Habsheim
@ Dozy
you misunderstood my post, which is probably my fault.
I'm with you in most of your posts in this thread here. My remark was pointed at those, who are either unable or unwilling to understand, that only in direct law the elevator deflection corresponds to the stick deflection, and that it is an basic principle of FBW C* law of AB (at least that's how i see it) that the pitch command of the stick represents an g command blended by pitch rate when applicable.
Therefore it is normal, that the pitch rate change of the increasing thrust enters the equation as just that, a pitch change. If the pilot commanded pitch change corrected by the protections is less than the actual pitch change by thrust, then the elevators will command a pitch reduction despite the stick position.
By the way, a pilot in the loop in a conventional aircraft in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!
Who wants to end like the B737 at Kazan?
you misunderstood my post, which is probably my fault.
I'm with you in most of your posts in this thread here. My remark was pointed at those, who are either unable or unwilling to understand, that only in direct law the elevator deflection corresponds to the stick deflection, and that it is an basic principle of FBW C* law of AB (at least that's how i see it) that the pitch command of the stick represents an g command blended by pitch rate when applicable.
Therefore it is normal, that the pitch rate change of the increasing thrust enters the equation as just that, a pitch change. If the pilot commanded pitch change corrected by the protections is less than the actual pitch change by thrust, then the elevators will command a pitch reduction despite the stick position.
By the way, a pilot in the loop in a conventional aircraft in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!
Who wants to end like the B737 at Kazan?
Last edited by RetiredF4; 2nd Jan 2014 at 17:17.
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Therefore it is normal, that the pitch rate change of the increasing thrust enters the equation as just that, a pitch change. If the commanded pitch change corrected by the protections is less than the actual pitch change by thrust, then the elevators will command a pitch reduction despite the stick position.
By the way, a pilot in the loop in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!
In a pre-FBW fighter or a specialist aerobatic aircraft, a pilot will be trained and given experience in recognising responses and making those split-second
decisions/inputs - and because those aircraft are designed to be lightweight and nimble above all else, momentum is less of a problem. Even smaller short/medium-haul airliners like the A320 are still big and unwieldy aircraft by comparison - and this is the reason high AoA protections were a very important thing to demonstrate in the early days, being a scenario no previous type could pull off safely. I'd be prepared to bet that if you asked any one of Yeager, Armstrong, Cunningham or Beamont et al. to get in a conventional jetliner and hold 15 degrees of AoA precisely while flying slowly (as in near stall speed) at 1000ft, let alone 100ft, their refusals would have been both emphatic and unprintable.
The flight envelope protections afforded by modern FBW systems have been consistently characterised by the doubters as being primarily a technological vote-of-no-confidence in piloting skills, and this has been a very difficult idea to dispel amongst pilots. Again at the risk of repeating myself, the overarching premise is nothing of the sort - the main impetus is in allowing pilots to have confidence in making positive maneouvres without having to worry about the aircraft's ability to stay in the air.
Thread Starter
Dozy,
To answer your recent question: yes, the (Habsheim) a/c was in a normal landing configuration, i.e., L/G down and "Flaps 3" (Slats 22 deg, Flaps 20 deg). BTW, the other normal landing config is "Flaps Full" (Slats 27 deg, Flaps 35).
Quote from RetiredF4:
"By the way, a pilot in the loop in a conventional aircraft in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!
"Who wants to end like the B737 at Kazan?"
Quite so, and - IIRC - there's a precedent on the A310. I imagine most twin-engined airliners with underslung engines have comparable characteristics in the go-around; partly because they are essentially overpowered when both donks are running, and it is normal practice to use TOGA thrust when going-around from a low height.
On the A310, which for this discussion we can regard as being in Direct Law, the amount of down-elevator to counter the pitch-couple effect of TOGA is cosiderable, even from Vref. This is highlighted best if the AP is off, when the PF has to exert an almost unsustainable forward force on the control column, manually trimming the THS forward from the landing setting as fast as possible to relieve the elevator load. (When in use, the AP and auto-trim obviously have the same task.) The down-elevator requirement would be much greater from Valpha-max. (BTW, the A310 also has an Alpha-Floor mode in A/THR, so the latter must have been demonstrated for certification, presumably at an aft CG.)
FWIW, I don't remember ever doing a G/A on an A320 in Direct Law (certainly not in anger, because in 14 years I never lost Normal Law). However, a glance at the relative profiles of the A320 and A310 suggest that the A320 elevators may enjoy a more effective pitch-moment than the A310's. (The A319 might be more comparable to the A310 in that respect, and the A320 more like the A300-600.)
