AF 447 Thread No. 9
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Cheers HazelNuts39,
All - the point I'm trying to get across is that we're in the realms of theory here, and there are too many variables about which we are unsure to be able to make a definitive assertion one way or the other. For example, we know the speeds came back at about 02:10:35 (shortly into the zoom climb) - did this affect the C*/pitch angle transition at any point?
My personal assumption is that the THS may have continued to trim NU to maintain the commanded FPA or pitch angle, but I think it was the continued attpemts to pull up during this process that caused it to happen so quickly.
All - the point I'm trying to get across is that we're in the realms of theory here, and there are too many variables about which we are unsure to be able to make a definitive assertion one way or the other. For example, we know the speeds came back at about 02:10:35 (shortly into the zoom climb) - did this affect the C*/pitch angle transition at any point?
My personal assumption is that the THS may have continued to trim NU to maintain the commanded FPA or pitch angle, but I think it was the continued attpemts to pull up during this process that caused it to happen so quickly.
@ henra
See gums post, according to that it maintains 1 g with hands off, which would not be the case if it would maintain pitch.
If you would be right, BEA has it wrong:
This aircraft would under your definition of the NZ law maintain pitch attitude, but go into an descent to compensate for the loss of speed. It would be partial speed stable. It would not stall from level flight and it would not try to maintain altitude.
From FCTM Cathay Pacific:
See gums post, according to that it maintains 1 g with hands off, which would not be the case if it would maintain pitch.
G command--Pitch-axis control law by which the pilot gets the same "g" for a particular amount of stick force, regardless of speed (energy permitting)
Pitch-rate command--Pitch-axis control law in which the pilot gets the same pitch rate for a particular amount of stick force (or deflection in some designs), regardless of speed.
henra
My understanding is the following:
At higher speeds (lets say above 230kts. it will keep 1 g at stick neutral and when deflecting the g load will always be the same for a certain stick position, independent of the speeds (so at 280 kts the same deflection will yield the same g as at 230kts).
Below a certain threshold (which was still quite a bit above stall speed IIRC) the law blends over to a pitch rate law. I.e. at SS neutral pitch rate is 0, thus pitch constant.
My understanding is the following:
At higher speeds (lets say above 230kts. it will keep 1 g at stick neutral and when deflecting the g load will always be the same for a certain stick position, independent of the speeds (so at 280 kts the same deflection will yield the same g as at 230kts).
Below a certain threshold (which was still quite a bit above stall speed IIRC) the law blends over to a pitch rate law. I.e. at SS neutral pitch rate is 0, thus pitch constant.
BEA FR
This is not the case on the A330 in alternate law. The specific consequence is that in this control law the aeroplane, placed in a configuration where the thrust is not sufficient to maintain speed on the flight path, would end up by stalling without any inputs on the sidestick.
This is not the case on the A330 in alternate law. The specific consequence is that in this control law the aeroplane, placed in a configuration where the thrust is not sufficient to maintain speed on the flight path, would end up by stalling without any inputs on the sidestick.
From FCTM Cathay Pacific:
Flight Mode
In pitch, when an input is made on the sidestick, the flight control computers
interpret this input as a “g” demand/pitch rate. Consequently, elevator deflection is not directly related to sidestick input. The aircraft responds to a sidestick order with a pitch rate at low speed and a flight path rate or “g” at high speed. When no input is made on the sidestick, the computers maintain a 1g flight path.
In pitch, when an input is made on the sidestick, the flight control computers
interpret this input as a “g” demand/pitch rate. Consequently, elevator deflection is not directly related to sidestick input. The aircraft responds to a sidestick order with a pitch rate at low speed and a flight path rate or “g” at high speed. When no input is made on the sidestick, the computers maintain a 1g flight path.
Last edited by RetiredF4; 8th Aug 2012 at 21:46.
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Putting to one side the ineptitude of this crew, what worries me is that after 1100+posts (and several years) we still cannot really establish what does what and when in an Airbus control system. It kind of takes away the chances of 'Mr Average' getting a 'feel' for it, doesn't it?
Many years ago, I think after the very early Indian AB crash, someone told me that 23 different control modes were effective in pitch between cruise and touchdown - all 'un-announced', of course.
Many years ago, I think after the very early Indian AB crash, someone told me that 23 different control modes were effective in pitch between cruise and touchdown - all 'un-announced', of course.
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@BOAC
We're talking about how the system is likely to behave in extremis based on the information we have - to be certain we'd need to have an Airbus insider to spell it out.
But in the same breath, a pilot didn't have to understand the inner workings of the artificial feel system in older aircraft to be able to fly the things. As you've pointed out, all this technical back-and-forth is rendered somewhat moot by the fact that the initiating event was sustained and repeated control inputs which were thoroughly inappropriate for the conditions.
