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AF 447 Thread No. 11

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AF 447 Thread No. 11

Old 8th Nov 2013, 12:02
  #701 (permalink)  
 
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Valpha prot is the AOA threshold from where the side stick will command AOA directly instead of load factor demand. Valpha max is the maximum AOA that flight control computers will let the side stick achieve and/or hold. With AP is off and side stick neutral if the aircraft is pitched up beyond Valpha prot due to disturbance it will try to pitch down to Valpha prot and not pitch up to alpha Max. If the pitch up is rapid then Phase advance will bring this threshold to a lesser value. The effect of activation of this protection mode is that trim freezes at entree point and the aircraft will stabilize at that AOA when stick free, But Pilot's action in A340 case would be same push forward to stop climb and that would bring flight controls out of protection mode to load factor demand which was not done and instead attempt was made to engage AP which does not engage when in protection. So this incident is not attributable to FBW characteristic.
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Old 8th Nov 2013, 13:34
  #702 (permalink)  
 
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Quote from vilas (my underscoring emphasis):
"With AP is off and side stick neutral if the aircraft is pitched up beyond Valpha prot due to disturbance it will try to pitch down to Valpha prot and not pitch up to alpha Max. If the pitch up is rapid then Phase advance will bring this threshold to a lesser value."

In an otherwise accurate and succinct analysis, I think you may still be confusing the phase-advanced threshold of an AoA-mode engagement with the target AoA once AoA mode is engaged. For the reasons I stated in my earlier reply to you, with neutral sidestick that target can only be Alpha-Prot itself; not the AoA value at its phase-advanced engagement, which has no further relevance.
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Old 8th Nov 2013, 14:32
  #703 (permalink)  
 
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Chris
I am in agreement with you that after initial lowering of threshold subsequently it will stabilize at actual V alpha prot because the purpose of phase advance is to preempt the change over point. I was explaining the normal change over at Valpha prot.
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Old 8th Nov 2013, 16:25
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Chris
Somewhere you have said "So my question remains: why does it not revert to Normal Law once the AoA falls to a safe value?" I take it you mean normal AOA (Vls)
Let me attempt to answer that. Basically the high angle of attack protection is provided to prevent stall while dealing with wind shear or terrain issues where you are likely to deliberatally invoke the protection. When the angle of attack has fallen to a safe value (I presume it as less than alpha Max) any attempt to increase speed beyond Valpha prot by lowering the pitch is left to the pilot as conscious decision. If it was automatically done without assessing the situation it would have been unsafe.
You will remember that when you change from NAV to HDG, CLB changes OPCLB but in descent it does not change to OPDES but to present VS. The reason is same. In CLB or OPCLB the thrust is in THR CLB but in DES it need not be in thrust idle and in such a case change to THR IDL as it would in OPDES will pitch the aircraft down increasing ROD. So this is left to the pilot as a conscious decision.

Last edited by vilas; 8th Nov 2013 at 16:48.
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Old 8th Nov 2013, 18:18
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Originally Posted by CONF iture
Please do not spread disinformation, that captain was definitely not relying on A-FLOOR to get the thrust on.
I don't think Chris was intentionally misinforming - let's face it, the story has been misremembered and re-told in slightly different ways so many times over the years - especially in the media - that deviations like that are pretty much the norm. It's pretty much only pedants like me and you who are militant about getting it right!

Originally Posted by CONF iture
DonH has now confirmed that earlier comment
Indeed - however your comment here:

Nothing is that simple with the airbus, a very complex machine indeed.
seems at odds with what was actually changed. It used to be that to cancel AoA protection - in which the nose comes up - one had to push the nose back down, which seems reasonably intuitive to me. The change meant that leaving the stick in neutral for a certain period of time would also return you to Normal Law - again, not exactly rocket science.
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Old 8th Nov 2013, 19:03
  #706 (permalink)  
 
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The change meant that leaving the stick in neutral for a certain period of time would also return you to Normal Law - again, not exactly rocket science.
It may just need a little help from atmospheric disturbances. Without those, it would just maintain alpha-prot in a phugoid motion which may be damped.