To answer your recent question: yes, the (Habsheim) a/c was in a normal landing configuration, i.e., L/G down and "Flaps 3" (Slats 22 deg, Flaps 20 deg). BTW, the other normal landing config is "Flaps Full" (Slats 27 deg, Flaps 35).
Quote from RetiredF4:
"By the way, a pilot in the loop in a conventional aircraft in such a pitch up situation due to increasing engine thrust would react how (without available protections)? By controlling the pitchup by use of proper stick input, which might be stick forward!
"Who wants to end like the B737 at Kazan?"
Quite so, and - IIRC - there's a precedent on the A310. I imagine most twin-engined airliners with underslung engines have comparable characteristics in the go-around; partly because they are essentially overpowered when both donks are running, and it is normal practice to use TOGA thrust when going-around from a low height.
On the A310, which for this discussion we can regard as being in Direct Law, the amount of down-elevator to counter the pitch-couple effect of TOGA is cosiderable, even from Vref. This is highlighted best if the AP is off, when the PF has to exert an almost unsustainable forward force on the control column, manually trimming the THS forward from the landing setting as fast as possible to relieve the elevator load. (When in use, the AP and auto-trim obviously have the same task.) The down-elevator requirement would be much greater from Valpha-max. (BTW, the A310 also has an Alpha-Floor mode in A/THR, so the latter must have been demonstrated for certification, presumably at an aft CG.)
FWIW, I don't remember ever doing a G/A on an A320 in Direct Law (certainly not in anger, because in 14 years I never lost Normal Law). However, a glance at the relative profiles of the A320 and A310 suggest that the A320 elevators may enjoy a more effective pitch-moment than the A310's. (The A319 might be more comparable to the A310 in that respect, and the A320 more like the A300-600.)
Last edited by Chris Scott; 3rd Jan 2014 at 00:45. Reason: "(Habsheim)" added
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In which case (and I'm looking for guidance here), does this not render the question of why the aircraft did not attain an AoA of 17.5 degrees somewhat moot?
Thread Starter
Dozy,
As you know, I'm not the best one to answer that, but I've no reason to doubt the BEA figure of Alpha-max = 17.5 deg, which is specific to Flaps 3.
AFAIK, the AoA is measured in relation to a specific part of the wing chord, ignoring the varying positions (angles) of the slats and flaps.
As you know, I'm not the best one to answer that, but I've no reason to doubt the BEA figure of Alpha-max = 17.5 deg, which is specific to Flaps 3.
AFAIK, the AoA is measured in relation to a specific part of the wing chord, ignoring the varying positions (angles) of the slats and flaps.
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Thanks for providing that info. What has tickled my grey cells is the "gear down" distinction. While not directly related to the calculation, would the increased drag from the gear require a reduction in maximum AoA in the protected mode to compensate?
Thread Starter
Hi HN39,
Re the flap settings. Regardless of briefings and configuration commands from PF to PNF, the DFDR shows that the fly-past was flown at Flaps/Config 3.
BEA Rapport Finale, Annexe VII, Tome 2
Published DFDR data start at 1244Z, Time mark 240.0 sec (this may be 240 secs from selection of TOGA at the start of T/O).
At 240.0 sec, Slats18/Flaps10 (= "Flaps 1+F", maintained from T/O).
At 263.0 sec, Slats22/Flaps15 (= "Flaps 2").
At 279.0 sec, Slats22/Flaps20 (= "Flaps 3").
At impact (334.0 or 335.0), Slats22/Flaps20 ("Flaps 3").
Dozy,
Re possible effect of L/G on alpha-max: again, I think the BEA would have done their homework. L/G increses drag, which offsets thrust, but I don't think the aerodynamics are changed significantly. For example, alpha-floor remains the same at 15.0 deg in that config, according to my contemporary FCOM, which unfortunately gives no equivalent figures for alpha-max. Placarded VLS (on the ASI) does not change, IIRC, with gear position, whereas it does with speedbrake extension.
PS
HN 39, I now see your exchanges with Dozy refer to the video of the Gordon Corps demo flight...
Re the flap settings. Regardless of briefings and configuration commands from PF to PNF, the DFDR shows that the fly-past was flown at Flaps/Config 3.
BEA Rapport Finale, Annexe VII, Tome 2
Published DFDR data start at 1244Z, Time mark 240.0 sec (this may be 240 secs from selection of TOGA at the start of T/O).
