Habsheim
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If not yet read .. you can read this Airbus report
"The A320 Habcheim Accident
An Airbus Industrie response to allegations made in television programme and other medias"
Rapport Airbus.pdf - Petit Fichier
"The A320 Habcheim Accident
An Airbus Industrie response to allegations made in television programme and other medias"
Rapport Airbus.pdf - Petit Fichier
Btw., for anyone frustrated with the poor quality PDF of the BEA report, here's a HTML version of it: Habsheim F-GFKC
Last edited by noske; 7th Jan 2014 at 12:43. Reason: unintentional double entendre
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Hi Chris,
I can't find a document to support it, except on page 17 of http://lessonslearned.faa.gov/Indian...0Condition.pdf
"Several commenters note that the new VS1 usage has not been uniformly applied throughout all Part 25 subparts. As configured for operation, the A320 is prevented from stalling by the incorporation of an angle-of-attack limiting feature. This feature would then not allow demonstration of the stall speeds used for structural design. During development flight testing, Airbus deactivated the stall protection feature and demonstrated compliance with the existing rules as currently interpreted both in the clean and flaps down configurations. Therefore, the special condition was not proposed for Subparts C&D."
Besides forcing the aircraft into ALT Law, how else do you think they deactivated the stall protection feature?
What is the source of info for your assertion that the stall test for certification was done in Pitch Alternate Law?
"Several commenters note that the new VS1 usage has not been uniformly applied throughout all Part 25 subparts. As configured for operation, the A320 is prevented from stalling by the incorporation of an angle-of-attack limiting feature. This feature would then not allow demonstration of the stall speeds used for structural design. During development flight testing, Airbus deactivated the stall protection feature and demonstrated compliance with the existing rules as currently interpreted both in the clean and flaps down configurations. Therefore, the special condition was not proposed for Subparts C&D."
Besides forcing the aircraft into ALT Law, how else do you think they deactivated the stall protection feature?
Thread Starter
Could alpha-max vary with Nz ?
Quote from CONF_iture:
At time 1035 in the video, the Airbus test pilot clearly states :
"We're close to 15 deg AoA which is the maximum for this configuration"
Thanks for reminding me. I assumed you had a paper reference.
Interpreting GC's introductory speech, the video seems to date from mid-1989. I agree with you that, in 1988/9, the standard landing configuration was Flaps/Config FULL, even single-engined (subject to its satisfying the WAT figure for approach-climb performance). So the only issue is whether GC's figure of 15 deg was a reference to alpha-max. Taken on face value, it would appear to be so, but - as rudderrudderrat and I have previously discussed here - the AoA indicator later exceeds 15 for some time while GC is demonstrating the ability of the a/c to bank with full back-stick.
During the deceleration, alpha-floor appears on the FMA as GC points out an AoA of +14 (time 9:54), which is close to my Tech Manual figure of 14.5, and is quickly cancelled. The AoA seems to stabilise at +15 while the wings are level, but quickly increases to a peak of nearly 17 deg when the a/c is rolled to about 25 deg of left bank. This increase is quickly acknowledged by GC, who gestures (10:53) at the AoA indicator with his pointer, saying "... slightly increased angle of attack, it's gone to sixteen degrees... maintain that bank angle..." without any indication of surprise in his intonation.
So, despite having stated previously that 15 degrees AoA was the "...maximum for this configuration", he was not surprised or concerned that it exceeded that figure as soon as bank was applied. Unless we can find evidence that alpha-max is merely a target maximum for the EFCS when the stick is fully back, rather than an absolute maximum, I think we need to treat with caution the 15 deg figure of alpha-max for Config FULL.
Is it possible that the EFCS might retain a margin of a couple of degrees from alpha-max when the wings are level, in order to retain manoeuvre capability? Such a margin from alpha-max would cater for the case where the pilot needs to turn suddenly and unexpectedly after alpha-max has already been reached, in which case allowing bank with an associated increase in Nz (normal acceleration) might lead to the stall. If such a margin was employed, that might explain the apparent shortfall of AoA in the (wings-level) Habsheim case.
At time 1035 in the video, the Airbus test pilot clearly states :
"We're close to 15 deg AoA which is the maximum for this configuration"
Thanks for reminding me. I assumed you had a paper reference.