P.S. I think you mean Nz-law. The 'normal' protections are included in Normal Law, and may not exist in Alternate Law.
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Old 8th Nov 2013, 21:32
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Agreed. Picky, picky, picky...
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Old 9th Nov 2013, 00:01
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Quote from Dozy Wannabe:
"I don't think Chris was intentionally misinforming - "

I don't think anyone will ever know for sure, so - more out of respect for Conf_iture than the gentleman concerned - I amended my post this morning to reflect fact, rather than supposition.

Hi vilas,

I like your explanation of why AoA mode used to remain engaged until a sidestick was displaced forward (as in the first two disengagement conditions listed in the FCOMs posted by DonH). The interesting development is that, sometime after the A340 AIRPROX incident of 2001, the introduction of the third disengagement condition seems to have been completed across the fleet.

The third disengagement condition is not unlike what I was proposing a few days ago in reaction to the A340 AIRPROX. (That was prior to HN39 drawing our attention to the BEA Report into the AF A340 Serious Incident of 2011.) I was concerned that a short-term up-gust had triggered AoA mode by phase-advance, and AoA mode had initiated the undesired climb with neutral sidestick which the crew were (admittedly) slow to correct.

Having studied the traces provided by the BEA of the AF A340 incident, where the third disengagement condition operated five times, I am reviewing my opinion. The first disengagement into normal (Nz) mode was, as you know, shortly followed by a significant exceedance of Alpha-MAX. It seems probable that the wing effectively stalled at that point, which probably would not have happened but for the temporary disengagement of AoA mode.
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Old 9th Nov 2013, 00:52
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Originally Posted by Chris Scott
Having studied the traces provided by the BEA of the AF A340 incident, where the third disengagement condition operated five times, I am reviewing my opinion.
Chris - the ultimate question at the end of the day is "how much hand-holding is a good idea to provide?". It's reasonable to assume that Alpha Prot and Alpha Floor as designed were in the main intended to assist a pilot who is already commanding nose-up with the stick, such that the pilot's command is followed to the best of the aircraft's ability while the aircraft remains flying. In this airprox incident we have a situation where one or both of the alpha protection modes were triggered by transient weather conditions, which were outside of the original spec.

Contrary to legend, the protections were designed to assist a pilot who, for whatever reason, has been forced to take the aircraft to the edge of the safe operating envelope - counteracting handling mistakes is and always was a secondary design consideration.

In this case we have an incident during which - for whatever reason - a protection mode has actually had a negative impact on the safety of the flight, hence the change in logic.
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Old 9th Nov 2013, 08:33
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Chris &Dozy
Some more thoughts on the A340 incident:

The report states “ The over speed warning(“OVERSPEED”) was triggered(3) andthe “Master Warning” warning light came on. The crew stated that they were very surprised by this warning. Mach reached 0.87.” The warning is set to trigger at MMO+.006 i.e.M.866 and it triggered correctly why was the crew surprised? The report says “during the event the high speed protection was not activated”. This is surprising as it should have activated and according to A340 FCOM “The autopilot disconnects when high speed protection goes active”.

The report says” F-GLZU recorded data showed that the AP would have remained connected if it had not been manually disengaged”.I don’t see any guarantee of this as high speed protection should have activated or was about to activate and that would have disconnected the AP.

Then the PF disconnected the AP and applied ¾ back stick which put the aircraft in high AOA protection.