As to your other point, the fact that there are only 6 control laws in the A320 causes me to question your source.
We're talking about how the system is likely to behave in extremis based on the information we have - to be certain we'd need to have an Airbus insider to spell it out.
But in the same breath, a pilot didn't have to understand the inner workings of the artificial feel system in older aircraft to be able to fly the things. As you've pointed out, all this technical back-and-forth is rendered somewhat moot by the fact that the initiating event was sustained and repeated control inputs which were thoroughly inappropriate for the conditions.
As to your other point, the fact that there are only 6 control laws in the A320 causes me to question your source.
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An opinion to which you are welcome, but looking at it from an engineering point of view, does it not make more sense to have the aircraft behave as close as it can to how it behaves when not in a degraded state?
Who is piloting a Airbus A330 .. the pilots or those who engineered the system ?
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@jcj:
OK, so let's look at it this way. As a pilot, do you want your aircraft to behave as it does more than 99.9(rec)% of the time if something goes wrong, or to throw you into a control regime with which you've had little hands-on experience since converting to the type on top of the failure that caused it?
OK, so let's look at it this way. As a pilot, do you want your aircraft to behave as it does more than 99.9(rec)% of the time if something goes wrong, or to throw you into a control regime with which you've had little hands-on experience since converting to the type on top of the failure that caused it?
Last edited by DozyWannabe; 8th Aug 2012 at 22:55.
Dozy, BOAC said "modes" and you responded with LAWS.
I'd read more carefully.
I refer you to the table (matrix) of A330 flight laws that was quite popular, though it is vintage 2005, as discussions on this mishap began.
I'd invite you to look at that chart again.
There is an interaction between behaviors a laws and which protection does or doesn't turn on or off or is modified, see the notes.
These modes include all that ... with varied PROTECTIONS
PITCH ATTITUDE
LOAD FACTOR
AOA
HIGH SPEED
LOW ENERGY
LOW SPEED STABILITY
HIGH SPEED STABILITY
ANGLE OF BANK
MAN’UVER LOAD ALLEVIAT-ION
TURBUL-ENCE DAMPING
YAW DAMPING
TURN COORD
NORMAL LAW
ALTERNATE LAW 1
ALTERNATE LAW 2 (with subtle degrees ...)
DIRECT LAW
MECH BACKUP
If BOAC had meant "laws" I suspect that's what he'd have said.
The picture he was trying to paint was of the complexity of what a pilot faced in knowing what he can expect his plane to do, or to be able to do.
That's how I read it anyway.
I'd read more carefully.
I refer you to the table (matrix) of A330 flight laws that was quite popular, though it is vintage 2005, as discussions on this mishap began.
I'd invite you to look at that chart again.
There is an interaction between behaviors a laws and which protection does or doesn't turn on or off or is modified, see the notes.
These modes include all that ... with varied PROTECTIONS
PITCH ATTITUDE
LOAD FACTOR
AOA
HIGH SPEED
LOW ENERGY
LOW SPEED STABILITY
HIGH SPEED STABILITY
ANGLE OF BANK
MAN’UVER LOAD ALLEVIAT-ION
TURBUL-ENCE DAMPING
YAW DAMPING
TURN COORD
NORMAL LAW
ALTERNATE LAW 1
ALTERNATE LAW 2 (with subtle degrees ...)
DIRECT LAW
MECH BACKUP
If BOAC had meant "laws" I suspect that's what he'd have said.
The picture he was trying to paint was of the complexity of what a pilot faced in knowing what he can expect his plane to do, or to be able to do.
That's how I read it anyway.
Last edited by Lonewolf_50; 8th Aug 2012 at 23:04.
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As I understood it, the crash in India was a case of FMC-based mode confusion similar to that which caused the Air Inter crash in the Vosges - flight control laws and modes never really came into it in terms of the priamry factors behind the accident.
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Response times of elevator and THS
Looking at the graph Dozy posted, noticed something I guess never computed before and apologize if it's been discussed to death and it just didn't make an impression. From 2:12.32 until about 2:12.49, pilot sitting in the right hand seat had predominately nose down inputs, with the first input going to the stop in the nose down direction. For this entire 17 seconds the elevators only moved to 15 degrees nose up from the bottom line of 30 degrees. The THS never moved.
Way too late, when the pilot in the left hand seat was finally given control, he input nose down almost to the stops from 2:13.45 to 2:13.52 so a total of perhaps 7 seconds until the dual inputs from the other guy evened out the inputs to around neutral. So full nose down for 7 seconds only moved the elevator to 15 degrees nose up and again, the THS never moved.
Just how long would a nose down input have to last to finally get the THS to full nose down? Took about a minute to roll to full nose up after predominately nose up side stick inputs from 2:10.47 to 2:11.45. Seems like an awful long time to have to wait in an emergency situation.
