AF 447 Thread No. 11
A340 AoA (protection) Law - disengagement conditions
The new (to this thread) BEA Report on the AF A340 Serious Incident of 2011 describes the conditions that allow AoA Law to disengage back into Normal Law (which it refers to correctly - in the context of high altitude - as "load-factor control"). The OR-gate conditions that it lists include a third condition that we seem to have been unaware of so far in our discussion of the A340 AIRPROX event of October 2000.
I raised some controversy recently when I questioned the disengagement logic of AoA Law, having been led to believe that, in the A340 AIRPROX case, once AoA Law was triggered by a short-term, phase-advanced (gust-induced) identification of alpha-prot, it remained engaged until the PF finally made a substantial push on the sidestick. This seemed to be despite the fact that - after the trigger-gust subsided - the AoA and phase-advanced AoA appear to have been below the trigger threshold for a couple of seconds until the EFCS (FBW) in AoA Law selected up-elevator to increase the AoA to alpha-prot.
Accordingly, I wrote:
"...Phase-advanced alpha-protection puts the FBW into AoA Protection Law. Meanwhile, the AoA has returned (fallen) to what it was before the gust. However, FBW uses up-elevator to increase the AoA to alpha-prot. The a/c climbs suddenly until I push the stick forward more than half-travel, OR use less-than-half forward stick for more than a second."
Well, in this new report into the 2011 incident, the BEA lists the disengagement conditions as follows (newbees, note that a sidestick is spring-loaded to neutral):
sidestick forward more than half-travel;
OR
sidestick forward less than half-travel for 1 s while AoA < Alpha MAX;
OR
sidestick neutral or forward for 0.5 s while AoA < Alpha Prot.
It seems now that, either my understanding of the disengagement conditions for the A340 in 2000 was wrong, or they were subsequently modified?
PS by EDIT
Re the report appendix, am looking forward to interpretation by those who are more experienced and better than I am in interpreting FDR traces. (Mentioning no names!) In the interim, I notice that AoA Law disengages (to Normal Law) and re-engages on 5 occasions prior to its final disengagement. So there are 6 periods of AoA Law.
The first period ended in disengagement after only 8 seconds, apparently because the sidestick was released to neutral and an up-gust ceased (or a down-gust was encountered), reducing the AoA below Alpha-Prot (see the third condition listed above) for > 0.5 s. Although this restoration of Normal Law lasted less than 3 seconds, the report suggests it would have been maintained, but for the turbulence and speedbrakes, with the sidestick neutral. During this short period of Normal Law, the pitch increased rapidly from about +5 to about +8.5. The increase in pitch rate and AoA rate was so great that, when AoA Law re-engaged, it was unable to stop the AoA briefly exceeding Alpha-MAX.
Notwithstanding my doubts, expressed a few days ago, about the logic of engagement/disengagement of AoA Law (see above), the DFDR traces in this BEA Report seem to show that the periods of its disengagement may have resulted in greater divergences from normal flight parameters than when it was engaged.
One assumes that, if AoA Law was maintained, the flight profile would approximate a phugoid? That view may be supported by the last (and longest) period of AoA-Law engagement, which started just after the FL382 apogee. During the descent, the VS increases gradually to about 4000 ft/min; then decreases gradually to zero (at FL365). It is at that point that the PF applies just under half forward-stick for more than a second, disengaging AoA law.
I raised some controversy recently when I questioned the disengagement logic of AoA Law, having been led to believe that, in the A340 AIRPROX case, once AoA Law was triggered by a short-term, phase-advanced (gust-induced) identification of alpha-prot, it remained engaged until the PF finally made a substantial push on the sidestick. This seemed to be despite the fact that - after the trigger-gust subsided - the AoA and phase-advanced AoA appear to have been below the trigger threshold for a couple of seconds until the EFCS (FBW) in AoA Law selected up-elevator to increase the AoA to alpha-prot.
Accordingly, I wrote:
"...Phase-advanced alpha-protection puts the FBW into AoA Protection Law. Meanwhile, the AoA has returned (fallen) to what it was before the gust. However, FBW uses up-elevator to increase the AoA to alpha-prot. The a/c climbs suddenly until I push the stick forward more than half-travel, OR use less-than-half forward stick for more than a second."
Well, in this new report into the 2011 incident, the BEA lists the disengagement conditions as follows (newbees, note that a sidestick is spring-loaded to neutral):
sidestick forward more than half-travel;
OR
sidestick forward less than half-travel for 1 s while AoA < Alpha MAX;
OR
sidestick neutral or forward for 0.5 s while AoA < Alpha Prot.
It seems now that, either my understanding of the disengagement conditions for the A340 in 2000 was wrong, or they were subsequently modified?
PS by EDIT
Re the report appendix, am looking forward to interpretation by those who are more experienced and better than I am in interpreting FDR traces. (Mentioning no names!) In the interim, I notice that AoA Law disengages (to Normal Law) and re-engages on 5 occasions prior to its final disengagement. So there are 6 periods of AoA Law.
The first period ended in disengagement after only 8 seconds, apparently because the sidestick was released to neutral and an up-gust ceased (or a down-gust was encountered), reducing the AoA below Alpha-Prot (see the third condition listed above) for > 0.5 s. Although this restoration of Normal Law lasted less than 3 seconds, the report suggests it would have been maintained, but for the turbulence and speedbrakes, with the sidestick neutral. During this short period of Normal Law, the pitch increased rapidly from about +5 to about +8.5. The increase in pitch rate and AoA rate was so great that, when AoA Law re-engaged, it was unable to stop the AoA briefly exceeding Alpha-MAX.