“From the start of thepitch-up input until point n°9, the high angle of attack protection was activated several times”. This statement has been misunderstood by some bloggers as automation causing unnecessary pitch up to Valpha MAX. This is not what happens in AOA protection rather it only can cause pitch down to Valpha prot with stick neutral. However if AOA is left at Valpha prot then the mode of pitch control may keep changing from load factor to AOA mode as the AOA changes due to disturbances. That is not same as pitching up and down. The only thing is if left alone, in AOA mode it will stabilize at alpha prot speed. In any case Pilot’s action should have been to push the nose down and recover the speed. So automation has not played any adverse role in this.

Last edited by vilas; 9th Nov 2013 at 09:21.
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Old 9th Nov 2013, 10:08
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Originally Posted by Dozy
The change meant that leaving the stick in neutral for a certain period of time would also return you to Normal Law - again, not exactly rocket science.
This has nothing to do with 'returning to Normal Law'.
Your approximations generate confusion on a complex System that on top of it keeps changing with time.
As 'it is not exactly rocket science' why do you get it wrong then ... ?
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Old 9th Nov 2013, 10:13
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Originally Posted by Chris Scott
AFAIK, Alpha-Floor is always higher than Alpha-Prot. As you know, it has always been necesssary to inhibit Alpha-Floor below a certain height on landing (IIRC, 50R on the A320, but I see 100R on the big 'buses), although it was widely thought that one fellow captain did not take that into account during his cavalier fly-by at Habsheim...
Sorry Chris but I just can't understand your thinking here as part of the procedure to present the Airbus at high AoA is specifically to inhibit A/THR to prevent Alpha Floor to spoil the demonstration.
How can you suggest the guy was waiting for Alpha-Floor to kick in when his initial intention was to prevent it to interfere in the first place ?
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Old 9th Nov 2013, 14:13
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DozyWannabe

"It used to be that to cancelAoA protection - in which the nose comes up - one had to push the nose backdown, which seems reasonably intuitive to me. The change meant that leaving thestick in neutral for a certain period of time would also return you to NormalLaw - again, not exactly rocket science".
I am sorry but it appears that you have completely misunderstood normal law and protections. Normal side stick fore and aft movement is a load factor demand and computers position elevator and stab to give you that. Pitch control through load factor demand has an advantage in that aircraft response remains same at all speeds. High angle of attack protection which begins at Valpha prot the pitch control is changed to directly achieve through AOA change and not through load factor demand. Valpha Max is maximum AOA aircraft can be pitched upto. When the angle of attack increases deliberately as in case of wind shear and GPWS issues or through atmospheric disturbance the pitch control is changed to AOA mode since that is more appropriate for the situation, this does not mean aircraft pitches to alpha max, rather it tries to pitch down to alpha prot which is the first low speed threshold and pilot has to overcome it by backward pressure if he wants more pitch up.. If you release the stick aircraft will pitch down and maintain that AOA (Valpha prot). If the contingecy is over and you want return to normal speed you push the stick forward as you would do in any aircraft and the pitch control changes back to load factor demand. You have to remember that only method of pitch control is changed aircraft doesn't pitch up on its own. During all this aircraft remains in normal law. In the incident we are discssing the protection mode had no negative impact whatsoever. It is definitely not rocket science but you have not understood it at all.
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Old 9th Nov 2013, 14:51
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Originally Posted by vilas
You have to remember that only method of pitch control is changed aircraft doesn't pitch up on its own. During all this aircraft remains in normal law.
With the sidestick left in neutral the airplane will pitch up on its own -

-- in AoA law at alpha=alpha-prot if the speed is greater than Valpha-prot
-- in Nz law when the pitch attitude is greater than the pitch which will maintain constant airspeed - AoA increases to maintain Nz=1 when airspeed reduces.
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Old 9th Nov 2013, 15:53
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HN39
With the sidestick left in neutral the airplane will pitch up on its own -