At 240.0 sec, Slats18/Flaps10 (= "Flaps 1+F", maintained from T/O).
At 263.0 sec, Slats22/Flaps15 (= "Flaps 2").
At 279.0 sec, Slats22/Flaps20 (= "Flaps 3").
At impact (334.0 or 335.0), Slats22/Flaps20 ("Flaps 3").
Dozy,
Re possible effect of L/G on alpha-max: again, I think the BEA would have done their homework. L/G increses drag, which offsets thrust, but I don't think the aerodynamics are changed significantly. For example, alpha-floor remains the same at 15.0 deg in that config, according to my contemporary FCOM, which unfortunately gives no equivalent figures for alpha-max. Placarded VLS (on the ASI) does not change, IIRC, with gear position, whereas it does with speedbrake extension.
PS
HN 39, I now see your exchanges with Dozy refer to the video of the Gordon Corps demo flight...
Last edited by Chris Scott; 3rd Jan 2014 at 00:48. Reason: PS added.
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To be somewhat brutal, the only person I've known to make a big deal about the 17.5 degree figure is CONF iture on these forums. The BEA alluded obliquely to the figure but did not question the systems' behavioural integrity, however at the same time neither the SNPL nor Capt. Asseline's legal team (who had access to the report and those figures) questioned it during the court case. If this apparent 2.5 degree discrepancy was the 'smoking gun' CONF claims it is then why was it not brought up at the time?
The way I see it there are two main possible conclusions - either Capt. Corps was mistaken when narrating the video, or the BEA included the 17.5 degree figure in their report as background information only, not intended for comparison with the aircraft's actual AoA during the sequence.
Thread Starter
Sorry Dozy, nothing I previously posted this evening refered to the Gordon Corps video, which I had not watched. I hadn't realised you and HN39 were refering to it in your exchanges about the landing config.
Have now watched the video, and cannot shed any light on which of the two standard landing configs Gordon was using, or why he implied that 15 degrees was about the maximum angle of attack for it. If he was using Flaps Full, I think alpha-max MIGHT be slightly less than for Flaps 3, but that guess is only based on the fact that my FCOM says the Flaps-Full alpha-floor is 14.5 deg - 0.5 deg less than at Flaps 3. That said, the small, electro-mechanical AoA indicator seems to exceed 15 deg occasionally in the video.
Have now watched the video, and cannot shed any light on which of the two standard landing configs Gordon was using, or why he implied that 15 degrees was about the maximum angle of attack for it. If he was using Flaps Full, I think alpha-max MIGHT be slightly less than for Flaps 3, but that guess is only based on the fact that my FCOM says the Flaps-Full alpha-floor is 14.5 deg - 0.5 deg less than at Flaps 3. That said, the small, electro-mechanical AoA indicator seems to exceed 15 deg occasionally in the video.
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Hi Chris,
I agree. It exceeds 15 momentarilly when he rolls on some bank.
During the steady turn, the AoA gauge then returns to 15 and when he levels the wings again, the AoA shows less than 15. All that is done with full back stick.
In level flight, the system seems to be limiting the AoA shy of Alpha Max - so that some bank can be applied with the same initial pitch attitude without exceeding Alpha Max.
From my FCOM:
• αfloor is activated through the A/THR system, when
α is greater than αfloor (9.5 ° in configuration 0; 15 ° in configuration 1, 2; 14 ° in configuration 3; 13 ° in configuration FULL)
FCOM FCB5 Avoiding Tail strikes; “Assuming an 8 kt speed decrease during flare, and a -1 ° flight path angle at touchdown, the pitch attitude will increase by approximately 4.5 °”
So during the flare, the flight path is changed by +2° (-3° to -1°) for a loss of 8kt. Therefore the angle of attack increases by 2.5° (4.5°-2°) for about 8 kts reduction in speed.
From the diagram, αfloor is about 8 kts from αMax, therefore in CONF 3, αMax would seem to be about 14° + 2.5° = 16.5°
the small, electro-mechanical AoA indicator seems to exceed 15 deg occasionally in the video.
During the steady turn, the AoA gauge then returns to 15 and when he levels the wings again, the AoA shows less than 15. All that is done with full back stick.
In level flight, the system seems to be limiting the AoA shy of Alpha Max - so that some bank can be applied with the same initial pitch attitude without exceeding Alpha Max.