Notwithstanding my doubts, expressed a few days ago, about the logic of engagement/disengagement of AoA Law (see above), the DFDR traces in this BEA Report seem to show that the periods of its disengagement may have resulted in greater divergences from normal flight parameters than when it was engaged.
One assumes that, if AoA Law was maintained, the flight profile would approximate a phugoid? That view may be supported by the last (and longest) period of AoA-Law engagement, which started just after the FL382 apogee. During the descent, the VS increases gradually to about 4000 ft/min; then decreases gradually to zero (at FL365). It is at that point that the PF applies just under half forward-stick for more than a second, disengaging AoA law.
Last edited by Chris Scott; 6th Nov 2013 at 19:30. Reason: PS added.
BOAC,
I thought that was what you meant - just checking!
As I said earlier, the "PNF" made (substantial and inappropriate) sidestick inputs, but IMO the BEA is right to continue to refer to him as PNF. We don't even know if his inputs were intentional.
Quote from OK465:
"....in the A346 & A332, the "AP OFF" ECAM message is red not amber when the A/P is disengaged with the takeover button. The red ones usually tend to get my attention.
Is it, or was it at one time, amber in the A343?....or is this a translation problem?....a tech writer problem?....or a report quality control problem?"
Must admit that red is my recollection from the A320, too, but any error is not in the translation.
Here's the French original (my emphasis):
"L’équipage n’a pas vu les différents changements d’affichage (message « AP OFF » ambre de l’ECAM, disparition de l’indication « AP1 » du bandeau FMA)."
I thought that was what you meant - just checking!
As I said earlier, the "PNF" made (substantial and inappropriate) sidestick inputs, but IMO the BEA is right to continue to refer to him as PNF. We don't even know if his inputs were intentional.
Quote from OK465:
"....in the A346 & A332, the "AP OFF" ECAM message is red not amber when the A/P is disengaged with the takeover button. The red ones usually tend to get my attention.
Is it, or was it at one time, amber in the A343?....or is this a translation problem?....a tech writer problem?....or a report quality control problem?"
Must admit that red is my recollection from the A320, too, but any error is not in the translation.
Here's the French original (my emphasis):
"L’équipage n’a pas vu les différents changements d’affichage (message « AP OFF » ambre de l’ECAM, disparition de l’indication « AP1 » du bandeau FMA)."
Last edited by Chris Scott; 6th Nov 2013 at 21:00. Reason: Quotes (re)italicised.
In this case, the copilot seems to have remained the PNF (i.e., as far as we know there was no formal handover of control), but made sidestick inputs - whether intentionally or not.
Chris, your estimation of the "chasing" of air data sensors by the A/P in such entries into turbulent air is confusing to me.
I learned a long time ago that if you were entering turbulent air that has a lot of vertical component, that you need to work on holding pitch and power for the proper "turbulent air penetration" airspeed. You could expect both A/S and altitude to vary a bit as the air mass bounces you about.
"Don't chase it!" was some of the teaching then.
Is that still current teaching?
Last edited by Lonewolf_50; 6th Nov 2013 at 19:45.
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Hello, Chris Scott, OK465, BOAC;
Since AP1 was engaged at the time of the event, and re-engaged subsequent to the recovery I think the PF was the captain. While not strictly SOP, normally a left seat PF engages AP1 and a right seat PF engages AP2.
However, the captain was eating dinner, with the retractable tray out and it is common for the PNF to temporarily take over PF duties as well as communications while the other crew member is having a meal. The AP is left in AP1, again informally.
Where something like this 'interruption in smooth flight' occurs, the "PNF" would continue the PF's duties and mind the store until the PF can put the meal and tray away and resume PF duties to deal with the event.
The Report mentions and describes the incorrect priorities and evident loss of CRM and the "command-control" process as both crew members were fussing with getting the dinner tray onto the observer's seat and getting out the PA handset for an announcement. Even as we don't have the CVR, this is clearly another breakdown in cockpit discipline, placing "Communicate" ahead of "Aviate, & Navigate". Without the CVR we dont' know if control was actually, verbally handed back to the PF, ("You have control....I have control", etc).
Further, it was the PNF's SS instinctive-disconnect button that was pressed and disconnected the AP but he clicked twice on the button which cancels the aural and ECAM warnings. From long observation, pressing the instinctive button to cancel the AP warning is not atypical behaviour; - this would have been out of habit, but here the cancellation robbed the PF of information that the AP had been disconnected and the altitude alert was also missed. The BEA Report states that such cancellation of the warning was an amber AP OFF, (OK465 you know, I do recall an amber "AP OFF" ECAM message somewhere in the dim past (perhaps on the A320?), but I can't find reference to it in either older A340 or recent A330 FCOMs. I can only find "red warnings" reference, (see below). Certainly, under the circumstances of this intentional, by the instinctive button disconnect, the warning was/is always red and the AP cavalry charge continues until the button is pressed again so the BEA Report is incorrect in its reference to the "amber AP OFF" ECAM message.
The PF who was distracted getting his meal put away, was unaware that the AP was disconnected and the airplane was under manual control. The PF remained unaware until he realized that the airplane had climbed well above its cleared altitude and when he went to set the AP to descend to their cleared altitude, did not see "AP" annunciated on the FMA, (p.5 of the Report).