So can someone explain why it takes so long to get control surfaces to give the actions requested by the pilot. As I see it, the pilot wanted the nose down for 17 seconds and got nothing more than 15 degree nose up then the other wanted FULL nose down for 7 seconds and the computers once again only gave him nose up elevator. Does this have something to do with the cumulative inputs? Can a set of bad inputs cause the airplane to not respond quickly as was needed in this case??
Way too late, when the pilot in the left hand seat was finally given control, he input nose down almost to the stops from 2:13.45 to 2:13.52 so a total of perhaps 7 seconds until the dual inputs from the other guy evened out the inputs to around neutral. So full nose down for 7 seconds only moved the elevator to 15 degrees nose up and again, the THS never moved.
Just how long would a nose down input have to last to finally get the THS to full nose down? Took about a minute to roll to full nose up after predominately nose up side stick inputs from 2:10.47 to 2:11.45. Seems like an awful long time to have to wait in an emergency situation.
So can someone explain why it takes so long to get control surfaces to give the actions requested by the pilot. As I see it, the pilot wanted the nose down for 17 seconds and got nothing more than 15 degree nose up then the other wanted FULL nose down for 7 seconds and the computers once again only gave him nose up elevator. Does this have something to do with the cumulative inputs? Can a set of bad inputs cause the airplane to not respond quickly as was needed in this case??
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OK, so let's look at it this way. As a pilot, do you want your aircraft to behave as it does more than 99.9(rec)% of the time if something goes wrong
The 99.9% is the behavior of the aircraft in flight which is operated by automatic systems , FBW and autopilot
The aircraft behave automatically .. that's is not of any help as experience when I must put my hand on the stick for fly it in another law than normal
When I have a problem (meaning that the flight management system is not more reliable at 100%) .. I'd rather manage myself (and with crew) and not have a hybrid system .. that can execute commands (eg trim auto) in my place
Hence the words many time hear when something go wrong:
It's my aircraft
PuraVidaTransport
So can someone explain why it takes so long to get control surfaces to give the actions requested by the pilot. As I see it, the pilot wanted the nose down for 17 seconds and got nothing more than 15 degree nose up then the other wanted FULL nose down for 7 seconds and the computers once again only gave him nose up elevator. Does this have something to do with the cumulative inputs? Can a set of bad inputs cause the airplane to not respond quickly as was needed in this case??
)
Last edited by jcjeant; 9th Aug 2012 at 03:03.
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PuraVidaTransport,
In both cases the ND inputs were not strong enough and sustained in the time to allow the elevators to go from 30 deg UP to a DOWN position.
For the second case, the ND input by the PNF was weakened by the NU input made by the PF.
In both cases the ND inputs were not strong enough and sustained in the time to allow the elevators to go from 30 deg UP to a DOWN position.
For the second case, the ND input by the PNF was weakened by the NU input made by the PF.
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Originally Posted by HN39
the sidestick was at an almost constant 21% nose-up position between 02:10:50 and 02:11:30, i.e. the time the THS was moving.
Originally Posted by HN39
I think it is both: The THS movement was driven by the continued NU SS input, and (after the stall) only a continued SS ND input would have stopped or reversed it.
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WHAT if Direct Law ?
If Direct Law rules after AP disconnect, the worst case scenario is the PF is still full back stick when the CPT is back, but the THS is still at 3 degrees, so the stall is not that developed and the stall warning keeps warning ...
Captain, what do you think now ?
I think we're in a stall. Sure I could guess better if I could see you are pulling like mad on your controls ...
Captain, what do you think now ?
I think we're in a stall. Sure I could guess better if I could see you are pulling like mad on your controls ...
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Just to keep the Oozlum Bird from disappearing "you know where", the following extract(s) from the Airbus A330 Instructor Support Manual provide the following:-
As an aside, the PNF must have said, "Alternate law, protections lost."
Characteristics in pitch
Don’t fight with the stick; if you feel you overcontrol, release the stick.
Indications
The degradation of control laws is indicated on ECAM as well as on PFD.
- On ECAM
in ALTN: ECAM EW/D FLT CTL ALTN LAW (PROT LOST)
MAX SPEED 305/.82
When acting on the stick the pilot commands a constant G load maneuver and the aircraft response is G Load / Pitch rate. The pilot order is therefore consistent with the aircraft response "naturally" expected by the pilot, Pitch rate at low speed / Flight Path rate or G at high speed.
Hence STICK FREE, the A/C maintains the flight path even in case of speed changes. Furthermore, STICK FREE in case of Configuration changes, or thrust variations, etc… the pitching moment effects are reduced by the feedbacks in the control law itself and compensated for by precommands. With STICK FREE in turbulence, small deviations do occur on the flight path but with a tendancy of the A/C to regain a steady condition.