-- in AoA law at alpha=alpha-prot if the speed is greater than Valpha-prot
-- in Nz law when the pitch attitude is greater than the pitch which will maintain constant airspeed - AoA increases to maintain Nz=1 when airspeed reduces.
No way. I think you should read flight controls chapter. It is high angle of attack protection. It begins at alpha prot. A protection can only be called protection when it prevents the pilot from doing it. In high speed protection if it pitches down it is not a protection it is destruction. Similarly in high AOA protection if AOA increases what kind of protection is that? Your first case, the speed is greater than Valpha prot you are not at all in any protection zone and side stick commands load factor so why would it go to alpha prot? If you intentionally keep pitching up it pitches up to alpha max and no more. if you release the SS it will not maintain alpha max but pitch down to alpha prot.
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Old 9th Nov 2013, 16:17
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AoA mode - engagement and disengagement criteria

vilas,

I agree with nearly all of your latest thoughts about the AF A340 Incident. You make some valid points, including the apparent discrepancy in relation to the overspeed protection. The latter had disengaged the AP in the A340 AIRPROX of 2001, but seemed already overdue to do so on the AF A340 when the backward movement of the F/O's sidestick manually disengaged it. Like you, I find the BEA statement that you quote to be confusing.

Where our respective understandings do still differ, however, is in what may happen after AoA-mode engages by phase-advance resulting from a brief up-gust.

It's worth noting here that the extra (3rd) disengagement condition available on the a/c in the AF A340 incident of 2011 caused the first 5 of the 6 reversions to Nz mode, for which the sidestick remained neutral. In the case of the A340 AIRPROX incident, where the 3rd condition had not been incorporated, AoA mode consequently persisted throughout the main event - even after a small push-forward on the sidestick for less than a second - until instantly disengaged by a selection of more than half-forward sidestick.

Quote (my emphasis):
"This is not what happens in AOA protection rather it only can cause pitch down to Valpha prot with stick neutral."

I disagree here, as I tried to explain in my last reply to you. A brief up-gust can cause the phase-advanced value of AoA to exceed alpha-prot briefly. Actual AoA can remain below alpha-prot, and when the up-gust ceases the phase-advanced value will return to the real AoA. Meanwhile, however, AoA mode has engaged. Under the present-day three-condition disengagement logic, AoA mode would disengage after 0.5 sec if both sidesticks were neutral. Under the two-condition disengagement logic in the A340 AIRPROX of 2001, it would remain engaged with neutral sidesticks. So in the latter case the EFCS will have to increase the AoA to Alpha-Prot, which will probably involve a pitch-up. This is what I think happened in the AIRPROX case, and why I questioned the disengagement criteria.

One of the vulnerabilities of my argument in the A340 AIRPROX case is that, AFAIK, we don't have any estimates for what would have happened to the pitch (and, therefore, the flightpath) if the AoA mode had reverted to Nz, delivering 1.0 G for the long period that the sidesticks remained neutral.

Quote:
"In any case Pilot’s action should have been to push the nose down and recover the speed. So automation has not played any adverse role in this."

Your first sentence is correct for both events. Your second sentence is arguable in the A340 AIRPROX case, as I've explained above. In the AF A340 case, however, the F/O's early action (pulling the stick) was - like AF447 - what started the zoom-climb. Once he had released the stick, the AoA mode was free to disengage each time the AoA happened to fall below Alpha-Prot for half a second. Each time, Normal mode targeted 1.0G, which on 5 occasions led to re-engagement of AoA mode. As previously noted, the first re-engagement was too late to prevent an exceedance of Alpha-MAX (perhaps in a gust).

These two events may present conflicting arguments for and against the third disengagement condition. I suspect that AI engineers may have been revisiting AoA-mode engagement and disengagement criteria since 2011.
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Old 9th Nov 2013, 16:40
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No way. I think you should read flight controls chapter. .
Vilas,

I think you will find that HN39 understands flight mechanics as well as any writing in these pages.
Although I agree with much of what you write, I think you nevertheless have some things wrong:

Alphaprot is NOT the same as normal law, so when you wrote:

You have to remember that only method of pitch control is changed aircraft doesn't pitch up on its own. During all this aircraft remains in normal law
this was not correct.