The Report explains the fact that it was the "PNF" who disconnected the AP, but leaves us there as to who was flying once the PNF made the HF call to advise New York Aeradio of the level bust, while the airplane was essentially still out-of-control), nor does the Report explain why the "PNF" chose to disconnect the AP and then do what he did, which was to pitch the airplane up.
The use of airborne radar has been a topic of considerable interest since June 1, 2009 particularly. I have noticed that air carriers began publishing in their FCOMs supplementary notes on the use of current radars including various techniques of using antenna tilt and gain. It's about time.
Given history and the formally recognized need to address certain cultural issues within the carrier*, I would hope that this got a thorough internal review (I wonder if their flight data program picked this up?), because, "If the AP had not been manually disengaged, it would have remained engaged; there would not have been a significant trajectory deviation, with a gain in altitude of about 200 ft.
Note: Without the high angle of attack protection, the aeroplane would have kept its ascending trajectory until the triggering of the stall warning. p.9"
*Operational Safety Review, Dec, 2009 - the contents of, and I believe, links to this review have been previously reported on PPRuNe.
Since AP1 was engaged at the time of the event, and re-engaged subsequent to the recovery I think the PF was the captain. While not strictly SOP, normally a left seat PF engages AP1 and a right seat PF engages AP2.
However, the captain was eating dinner, with the retractable tray out and it is common for the PNF to temporarily take over PF duties as well as communications while the other crew member is having a meal. The AP is left in AP1, again informally.
Where something like this 'interruption in smooth flight' occurs, the "PNF" would continue the PF's duties and mind the store until the PF can put the meal and tray away and resume PF duties to deal with the event.
The Report mentions and describes the incorrect priorities and evident loss of CRM and the "command-control" process as both crew members were fussing with getting the dinner tray onto the observer's seat and getting out the PA handset for an announcement. Even as we don't have the CVR, this is clearly another breakdown in cockpit discipline, placing "Communicate" ahead of "Aviate, & Navigate". Without the CVR we dont' know if control was actually, verbally handed back to the PF, ("You have control....I have control", etc).
Further, it was the PNF's SS instinctive-disconnect button that was pressed and disconnected the AP but he clicked twice on the button which cancels the aural and ECAM warnings. From long observation, pressing the instinctive button to cancel the AP warning is not atypical behaviour; - this would have been out of habit, but here the cancellation robbed the PF of information that the AP had been disconnected and the altitude alert was also missed. The BEA Report states that such cancellation of the warning was an amber AP OFF, (OK465 you know, I do recall an amber "AP OFF" ECAM message somewhere in the dim past (perhaps on the A320?), but I can't find reference to it in either older A340 or recent A330 FCOMs. I can only find "red warnings" reference, (see below). Certainly, under the circumstances of this intentional, by the instinctive button disconnect, the warning was/is always red and the AP cavalry charge continues until the button is pressed again so the BEA Report is incorrect in its reference to the "amber AP OFF" ECAM message.
The PF who was distracted getting his meal put away, was unaware that the AP was disconnected and the airplane was under manual control. The PF remained unaware until he realized that the airplane had climbed well above its cleared altitude and when he went to set the AP to descend to their cleared altitude, did not see "AP" annunciated on the FMA, (p.5 of the Report).
The Report explains the fact that it was the "PNF" who disconnected the AP, but leaves us there as to who was flying once the PNF made the HF call to advise New York Aeradio of the level bust, while the airplane was essentially still out-of-control), nor does the Report explain why the "PNF" chose to disconnect the AP and then do what he did, which was to pitch the airplane up.
The use of airborne radar has been a topic of considerable interest since June 1, 2009 particularly. I have noticed that air carriers began publishing in their FCOMs supplementary notes on the use of current radars including various techniques of using antenna tilt and gain. It's about time.
Given history and the formally recognized need to address certain cultural issues within the carrier*, I would hope that this got a thorough internal review (I wonder if their flight data program picked this up?), because, "If the AP had not been manually disengaged, it would have remained engaged; there would not have been a significant trajectory deviation, with a gain in altitude of about 200 ft.
Note: Without the high angle of attack protection, the aeroplane would have kept its ascending trajectory until the triggering of the stall warning. p.9"
*Operational Safety Review, Dec, 2009 - the contents of, and I believe, links to this review have been previously reported on PPRuNe.
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Chris,
Both incidents have been discussed on PPRuNe when the respective reports were published. I do remember that the disengagement conditions were modified after the AIRPROX incident, but I don't remember the details.
It seems now that, either my understanding of the disengagement conditions for the A340 in 2000 was wrong, or they were subsequently modified?
Hi DonH,
Think the report says the copilot only pressed the AP instinctive-disconnect PB on his sidestick once (top of Page 13)?
Am in full accord with your analysis of CRM aspects. Quite shocking mis-prioritisation of actions from start to finish of the incident. Hard to know where to start...
Yes, it also reflects my experience of the common underuse of wx radar, and misunderstanding of how to use it effectively to avoid flying into or over the upper parts of Cbs.
'Evening HN39,
Quote:
I do remember that the disengagement conditions were modified after the AIRPROX incident, but I don't remember the details.
Thanks, but it was news to me. The only recommendations in the AAIB Bulletin relate to RVSM procedures.
Think the report says the copilot only pressed the AP instinctive-disconnect PB on his sidestick once (top of Page 13)?
Am in full accord with your analysis of CRM aspects. Quite shocking mis-prioritisation of actions from start to finish of the incident. Hard to know where to start...