As a consequence the A/C is a STABLE PLATFORM and AUTOTRIMMED; it needs to be flown with minor corrections from the pilot on the stick, when the A/C deviates from its intended flight path.Hence STICK FREE, the A/C maintains the flight path even in case of speed changes. Furthermore, STICK FREE in case of Configuration changes, or thrust variations, etc… the pitching moment effects are reduced by the feedbacks in the control law itself and compensated for by precommands. With STICK FREE in turbulence, small deviations do occur on the flight path but with a tendancy of the A/C to regain a steady condition.
Don’t fight with the stick; if you feel you overcontrol, release the stick.
Indications
The degradation of control laws is indicated on ECAM as well as on PFD.
- On ECAM
in ALTN: ECAM EW/D FLT CTL ALTN LAW (PROT LOST)
MAX SPEED 305/.82
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Originally Posted by DW
As I understood it, the crash in India was a case of FMC-based mode confusion
Response times of elevator and THS
PuraVidaTransport
So can someone explain why it takes so long to get control surfaces to give the actions requested by the pilot. As I see it, the pilot wanted the nose down for 17 seconds and got nothing more than 15 degree nose up then the other wanted FULL nose down for 7 seconds and the computers once again only gave him nose up elevator. Does this have something to do with the cumulative inputs? Can a set of bad inputs cause the airplane to not respond quickly as was needed in this case??
So can someone explain why it takes so long to get control surfaces to give the actions requested by the pilot. As I see it, the pilot wanted the nose down for 17 seconds and got nothing more than 15 degree nose up then the other wanted FULL nose down for 7 seconds and the computers once again only gave him nose up elevator. Does this have something to do with the cumulative inputs? Can a set of bad inputs cause the airplane to not respond quickly as was needed in this case??
Load factor protection was the only protection left, and again, i dont know wether it has some speed dependent gains as well.
Maybe an overlay of the graph you mentioned with actual normal acceleration values would shed some light.
Last edited by RetiredF4; 9th Aug 2012 at 08:40.
Edit: Maybe just to clarify: When I say attitude I refer to pitch and not to AoA.
edit2: I assume the notion of Flight Path in BEA's description might be a traslation thing. Per my understanding the FCS does not care about Flight Path in the proper sense. That's the FMS which does that. FCS cares about behaviour relative to the air, not navigational.
Let's assume you place the nose 10° above the horizon. My understanding is it will try to keep the nose pointed at 10° no matter what the AoA. Given the fact that thrust will not be sufficient at that altitude to maintain speed in that attitude and given the progressively increasing drag with decreasing speed it will go into the stall and will resist any natural tendency to drop the nose with elevator and once its authority limit is reached it will call the trim for help.
Could you point my to where my assumption differs from BEA's?
Last edited by henra; 9th Aug 2012 at 08:59.
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Let's assume you place the nose 10° above the horizon. My understanding is it will try to keep the nose pointed at 10° no matter what the AoA.
HN, franzl, henra:
As this discussion of C* and its nuances progresses. I begin to get the impression, that attitude flying isn't an allowable mode in the A330, unless you transition to Direct Law. Or, in the case of pitch control, force the issue by use of Trim Wheels.
That does not make sense to me, and means that I misunderstand something essential. Also, based on the comments of those who fly Airbus aircraft, I infer that you can fly the A330 like any other aircraft.
As noted a few posts up, when PNF made a significant nose down input, and it was reduced in scope by PF coming back onto the controls, we have a non trivial CRM issue: two pilots fighting over the controls.
I have the controls
You have the controls
Not sure if PNF (LHS pilot) would have saved the day had his inputs not been interfered with, but I don't care what kind of aircraft you are flying in: If there are two pilots, and two sets of controls, and there are two people making inputs in opposition to one another, the odds of it all ending in tears goes up by orders of magnitude.
As this discussion of C* and its nuances progresses. I begin to get the impression, that attitude flying isn't an allowable mode in the A330, unless you transition to Direct Law. Or, in the case of pitch control, force the issue by use of Trim Wheels.
That does not make sense to me, and means that I misunderstand something essential. Also, based on the comments of those who fly Airbus aircraft, I infer that you can fly the A330 like any other aircraft.
As noted a few posts up, when PNF made a significant nose down input, and it was reduced in scope by PF coming back onto the controls, we have a non trivial CRM issue: two pilots fighting over the controls.
I have the controls
You have the controls
Not sure if PNF (LHS pilot) would have saved the day had his inputs not been interfered with, but I don't care what kind of aircraft you are flying in: If there are two pilots, and two sets of controls, and there are two people making inputs in opposition to one another, the odds of it all ending in tears goes up by orders of magnitude.