I think you are failing to distinguish between angle of attack and pitch. When HN39 writes
With the sidestick left in neutral the airplane will pitch up on its own -

-- in AoA law at alpha=alpha-prot if the speed is greater than Valpha-prot
he is properly relating the flight dynamics. With stick neutral, alphaprot will control the aircraft to a constant AoA, to whit the alphaprot for the applicable Mach Number. It controls AoA; it does NOT control pitch. If the airspeed at this time is greater than that appropriate to 1g at alphaprot then the aircraft will develop a normal acceleration and start to change its flight path. Unless corrected by a change in pitch this will result in a change in AoA. But the system is trying to control to a constant AoA so it must apply a pitch up to compensate. This, I think, is what HN39 is alluding to.
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Old 9th Nov 2013, 16:51
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Quote from Dozy Wannabe (my insertions in square brackets):
"In this [A340 AIRPROX] case we have [had] an incident during which - for whatever reason - a protection mode has actually had a negative impact on the safety of the flight, hence the change in logic."

Hi Dozy,

Prior to our attention being drawn to the AF A340 case, I caused some controversy here by opining that the disengagement criteria for what we are now calling the AoA mode (of Normal Law) should be relaxed. I'm now having second thoughts, because the new (third) criterion MAY have allowed the AF A340 to stall during the second engagement of AoA mode. This followed a brief (3-second) resumption of Nz mode, during which the a/c pitched up by about 4 degrees.

Last edited by Chris Scott; 9th Nov 2013 at 18:30. Reason: Typo.
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Old 9th Nov 2013, 17:46
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Owain Glyndwr
My comments are strictly based on Airbus documents and I don't lay any claims to greatness. I am aware that some of the people who comment here have better qualifications than i have.
The way FCOM describes this chapter is responsible for some confusion. First these are protections and they are available only in normal law. What is described as load factor law or AOA law are in facts modes of normal law Valpha prot is the defining boundary. FCOM is quoted below
HIGH ANGLE OF ATTACK PROTECTION
Innormal law, when the angle-of-attack becomes greater than α PROT, the systemswitches the elevator control from normal mode to a protection mode, in which the angle-of-attack is proportional to sidestick deflection. That is, in the α PROT range, from α PROT to α MAX, the sidestick commands α directly. However the angle-of-attack will not exceed αMAX, even if the pilot gently pulls the sidestick all the way back. If the pilot releases the sidestick, the
angle-of-attack returns to α PROT and stays there.

I don't see where is the problem?
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Old 9th Nov 2013, 17:50
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laws, modes, backup modes/laws

OG has the best description of the "laws" I have seen in many moons.

The discussion here amongst the heavy pilots emphasizes a point I have made ( fire a way Doze) , and several others, that you must have a simple degredation sequence when so many aspects of flying the jet are not pure hydraulics with minimal inputs like dampers and such.

I would think that the primary "law" would be related to AoA. You know, the basic parameter that keeps us using all those lifties. Overspeed comments here puzzle me, as most bent wing jets airfoils can safely get to higher mach than is normal for cruise. Even near "critical mach", there should be some aero feedback to the pilot that controls are reversing, an unusual buffet, and so forth. I only flew one jet that had a nasty tendency if close to the critical mach, and the sucker would "tuck" and unless you could pull back power and extend speed brakes then you were toast. It was a straight wing attack jet. 20 or 30 knots below the bad zone you would start having aileron problems, so I never flew the thing that fast again on bombing runs.

My main point is that the 'bus has so many "protections" and so many conditions when one "protection" is active and another is not. Sheesh.

IMHO, pilots need to have control logic that they can hang their hat on. I don't think I could pass a test on all the reversion modes of the 'bus and the sub-modes. Of course, none of this applies to those jets that are not fully FBW and have a plethora of Otto inputs to the same.
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