Yes, it also reflects my experience of the common underuse of wx radar, and misunderstanding of how to use it effectively to avoid flying into or over the upper parts of Cbs.
'Evening HN39,
Quote:
I do remember that the disengagement conditions were modified after the AIRPROX incident, but I don't remember the details.
Thanks, but it was news to me. The only recommendations in the AAIB Bulletin relate to RVSM procedures.
Last edited by Chris Scott; 6th Nov 2013 at 20:50. Reason: Reply to HN39 added.
Quote from Lonewolf_50:
"I learned a long time ago that if you were entering turbulent air that has a lot of vertical component, that you need to work on holding pitch and power for the proper "turbulent air penetration" airspeed. You could expect both A/S and altitude to vary a bit as the air mass bounces you about.
'Don't chase it!' was some of the teaching then. Is that still current teaching?"
Afraid I'm the wrong person to ask about current teaching! But you raise a very valid point - in line with my own experience.
However, I think the only "chasing" by the EFCS/FBW visible in the DFDR traces is of the AoA when it is in AoA Law? I can only suggest that perhaps that is inevitable, bearing in mind the (presumably) narrow margin between Alpha-Prot and Alpha-MAX at cruise Mach - not to mention the stall. (That is: I'm out of my depth here, call an aerodynamicist!)
I doubt that, at normal cruise speeds and AoAs, Normal Law would be "chasing" the AoA, or even the Nz that it has been commanded to achieve by the sidestick position.
"I learned a long time ago that if you were entering turbulent air that has a lot of vertical component, that you need to work on holding pitch and power for the proper "turbulent air penetration" airspeed. You could expect both A/S and altitude to vary a bit as the air mass bounces you about.
'Don't chase it!' was some of the teaching then. Is that still current teaching?"
Afraid I'm the wrong person to ask about current teaching! But you raise a very valid point - in line with my own experience.
However, I think the only "chasing" by the EFCS/FBW visible in the DFDR traces is of the AoA when it is in AoA Law? I can only suggest that perhaps that is inevitable, bearing in mind the (presumably) narrow margin between Alpha-Prot and Alpha-MAX at cruise Mach - not to mention the stall. (That is: I'm out of my depth here, call an aerodynamicist!)
I doubt that, at normal cruise speeds and AoAs, Normal Law would be "chasing" the AoA, or even the Nz that it has been commanded to achieve by the sidestick position.
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Aeroperu 601 with blocked static ports worse than AF447
Although the post to which you were replying incorrectly stated that the Aeroperu 603 crash was caused by the aircraft stalling on account of the wrong PITCH was incorrect, the aircraft did finally stall.
As pointed out in my book Air Crashes and Miracle Landings--60 Narratives the Aeroperu pilots flew out over the sea for safety (and presumably because there would be no mountains there did not use their radio altimeter). Not realizing that the altitude of 9,700 ft confirmed by ATC was merely that given out by their transponder in turn dependent on the barometric altimeter (not working properly due to the blocked static ports) flew into the sea.
The aircraft bounced and rose 200 ft before STALLING, inverting and falling back into the sea. Although one of engines had ingested some water on initial impact with the sea and was impaired, it might have been have been a different story had the engines not already been throttled back due to a spurious overspeed warning and had no time to spool up.
As you rightly said, blockage of the static ports is even more serious than blockage of the pitots. The AF447 pilots could see that after a considerable climb (causing the a/c to stall) they were falling downwards which is more than the Aeroperu pilots could. In the circumstances they (Aeroperu pilots) did well.
As pointed out in my book Air Crashes and Miracle Landings--60 Narratives the Aeroperu pilots flew out over the sea for safety (and presumably because there would be no mountains there did not use their radio altimeter). Not realizing that the altitude of 9,700 ft confirmed by ATC was merely that given out by their transponder in turn dependent on the barometric altimeter (not working properly due to the blocked static ports) flew into the sea.
The aircraft bounced and rose 200 ft before STALLING, inverting and falling back into the sea. Although one of engines had ingested some water on initial impact with the sea and was impaired, it might have been have been a different story had the engines not already been throttled back due to a spurious overspeed warning and had no time to spool up.
As you rightly said, blockage of the static ports is even more serious than blockage of the pitots. The AF447 pilots could see that after a considerable climb (causing the a/c to stall) they were falling downwards which is more than the Aeroperu pilots could. In the circumstances they (Aeroperu pilots) did well.
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ChrisScott
In posts abot A340 incidents I want to clarify some misunderstanding about FBW control laws.
Alpha Lock function cannot be triggerred in clean configuration. If anything it was alpha floor protection and experienced pilot should be able to recall if it was engaged because he would have manually disconnected it.
In normal law from Valpha prot to alpha Max side stick commands alpha directly which is desirable because it gives the pilot direct control over AOA for recovery.
Alpha prot is not same value when below Vls and when at cruise speeds. In cruise it is about 5 degrees.
Once in alpha prot with stick neutral AOA does not increase but is decreased to Valpha prot. In this case phase advanced means it triggered before 5 degrees may be 2or 3 degrees. So FBW will try to maintain that and not increase as you suggest.
Manual flying skill is not a panacea for automated aircraft problems. Manually flying Airbus is very easy but it is more important to understand automation thoroughly. in this case the experienced pilot had he understood the auotmation he would not have wasted precious time trying to engage autopilot which does not engage when the protection is active.