Interpreting GC's introductory speech, the video seems to date from mid-1989. I agree with you that, in 1988/9, the standard landing configuration was Flaps/Config FULL, even single-engined (subject to its satisfying the WAT figure for approach-climb performance). So the only issue is whether GC's figure of 15 deg was a reference to alpha-max. Taken on face value, it would appear to be so, but - as rudderrudderrat and I have previously discussed here - the AoA indicator later exceeds 15 for some time while GC is demonstrating the ability of the a/c to bank with full back-stick.
During the deceleration, alpha-floor appears on the FMA as GC points out an AoA of +14 (time 9:54), which is close to my Tech Manual figure of 14.5, and is quickly cancelled. The AoA seems to stabilise at +15 while the wings are level, but quickly increases to a peak of nearly 17 deg when the a/c is rolled to about 25 deg of left bank. This increase is quickly acknowledged by GC, who gestures (10:53) at the AoA indicator with his pointer, saying "... slightly increased angle of attack, it's gone to sixteen degrees... maintain that bank angle..." without any indication of surprise in his intonation.
So, despite having stated previously that 15 degrees AoA was the "...maximum for this configuration", he was not surprised or concerned that it exceeded that figure as soon as bank was applied. Unless we can find evidence that alpha-max is merely a target maximum for the EFCS when the stick is fully back, rather than an absolute maximum, I think we need to treat with caution the 15 deg figure of alpha-max for Config FULL.
Is it possible that the EFCS might retain a margin of a couple of degrees from alpha-max when the wings are level, in order to retain manoeuvre capability? Such a margin from alpha-max would cater for the case where the pilot needs to turn suddenly and unexpectedly after alpha-max has already been reached, in which case allowing bank with an associated increase in Nz (normal acceleration) might lead to the stall. If such a margin was employed, that might explain the apparent shortfall of AoA in the (wings-level) Habsheim case.
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rrr
It is not unusual for the software used on flight test airplanes to be more flexible than the production version. This is to allow gains etc to be tweaked easily for test purposes. So it may be that they simply inhibited the incidence protection software for stall tests.
Besides forcing the aircraft into ALT Law, how else do you think they deactivated the stall protection feature?
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The question is not whether the airplane was tested beyond the arbitrarily chosen alpha-max incorporated in the FCS software in normal law. Of course it was flight-tested beyond alpha-max, either in alternate law, direct law or with some 'experimental' development version of the FCS software as suggested by OwainG.
The surprise for me is that apparently the special conditions under which the A320 was certified specify operational speeds such as V2 and Vref referenced to a Vs1g that can not be demonstrated with its systems operating as designed, i.e. in normal law. In other words, alpha-max is not determining Vs1g as it would have been under later regulation, but was selected somewhat independently.
The surprise for me is that apparently the special conditions under which the A320 was certified specify operational speeds such as V2 and Vref referenced to a Vs1g that can not be demonstrated with its systems operating as designed, i.e. in normal law. In other words, alpha-max is not determining Vs1g as it would have been under later regulation, but was selected somewhat independently.
Thread Starter
Certification of Vs
Hi rudderrudderrat,
Thanks for that very interesting link, and I note Owain Glyndwr's quick response that reversion to Pitch Alternate may not have been necessary during the certification flights. Experience suggests he is usually right, in which case I infer that Normal Law and Alpha-Protection would have been retained, but alpha-max modified to a figure at or above Cz-max. Alpha-Floor would also be inhibited. If so, this would have the advantage that THS (pitch) up-trimming would cease at alpha-prot. That would make the exercise much safer than using Pitch Alternate, in which - as you have pointed out - the THS would continue trimming to the stall and beyond (as it did later on AF447).
I am told that there have traditionally been three alternative scenarios for establishing figures of Vs during the certification of aeroplanes. In each case the a/c is flown straight and level (Nz = 1.0g) for as long as possible with a deceleration of 1 kt/sec. The following are very rough, layman's descriptions for OG, HN39 and others to correct/amend/expand:
(1) If full back stick/column is reached before the stall, Vs is defined as the lowest speed achieved. (Normally, that definition applies only if elevator authority is insufficient to achieve a classic stall. However, it would have applied to the A320 in the Alpha-Protection mode of Normal Law for artificial reasons.)
(2) The a/c is maintained at 1g until the stall is denoted by a marked "break" (nose-drop), at which point the speed is nominated as the Vs. (This was the traditional British method for BCAR certification.)
(3) As for (2), but the a/c was allowed to execute a mild bunt, probably starting momentarily before the 1-g break, with continuing deceleration, until the Nz fell to a certain level. At that point the resulting speed was nominated as the Vs, sometimes described as the "minimum speed in the stall", provided it was not more than a certain percentage below the Vs1g. (This was the traditional American method for FAR certification.)