From my FCOM:
• αfloor is activated through the A/THR system, when
α is greater than αfloor (9.5 ° in configuration 0; 15 ° in configuration 1, 2; 14 ° in configuration 3; 13 ° in configuration FULL)
FCOM FCB5 Avoiding Tail strikes; “Assuming an 8 kt speed decrease during flare, and a -1 ° flight path angle at touchdown, the pitch attitude will increase by approximately 4.5 °”
So during the flare, the flight path is changed by +2° (-3° to -1°) for a loss of 8kt. Therefore the angle of attack increases by 2.5° (4.5°-2°) for about 8 kts reduction in speed.
From the diagram, αfloor is about 8 kts from αMax, therefore in CONF 3, αMax would seem to be about 14° + 2.5° = 16.5°
Thread Starter
Contemporary FCOM extracts
Quote from rudderrudderrat:
"From my FCOM:
• αfloor is activated through the A/THR system, when
α is greater than αfloor (9.5 ° in configuration 0; 15 ° in configuration 1, 2; 14 ° in configuration 3; 13 ° in configuration FULL)"
I guess that must be a FCOM from a later era than June 1988? Here is the equivalent paragraph from the BCAL Tech Manual, dated 17FEB1988, around the time of A320-100 type-certification. [I'm stuck with transcribing it in plain script.]
Alpha-floor is activated when:
- alpha > alpha-floor (9.5 deg in conf 0; 15 deg in conf 1, 2, 3; 14.5 deg in conf FULL, or
- sidestick deflection > 14 deg nose-up and pitch-attitude > 25 deg or in angle-of-attack protection [sic...].
[The ambiguous mix of "and" and "or" in the final, un-punctuated sentence is as printed.]
A slightly earlier, late-1987 edition of the Airbus FCOM Flight Controls chapter (1.09.10 P8, REV 03, SEQ 001) omits any specific alpha figures; instead providing the following figures of the relationship between the alphas at the protection stages and the alpha at Vs.
All configurations at low speed:
alpha-prot >= alpha (1.13 Vs)
alpha-floor = alpha (1.1 Vs)
alpha-max = alpha (1.06 Vs)
Clean configuration at high speed:
alpha-prot = alpha (buffet)
alpha-max = Czmax
When alpha protection is active nose-up auto-pitch-trim is inhibited.
One might assume in the above that Vs is a speed associated with the alpha at CL -max, but it does not state whether that Vs is at 1.0G or a lower figure. And whether it is the same value used in the "Stalling Speeds" graph (FCOM 3.01.20 P5) is unclear.
Therefore, I think any attempt, using a combination of the above sources, to infer specific values of alpha for the three different protection levels might be ill-founded. Also, the two versions may originate either side of the FBW certification process.
Hi Jesse,
"A FLOOR doesn't work below 100'RA."
Yes, we have discussed that previously, although my 1988 manuals only seemed to mention it in the Power Plant section (A/THR automatic-engagement criteria). The Habsheim crew had planned to disable Alpha-Floor to ensure it did not intervene at their briefed display height of 100ft.
"From my FCOM:
• αfloor is activated through the A/THR system, when
α is greater than αfloor (9.5 ° in configuration 0; 15 ° in configuration 1, 2; 14 ° in configuration 3; 13 ° in configuration FULL)"
I guess that must be a FCOM from a later era than June 1988? Here is the equivalent paragraph from the BCAL Tech Manual, dated 17FEB1988, around the time of A320-100 type-certification. [I'm stuck with transcribing it in plain script.]
Alpha-floor is activated when:
- alpha > alpha-floor (9.5 deg in conf 0; 15 deg in conf 1, 2, 3; 14.5 deg in conf FULL, or
- sidestick deflection > 14 deg nose-up and pitch-attitude > 25 deg or in angle-of-attack protection [sic...].
[The ambiguous mix of "and" and "or" in the final, un-punctuated sentence is as printed.]
A slightly earlier, late-1987 edition of the Airbus FCOM Flight Controls chapter (1.09.10 P8, REV 03, SEQ 001) omits any specific alpha figures; instead providing the following figures of the relationship between the alphas at the protection stages and the alpha at Vs.
All configurations at low speed:
alpha-prot >= alpha (1.13 Vs)
alpha-floor = alpha (1.1 Vs)
alpha-max = alpha (1.06 Vs)
Clean configuration at high speed:
alpha-prot = alpha (buffet)
alpha-max = Czmax
When alpha protection is active nose-up auto-pitch-trim is inhibited.