It is necessary to study each line and between the lines of Autoflight system and Flight control system. Merely Flying 4 sectors manually everyday does not necessarily make you a better Airbus pilot. When the speed is below V alpha prot you should know that side stick is commanding alpha there is no need for any further display.
In posts abot A340 incidents I want to clarify some misunderstanding about FBW control laws.
Alpha Lock function cannot be triggerred in clean configuration. If anything it was alpha floor protection and experienced pilot should be able to recall if it was engaged because he would have manually disconnected it.
In normal law from Valpha prot to alpha Max side stick commands alpha directly which is desirable because it gives the pilot direct control over AOA for recovery.
Alpha prot is not same value when below Vls and when at cruise speeds. In cruise it is about 5 degrees.
Once in alpha prot with stick neutral AOA does not increase but is decreased to Valpha prot. In this case phase advanced means it triggered before 5 degrees may be 2or 3 degrees. So FBW will try to maintain that and not increase as you suggest.
Manual flying skill is not a panacea for automated aircraft problems. Manually flying Airbus is very easy but it is more important to understand automation thoroughly. in this case the experienced pilot had he understood the auotmation he would not have wasted precious time trying to engage autopilot which does not engage when the protection is active.
It is necessary to study each line and between the lines of Autoflight system and Flight control system. Merely Flying 4 sectors manually everyday does not necessarily make you a better Airbus pilot. When the speed is below V alpha prot you should know that side stick is commanding alpha there is no need for any further display.
Thanks to Chris, Don, and vilas. Makes a bit more sense now, with all of that presented. 
I'm having flashbacks to the line in "A Few Good Men" when Tom Cruise is asked, "Is that clear?"
And he says, "Crystal".
Well I'll tell ya folks, even if you pitch up from 35000 feet and 265 KCAS to 20 degrees nose high with idle thrust, the lowest Mach you will see before 'prot' finally gets the nose down is about 0.58....no alpha floor.
And 0.67 was the lowest Mach recorded in the near miss.
Someone please 'splain' to me how it's possible to get alpha floor activated above 0.53.
And he says, "Crystal".
Well I'll tell ya folks, even if you pitch up from 35000 feet and 265 KCAS to 20 degrees nose high with idle thrust, the lowest Mach you will see before 'prot' finally gets the nose down is about 0.58....no alpha floor.
And 0.67 was the lowest Mach recorded in the near miss.
Someone please 'splain' to me how it's possible to get alpha floor activated above 0.53.
Hello vilas,
Thanks for listing many of the areas we have been discussing in relation to the A340 AIRPROX and (since yesterday) the AF A340 Incident, and summarising your understanding of them.
I particularly welcome your explanation of the Alpha Lock and Alpha Floor functions in relation to the captain's statement to the UK AAIB after the A340 AIRPROX incident. Some of us are still trying to confirm that Alpha Floor could be engaged at a speed greater than M0.53. Do you have any information?
Some of your points, however, contain significant differences from my understanding. I shall now quote them in the order of your post, and respond.
"In normal law from Valpha prot to alpha Max side stick commands alpha directly..."
I think you are referring to AoA (Protection) Law, not Normal Law? ***
"Once in alpha prot with stick neutral AOA does not increase but is decreased to Valpha prot."
I think that would be true if AoA (Protection) Law was triggered as the AoA was increasing through Alpha-Prot, but not necessarily true if the trigger criterion was phase-advanced Alpha-Prot, i.e., below Alpha-Prot.
"...phase advanced means it [is] triggered before 5 degrees may be 2or 3 degrees. So FBW will try to maintain that and not increase as you suggest."
How could it do that? I think you may be misunderstanding the principle of phase-advanced Alpha-Prot. Even if you do understand it, I think it may be helpful at this point to share my own (tentative) understanding in case it helps some of our readers. No doubt better brains than mine will correct me if necessary.
"Phase-advanced" stall-protection existed, to my knowledge, nearly 50 years ago on the VC10 and BAC 1-11. On Airbus FBW, when the AoA is increasing rapidly in Normal Law, the EFCS/FBW needs to anticipate the exceedance of Alpha-Prot and trigger/engage AoA (Protection) Law in "advance". So it is constantly extrapolating what the AoA may be a short time after the present. We do not know the precise figure of that time-period, but it is probably between 1 and 2 seconds. That means that the precise AoA at the moment of engagement is a function of the rate-of-change of AoA (i.e., delta-alpha), as well as the current value of Alpha-Prot. Delta-alpha obviously depends on a number of variables, including recent/current pilot inputs and gusts. Therefore, for the EFCS to use the engagement AoA as the target for the duration of the period of AoA Law would seem unlikely and inappropriate.
I stand to be corrected, but my money is on the EFCS targeting current Alpha-Prot - which is variable for the reasons you have implied.
"When the speed is below V alpha prot you should know that side stick is commanding alpha there is no need for any further display."
True but - sitting comfortably here on terra-firma - I would simply remind you that the speed indications on the PFD, excellent as they are, may be quite difficult to interpret when you and they are jumping up and down in severe turbulence?
*** (added by edit)
Apologies, vilas! I now see from DonH's post (below) that you are right. Although the AAIB Bulletin into the A340 AIRPROX incident of 2001 refers to it as "AoA (Protection) Law", the FCOM extracts from both 2001 and 2010 that DonH displays clearly refer to it as the "High Angle of Attack Protection" mode of Normal Law.
Thanks for listing many of the areas we have been discussing in relation to the A340 AIRPROX and (since yesterday) the AF A340 Incident, and summarising your understanding of them.