The FAR method (3), with its lower values of Vs, gave American a/c a commercial advantage over British a/c when field performance was being considered, because the different regulators applied the same factors to Vs for the calculations of V2 (1.2 Vs) and Vref (1.3 Vs). I always wondered why - on a calm day when the numbers were right - one could confidently close the throttles at 50 ft on a VC10 or BAC1-11, but wouldn't even consider it on a B707.
Sometime in the 1980s, there is said to have been a levelling of the playing field by agreement, but I notice that my stall graphs for the A310 - the last Airbus type to be certificated before the A320 - are headed "VS MINIMUM". So did the DGAC use FAR criteria for the A310 certification?
So to the A320. The version of the cryptic, ubiquitous curve of CL-versus-alpha that you posted 4 days ago is annotated "1G stall" over the apogee associated with CL max (unlike the versions of Cz-versus-alpha in the early FCOMs, which omit it). Although John Farley pointed out (on seeing it earlier) that the caption was redundant and potentially misleading, AI was presumably trying to send a message that their Vs is taken (a) at CLmax and (b) at an Nz of 1G, not a lower figure.
Thanks for that very interesting link, and I note Owain Glyndwr's quick response that reversion to Pitch Alternate may not have been necessary during the certification flights. Experience suggests he is usually right, in which case I infer that Normal Law and Alpha-Protection would have been retained, but alpha-max modified to a figure at or above Cz-max. Alpha-Floor would also be inhibited. If so, this would have the advantage that THS (pitch) up-trimming would cease at alpha-prot. That would make the exercise much safer than using Pitch Alternate, in which - as you have pointed out - the THS would continue trimming to the stall and beyond (as it did later on AF447).
I am told that there have traditionally been three alternative scenarios for establishing figures of Vs during the certification of aeroplanes. In each case the a/c is flown straight and level (Nz = 1.0g) for as long as possible with a deceleration of 1 kt/sec. The following are very rough, layman's descriptions for OG, HN39 and others to correct/amend/expand:
(1) If full back stick/column is reached before the stall, Vs is defined as the lowest speed achieved. (Normally, that definition applies only if elevator authority is insufficient to achieve a classic stall. However, it would have applied to the A320 in the Alpha-Protection mode of Normal Law for artificial reasons.)
(2) The a/c is maintained at 1g until the stall is denoted by a marked "break" (nose-drop), at which point the speed is nominated as the Vs. (This was the traditional British method for BCAR certification.)
(3) As for (2), but the a/c was allowed to execute a mild bunt, probably starting momentarily before the 1-g break, with continuing deceleration, until the Nz fell to a certain level. At that point the resulting speed was nominated as the Vs, sometimes described as the "minimum speed in the stall", provided it was not more than a certain percentage below the Vs1g. (This was the traditional American method for FAR certification.)
The FAR method (3), with its lower values of Vs, gave American a/c a commercial advantage over British a/c when field performance was being considered, because the different regulators applied the same factors to Vs for the calculations of V2 (1.2 Vs) and Vref (1.3 Vs). I always wondered why - on a calm day when the numbers were right - one could confidently close the throttles at 50 ft on a VC10 or BAC1-11, but wouldn't even consider it on a B707.
Sometime in the 1980s, there is said to have been a levelling of the playing field by agreement, but I notice that my stall graphs for the A310 - the last Airbus type to be certificated before the A320 - are headed "VS MINIMUM". So did the DGAC use FAR criteria for the A310 certification?
So to the A320. The version of the cryptic, ubiquitous curve of CL-versus-alpha that you posted 4 days ago is annotated "1G stall" over the apogee associated with CL max (unlike the versions of Cz-versus-alpha in the early FCOMs, which omit it). Although John Farley pointed out (on seeing it earlier) that the caption was redundant and potentially misleading, AI was presumably trying to send a message that their Vs is taken (a) at CLmax and (b) at an Nz of 1G, not a lower figure.
Last edited by Chris Scott; 9th Jan 2014 at 19:49. Reason: Codition (1) extended. Condition (3) corrected in the light of HN39's post. 9/1: amendments to (1), and speculative values removed from (3) and last para.
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Just for accuracy, a few comments in response to Chris Scott's post #270):
Regarding Chris' stall condition (1), I don't think it is such a 'rarity' even for conventional airplanes.
