old 9th Aug 2011, 17:23
  #1800 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
I think a new picture is nice. But, for who? For you? The answer is not in new. Old is the secret for this survival. Old pilot named Captain. Too much lights, too many colors, no one understands, and Captain is gone. More lights and pretty pictures....that is the tickets?
Lyman is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright İ 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.