I particularly welcome your explanation of the Alpha Lock and Alpha Floor functions in relation to the captain's statement to the UK AAIB after the A340 AIRPROX incident. Some of us are still trying to confirm that Alpha Floor could be engaged at a speed greater than M0.53. Do you have any information?
Some of your points, however, contain significant differences from my understanding. I shall now quote them in the order of your post, and respond.
"In normal law from Valpha prot to alpha Max side stick commands alpha directly..."
I think you are referring to AoA (Protection) Law, not Normal Law? ***
"Once in alpha prot with stick neutral AOA does not increase but is decreased to Valpha prot."
I think that would be true if AoA (Protection) Law was triggered as the AoA was increasing through Alpha-Prot, but not necessarily true if the trigger criterion was phase-advanced Alpha-Prot, i.e., below Alpha-Prot.
"...phase advanced means it [is] triggered before 5 degrees may be 2or 3 degrees. So FBW will try to maintain that and not increase as you suggest."
How could it do that? I think you may be misunderstanding the principle of phase-advanced Alpha-Prot. Even if you do understand it, I think it may be helpful at this point to share my own (tentative) understanding in case it helps some of our readers. No doubt better brains than mine will correct me if necessary.
"Phase-advanced" stall-protection existed, to my knowledge, nearly 50 years ago on the VC10 and BAC 1-11. On Airbus FBW, when the AoA is increasing rapidly in Normal Law, the EFCS/FBW needs to anticipate the exceedance of Alpha-Prot and trigger/engage AoA (Protection) Law in "advance". So it is constantly extrapolating what the AoA may be a short time after the present. We do not know the precise figure of that time-period, but it is probably between 1 and 2 seconds. That means that the precise AoA at the moment of engagement is a function of the rate-of-change of AoA (i.e., delta-alpha), as well as the current value of Alpha-Prot. Delta-alpha obviously depends on a number of variables, including recent/current pilot inputs and gusts. Therefore, for the EFCS to use the engagement AoA as the target for the duration of the period of AoA Law would seem unlikely and inappropriate.
I stand to be corrected, but my money is on the EFCS targeting current Alpha-Prot - which is variable for the reasons you have implied.
"When the speed is below V alpha prot you should know that side stick is commanding alpha there is no need for any further display."
True but - sitting comfortably here on terra-firma - I would simply remind you that the speed indications on the PFD, excellent as they are, may be quite difficult to interpret when you and they are jumping up and down in severe turbulence?
*** (added by edit)
Apologies, vilas! I now see from DonH's post (below) that you are right. Although the AAIB Bulletin into the A340 AIRPROX incident of 2001 refers to it as "AoA (Protection) Law", the FCOM extracts from both 2001 and 2010 that DonH displays clearly refer to it as the "High Angle of Attack Protection" mode of Normal Law.
Last edited by Chris Scott; 7th Nov 2013 at 20:03. Reason: *** added.
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Chris Scott, Re, "Think the report says the copilot only pressed the AP instinctive-disconnect PB on his sidestick once (top of Page 13)?"
Yes, agree thanks - I had misread the para. on P13, believing that the button had been pressed a second time.
To clarify for OK465 regarding the "AP OFF" ECAM message, some collective in-depth research through historical FCOMs has revealed nothing in the way of an amber ECAM message - the "AP OFF" ECAM message is red, no matter what conditions, (instinctive disconnect on the SS, pushing the AP1/2 button, moving the SS when AP engaged, aircraft self-disconnect) the AP was disconnected under.
- edited to add: OK465, Re, "Someone please 'splain' to me how it's possible to get alpha floor activated above 0.53.
"
Alpha-floor is an A/THR mode - (in that the A/THR is provided with AoA information and invokes Alpha-floor when other conditions are satisfied). Alpha floor is inhibited above M0.53 because, (I'm guessing here!) there probably isn't sufficient thrust to respond alone to an alpha-prot event, (meaning, the pitch must also be reduced to unload the wing/reduce AoA).
Alpha-prot on the other hand is a pitch flight-law reversion from Nz (load factor) to AoA and is not inhibited at any time, (but, logically, isn't available outside of Normal Law). This may explain why the recent "A340-2011" event moved into, and out of "Alpha-prot" several times.
-end edit
HN39, Re, "I do remember that the disengagement conditions were modified after the AIRPROX incident, but I don't remember the details. "
It appears as though the disengagement conditions were changed some time after June 01 2001 and prior to July 01 2010:
Yes, agree thanks - I had misread the para. on P13, believing that the button had been pressed a second time.
To clarify for OK465 regarding the "AP OFF" ECAM message, some collective in-depth research through historical FCOMs has revealed nothing in the way of an amber ECAM message - the "AP OFF" ECAM message is red, no matter what conditions, (instinctive disconnect on the SS, pushing the AP1/2 button, moving the SS when AP engaged, aircraft self-disconnect) the AP was disconnected under.
- edited to add: OK465, Re, "Someone please 'splain' to me how it's possible to get alpha floor activated above 0.53.
"Alpha-floor is an A/THR mode - (in that the A/THR is provided with AoA information and invokes Alpha-floor when other conditions are satisfied). Alpha floor is inhibited above M0.53 because, (I'm guessing here!) there probably isn't sufficient thrust to respond alone to an alpha-prot event, (meaning, the pitch must also be reduced to unload the wing/reduce AoA).