Regarding Chris' (2) I am somewhat sceptical but can't comment because I wasn't involved in BCAR certifications in those days. However, strictly speaking the so called g-break speed is close to but not equal to the 1-g stall speed, and both are not obtained at 1-g in the stall maneuver. The 1-g stall speed is the lowest speed at which wing lift alone can support the weight of the airplane in 1-g flight. It is defined by the point in the stall maneuver at which the lift coefficient cL is at a maximum. At that point the normal acceleration is usually not equal to 1, and the reference stall speed is then calculated as Vs1g = V(cLmax) / √(nz)
Regarding Chris' (3) I would observe that, although the minimum speed in the stall maneuver is usually at less than 1 g, it is not obtained at 0.94 g, nor at √0.94 g.
Regarding Chris' stall condition (1), I don't think it is such a 'rarity' even for conventional airplanes.
Regarding Chris' (2) I am somewhat sceptical but can't comment because I wasn't involved in BCAR certifications in those days. However, strictly speaking the so called g-break speed is close to but not equal to the 1-g stall speed, and both are not obtained at 1-g in the stall maneuver. The 1-g stall speed is the lowest speed at which wing lift alone can support the weight of the airplane in 1-g flight. It is defined by the point in the stall maneuver at which the lift coefficient cL is at a maximum. At that point the normal acceleration is usually not equal to 1, and the reference stall speed is then calculated as Vs1g = V(cLmax) / √(nz)
Regarding Chris' (3) I would observe that, although the minimum speed in the stall maneuver is usually at less than 1 g, it is not obtained at 0.94 g, nor at √0.94 g.
Last edited by HazelNuts39; 8th Jan 2014 at 08:57.
Thread Starter
Hello HN39,
Thanks for the useful corrections and clarifications to my attempted layman's guide to stall certification. particularly for pointing out my mistake in condition (3), in which my figure of minimum Nz appears to have been incorrect. I have amended my post, while noting that you will remain not entirely in agreement with it in principle.
Thanks also for reminding me that the British ARB's equivalent of FARs (and since replaced by European JARs), were the BCARs (not BARs, as I stated).
Thanks for the useful corrections and clarifications to my attempted layman's guide to stall certification. particularly for pointing out my mistake in condition (3), in which my figure of minimum Nz appears to have been incorrect. I have amended my post, while noting that you will remain not entirely in agreement with it in principle.
Thanks also for reminding me that the British ARB's equivalent of FARs (and since replaced by European JARs), were the BCARs (not BARs, as I stated).
Last edited by Chris Scott; 9th Jan 2014 at 20:10. Reason: Speculative figures of Nz and VSmin/VS1g deleted.
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What this means is that the Direct Law implementation (where PFC commands are directly translated to the surfaces) serves as the operational "core" of the EFCS. What came to be known as the Alternate Laws (C* functions and soft protections) were built on top of the Direct Law core, and Normal Law (hard protections and tweaked rate functions) built on top of those.
As OG hints at, the software used in development, which would have occurred both on the A300 testbed and A320 prototypes*, would likely have had logical "hooks" linked to switches on the test consoles and possibly in the flight deck allowing for easy transfer between the different laws, modes and the two distinct implementations - not just for testing and certification purposes, but also as a failsafe safety measure should there be a significant software problem with the 'higher' level implementations during testing. The software used in production would be identical and still have those logical hooks - but would have nothing connected to them on the production aircraft.
* - Along with the A330/340/380/350 prototypes later on.
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Originally Posted by Chris Scott
Is it possible that the EFCS might retain a margin of a couple of degrees from alpha-max when the wings are level, in order to retain manoeuvre capability?
A very temporary excursion over alpha max is acceptable and is part of the protection mode. The system will not try to achieve anything more than alpha max but it will tolerate an overshoot if turbulence or sudden turn happen when already established at alpha max and of course the system will correct to bring back the AoA where it should be, its alpha max value.
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Thanks for that very interesting link, and I note Owain Glyndwr's quick response that reversion to Pitch Alternate may not have been necessary during the certification flights. Experience suggests he is usually right, in which case I infer that Normal Law and Alpha-Protection would have been retained, but alpha-max modified to a figure at or above Cz-max. Alpha-Floor would also be inhibited. If so, this would have the advantage that THS (pitch) up-trimming would cease at alpha-prot. That would make the exercise much safer than using Pitch Alternate, in which - as you have pointed out - the THS would continue trimming to the stall and beyond (as it did later on AF447).
prototypes*, would likely have had logical "hooks" linked to switches on the test consoles and possibly in the flight deck allowing for easy transfer between the different laws, modes
to the "ordinary" pilots (and gums who is not an ordinary pilot !) ?
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The margin is already in alpha max and that is why alpha max has not been set to alpha stall and V alpha max is not Vs1g.
A very temporary excursion over alpha max is acceptable and is part of the protection mode. The system will not try to achieve anything more than alpha max but it will tolerate an overshoot if turbulence or sudden turn happen when already established at alpha max...
A very temporary excursion over alpha max is acceptable and is part of the protection mode. The system will not try to achieve anything more than alpha max but it will tolerate an overshoot if turbulence or sudden turn happen when already established at alpha max...
You mention the Airbus demonstrator saying/showing "the airplane will rapidly reach alpha max", but I bet the demonstrator said nothing about it reaching a precise 17.5 degrees of AoA with the same urgency. The manuals, both FCOM and FCTM, seem to shy away from giving precise quantitative values for limit and rate when describing the procedure and functions, and this makes sense because there will be occasions where to do so would be inadvisable (e.g. adverse weather and/or a decelerating aircraft).
I mentioned this before, but I'll point out again that the difference between Bilbao (where phugoid damping was an issue) and Habsheim (where it most likely wasn't) was a gusting tailwind causing a sudden (approx. 10kt in under a second) drop in IAS in the former case.
The demonstration video by Capt. Corps would likely have been seen by Capt. Asseline, and in all probability he'd have sat in on a similar demonstration using the prototype (with the AoA gauge) - therefore he'd likely have been aware that immediate response to full back-stick in the landing configurations with protection active would have been in the 15-16 degree range.
The absolute Alpha Max value of 17.5 degrees was mentioned (albeit obliquely) in the BEA report to which both Asseline and his legal team would have had access prior to the court cases. Given the myriad ways in which they tried to draw attention to Airbus and possible technical issues, one would think that the apparent discrepancy between that absolute value and the AoA achieved would have made a cornerstone of their case, but it wasn't even touched upon. The only reasons I can think of that this would be the case are either that they missed it (IMO unlikely), or that they knew such behaviour was normal and therefore not a technical fault or design flaw.
[EDIT
Why not put these switches on the consoles again to give observability and controllability to the "ordinary" pilots?
to the "ordinary" pilots?
PS. Gums is certainly not an ordinary pilot - I'd love to see him get a go in the A320 prototype one day to help him understand how it all fits together and why.
]
Last edited by DozyWannabe; 8th Jan 2014 at 15:54.
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CONF, (re: #274) agreed for turbulences, but we were discussing turns (which are not so "very temporary").
Dozy, as an attempt to illustrate your #273, here are two pictures of A400M cockpits :
- test aircraft: Photos: Airbus A400M Grizzly Aircraft Pictures | Airliners.net
- production aircraft: Photos: Airbus A400M Grizzly Aircraft Pictures | Airliners.net
On the test aircraft, between the standby ADI and the gear indicator, there are two guarded switches. The right one is labelled "FLT CTL DIR-LAW", with 2 positions: DIR and NORM.
Thoses switches are absent from the production aircraft, the second delivered to the French Air Force.
Dozy, as an attempt to illustrate your #273, here are two pictures of A400M cockpits :
- test aircraft: Photos: Airbus A400M Grizzly Aircraft Pictures | Airliners.net
- production aircraft: Photos: Airbus A400M Grizzly Aircraft Pictures | Airliners.net
On the test aircraft, between the standby ADI and the gear indicator, there are two guarded switches. The right one is labelled "FLT CTL DIR-LAW", with 2 positions: DIR and NORM.
Thoses switches are absent from the production aircraft, the second delivered to the French Air Force.
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Agreed, and I'm with you. The difference between your input (as I understand it) and CONF iture's is that he seems to interpret the term "alpha max" as adhering strictly to 17.5 degrees AoA and believes anything other than a swift acquisition and maintaining of that value to be a technical fault or anomaly.
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Originally Posted by Dozy
Honest conjecture - please don't be offended, but that's your interpretation and yours only.
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Originally Posted by Dozy
If so, at what point in the video does the AoA gauge hit and stay at 17.5 degrees precisely?
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Would you be willing to at least consider the possibility that there are many valid reasons why the aircraft won't attain 17.5deg in a hurry?
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Originally Posted by Dozy
Because that value is briefly exceeded
The problem is ... ?
and right now we only have your supposition that the immediate alpha max value at Habsheim should have been 17.5 degrees due to CONF setting...
Regarding the 'supposition' it is Yours not Mine ... mine is still here.