Alpha-prot on the other hand is a pitch flight-law reversion from Nz (load factor) to AoA and is not inhibited at any time, (but, logically, isn't available outside of Normal Law). This may explain why the recent "A340-2011" event moved into, and out of "Alpha-prot" several times.
-end edit
HN39, Re, "I do remember that the disengagement conditions were modified after the AIRPROX incident, but I don't remember the details. "
It appears as though the disengagement conditions were changed some time after June 01 2001 and prior to July 01 2010:
Last edited by Jetdriver; 7th Nov 2013 at 23:39.
High AoA Protection mode - disengagement conditions (cont'd)
'Evening DonH,
See you've finally confirmed by the 2010 FCOM the third condition that was in the BEA Report of 2011 into the AF A340 Incident - but absent from the AAIB Bulletin about the A340 AIRPROX.
Thanks for posting the relevant extracts of a 2010 FCOM and a 2001 FCOM. We can infer that the 3rd condition/criterion may have been added as a result of the A340 AIRPROX event, although the AAIB's recommendations did not include it.
As one who has recently been arguing for easier disengagement conditions than those that seem to have applied at the time of the A340 AIRPROX, I must reiterate that the AF A340 Incident of 2011 does little to support that contention, as I admitted yesterday.
Re. Alpha Floor inhibition
Do you have access to FCOM 2.22.30? Does it mention an inhibition at speed > M0.53?
See you've finally confirmed by the 2010 FCOM the third condition that was in the BEA Report of 2011 into the AF A340 Incident - but absent from the AAIB Bulletin about the A340 AIRPROX.
Thanks for posting the relevant extracts of a 2010 FCOM and a 2001 FCOM. We can infer that the 3rd condition/criterion may have been added as a result of the A340 AIRPROX event, although the AAIB's recommendations did not include it.
As one who has recently been arguing for easier disengagement conditions than those that seem to have applied at the time of the A340 AIRPROX, I must reiterate that the AF A340 Incident of 2011 does little to support that contention, as I admitted yesterday.
Re. Alpha Floor inhibition
Do you have access to FCOM 2.22.30? Does it mention an inhibition at speed > M0.53?
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Hello Chris Scott, Re, "Re. Alpha Floor inhibition
Do you have access to FCOM 2.22.30? Does it mention an inhibition at speed > M0.53? "
Yes, M0.53 is mentioned in relation to an A/THR Reversion Mode, (but not a change from Nz to AoA law - that is a different matter, I think): - from 2.22.30 in an FCOM:
I have to say that the differences between "Alpha-prot" and "Alpha-floor" have eluded me all these years, perhaps even still !
In addition to the FCOM descriptions of both, I think there is some imprecise use of terms in the FCOM, specifically the word, "protection". The word is used in both contexts to describe "prot" and "floor" and so is informal, whereas the terms alpha-prot, (or "α prot") and alpha-floor are formal terms.
I think the words, "prot" and "floor" are being understood as inter-changeable but am thinking that they are not. I think it is significant that "alpha-floor" is discussed only in ATA22 (Autoflight) but "alpha-prot" is not. Alpha-prot is only discussed in ATA27 (Flight Controls), with "alpha-floor" only being used in the context of, and with reference to, "alpha-prot".
Put differently, I think one can be in "alpha-prot", (a pitch-law mode) without being in alpha-floor, (an A/THR mode), but one cannot be in alpha-floor without first having "gone through" alpha-prot. The schematic in the graphic above even shows this.
So you can be in "alpha-prot" at any Mach number, but alpha-floor mode is restricted as described in the para. from the FCOM.
The important key to understanding this and using all this information from a pilot's pov is to remember what Chris Scott has outlined in the discussion on "disengagement of the AoA law". Both the AirProx and the A340 2011 Events were controllable either by leaving the stick alone, (but that leaves the airplane to "select" Max AoA in a phase-advanced scenario and is therefore not a good option), or to take control to gently push the stick forward to take the airplane back to Nz law (because, as has been observed previously, one is 'unloading the wing', so to speak) and deal with the airprox on the one hand, and sort out the mess the crew of the A340 - 2011 made for themselves on the other.
Do you have access to FCOM 2.22.30? Does it mention an inhibition at speed > M0.53? "
Yes, M0.53 is mentioned in relation to an A/THR Reversion Mode, (but not a change from Nz to AoA law - that is a different matter, I think): - from 2.22.30 in an FCOM:
A/THR MODE REVERSION
For detail refer to “Speed mode protection”.
Alpha floor
The ALPHA FLOOR protection is triggered when the FMGCs receive a signal elaborated by the PRIMs. This signal is sent when the aircraft angle of attack is above a predetermined threshold function of the aircraft configuration. The A/THR is automatically activated and commands TOGA thrust regardless of thrust lever positions. This protection is available from lift off to 100 feet RA in approach.
Following indications are then provided :
– A-FLOOR on the FMA and on the EWD as long as α floor conditions are met;
– TOGA LK on the FMA when the aircraft leaves the α floor conditions.
TOGA thrust is then frozen.
A FLOOR and TOGA LK are displayed in green and surrounded by an amber flashing box.
In order to cancel the ALPHA FLOOR or TOGA LK thrust, disconnect the A/THR.
Note: Alpha Floor is inhibited :
– in case of engine failure with flaps extended
– in case of engine failure with derated TO selected
– below 100 feet at landing
– above M.53
Alpha Floor protection is lost in case of A/THR failure.
For detail refer to “Speed mode protection”.
Alpha floor
The ALPHA FLOOR protection is triggered when the FMGCs receive a signal elaborated by the PRIMs. This signal is sent when the aircraft angle of attack is above a predetermined threshold function of the aircraft configuration. The A/THR is automatically activated and commands TOGA thrust regardless of thrust lever positions. This protection is available from lift off to 100 feet RA in approach.
Following indications are then provided :
– A-FLOOR on the FMA and on the EWD as long as α floor conditions are met;
– TOGA LK on the FMA when the aircraft leaves the α floor conditions.
TOGA thrust is then frozen.
A FLOOR and TOGA LK are displayed in green and surrounded by an amber flashing box.
In order to cancel the ALPHA FLOOR or TOGA LK thrust, disconnect the A/THR.
Note: Alpha Floor is inhibited :
– in case of engine failure with flaps extended
– in case of engine failure with derated TO selected
– below 100 feet at landing
– above M.53
Alpha Floor protection is lost in case of A/THR failure.
In addition to the FCOM descriptions of both, I think there is some imprecise use of terms in the FCOM, specifically the word, "protection". The word is used in both contexts to describe "prot" and "floor" and so is informal, whereas the terms alpha-prot, (or "α prot") and alpha-floor are formal terms.
I think the words, "prot" and "floor" are being understood as inter-changeable but am thinking that they are not. I think it is significant that "alpha-floor" is discussed only in ATA22 (Autoflight) but "alpha-prot" is not. Alpha-prot is only discussed in ATA27 (Flight Controls), with "alpha-floor" only being used in the context of, and with reference to, "alpha-prot".
Put differently, I think one can be in "alpha-prot", (a pitch-law mode) without being in alpha-floor, (an A/THR mode), but one cannot be in alpha-floor without first having "gone through" alpha-prot. The schematic in the graphic above even shows this.
So you can be in "alpha-prot" at any Mach number, but alpha-floor mode is restricted as described in the para. from the FCOM.
The important key to understanding this and using all this information from a pilot's pov is to remember what Chris Scott has outlined in the discussion on "disengagement of the AoA law". Both the AirProx and the A340 2011 Events were controllable either by leaving the stick alone, (but that leaves the airplane to "select" Max AoA in a phase-advanced scenario and is therefore not a good option), or to take control to gently push the stick forward to take the airplane back to Nz law (because, as has been observed previously, one is 'unloading the wing', so to speak) and deal with the airprox on the one hand, and sort out the mess the crew of the A340 - 2011 made for themselves on the other.
Alpha-Floor inhibition above Mach 0.53
Thanks Don,
That nails it in line with what OK465 has been telling us all along?
My (perhaps simplistic) understanding has always been that Alpha-Prot and Alpha-MAX relate to the flight control laws, whereas Alpha-Floor is purely an A/THR mode, albeit an extremely powerful one at low altitudes.***
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...
Quote from OK465:
Well I'll tell ya folks, even if you pitch up from 35000 feet and 265 KCAS to 20 degrees nose high with idle thrust, the lowest Mach you will see before 'prot' finally gets the nose down is about 0.58....no alpha floor.
YES...!
***(by edit)
You may not be aware that AI originally introduced Alpha-Floor on the A310 in 1983. The A310 has conventional flying controls (except for the roll-spoilers, which are FBW). So, on the A310 and A300-600, you have to push very hard if Alpha-Floor engages...
That nails it in line with what OK465 has been telling us all along?
My (perhaps simplistic) understanding has always been that Alpha-Prot and Alpha-MAX relate to the flight control laws, whereas Alpha-Floor is purely an A/THR mode, albeit an extremely powerful one at low altitudes.***
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...
Quote from OK465:
Well I'll tell ya folks, even if you pitch up from 35000 feet and 265 KCAS to 20 degrees nose high with idle thrust, the lowest Mach you will see before 'prot' finally gets the nose down is about 0.58....no alpha floor.
YES...!
***(by edit)
You may not be aware that AI originally introduced Alpha-Floor on the A310 in 1983. The A310 has conventional flying controls (except for the roll-spoilers, which are FBW). So, on the A310 and A300-600, you have to push very hard if Alpha-Floor engages...
Last edited by Chris Scott; 8th Nov 2013 at 09:48. Reason: "...at low altitudes" added. *** added. "...it was widely thought that..." added.
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Hello Chris;
Re, "That nails it in line with what OK465 has been telling us all along?"
Yes, I think so! So I probably mis-read his comment about 'splainin' how alpha-floor can occur below M0.53! - it was a statement, not a question! Ahh, me.
Re, "That nails it in line with what OK465 has been telling us all along?"
Yes, I think so! So I probably mis-read his comment about 'splainin' how alpha-floor can occur below M0.53! - it was a statement, not a question! Ahh, me.
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Originally Posted by Chris Scott
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 one fellow captain did not take that into account during his cavalier fly-by at Habsheim...
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Originally Posted by HN39
Both incidents have been discussed on PPRuNe when the respective reports were published. I do remember that the disengagement conditions were modified after the AIRPROX incident, but I don't remember the details.
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@ DonH
AlphaFoor is strictly A/THR and only triggers within certain parameters.
It also triggers based on a set of variables not just indicated airspeed.
No relation to AlphaProts in flight controls other than Normal Law must be active.
AlphaFoor is strictly A/THR and only triggers within certain parameters.
It also triggers based on a set of variables not just indicated airspeed.
No relation to AlphaProts in flight controls other than Normal Law must be active.