One might assume in the above that Vs is a speed associated with the alpha at CL -max, but it does not state whether that Vs is at 1.0G or a lower figure. And whether it is the same value used in the "Stalling Speeds" graph (FCOM 3.01.20 P5) is unclear.
Therefore, I think any attempt, using a combination of the above sources, to infer specific values of alpha for the three different protection levels might be ill-founded. Also, the two versions may originate either side of the FBW certification process.
Hi Jesse,
"A FLOOR doesn't work below 100'RA."
Yes, we have discussed that previously, although my 1988 manuals only seemed to mention it in the Power Plant section (A/THR automatic-engagement criteria). The Habsheim crew had planned to disable Alpha-Floor to ensure it did not intervene at their briefed display height of 100ft.
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For a non-professional it is fun to read all these comments about official manuals for the same type of aircraft that change whenever the manufacturer decides and when those are used by professionals to analyze a sequence of a particular flight they come in of conlusions that contain the words .. ill founded ... ambiguous ... unclear ... probable ... etc ...
So I guess the results when everything is explained in a court where everyone tries to blame the other party
So I guess the results when everything is explained in a court where everyone tries to blame the other party
Thread Starter
Quote from jcjeant:
"For a non-professional it is fun to read all these comments about official manuals for the same type of aircraft that change whenever the manufacturer decides and when those are used by professionals to analyze a sequence of a particular flight they come in of conlusions that contain the words .. ill founded ... ambiguous ... unclear ... probable ... etc ..."
Well, that's not entirely true. Haven't noticed any (ex) professional recently using the word "probable" here...
"For a non-professional it is fun to read all these comments about official manuals for the same type of aircraft that change whenever the manufacturer decides and when those are used by professionals to analyze a sequence of a particular flight they come in of conlusions that contain the words .. ill founded ... ambiguous ... unclear ... probable ... etc ..."
Well, that's not entirely true. Haven't noticed any (ex) professional recently using the word "probable" here...
Thread Starter
Quote from rudderrudderrat:
FCOM FCB5 Avoiding Tail strikes; “Assuming an 8 kt speed decrease during flare, and a -1 ° flight path angle at touchdown, the pitch attitude will increase by approximately 4.5 °”
"So during the flare, the flight path is changed by +2° (-3° to -1°) for a loss of 8kt. Therefore the angle of attack increases by 2.5° (4.5°-2°) for about 8 kts reduction in speed.
"From the diagram, αfloor is about 8 kts from αMax, therefore in CONF 3, αMax would seem to be about 14° + 2.5° = 16.5°
I take it you are assuming Nz is 1G before and after the flare? I'm wondering about the possible change of ground effect on the 1G-AoAs between (say) 30'R and 5'R.
FCOM FCB5 Avoiding Tail strikes; “Assuming an 8 kt speed decrease during flare, and a -1 ° flight path angle at touchdown, the pitch attitude will increase by approximately 4.5 °”
"So during the flare, the flight path is changed by +2° (-3° to -1°) for a loss of 8kt. Therefore the angle of attack increases by 2.5° (4.5°-2°) for about 8 kts reduction in speed.
"From the diagram, αfloor is about 8 kts from αMax, therefore in CONF 3, αMax would seem to be about 14° + 2.5° = 16.5°
I take it you are assuming Nz is 1G before and after the flare? I'm wondering about the possible change of ground effect on the 1G-AoAs between (say) 30'R and 5'R.
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Originally Posted by rudderrudderrat
Please have a look at Airbus A320 Fly By Wire Demo
The presence of the AoA indicator is perfect for the demo, only a simultaneous view on the sidestick and the flight controls page could be better.
The AoA never exceeds its design limit (apparently 15 degs mentioned around time 10:00)
During the demonstrated pull up manoeuvre, the AoA increases rapidly to 15 degs during the +ve delta g.
Why no such intention in Habsheim ?
Thread Starter
Quote from CONF_iture:
Alpha max is at 15 deg in landing CONF FULL but 17.5 CONF 3
That seems a big difference between the two configurations, particularly as my contemporary FCOM states that the alpha-floor figures are 14.5 deg and 15 deg respectively (see my post above).
Alpha max is at 15 deg in landing CONF FULL but 17.5 CONF 3
That seems a big difference between the two configurations, particularly as my contemporary FCOM states that the alpha-floor figures are 14.5 deg and 15 deg respectively (see my post above).
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Originally Posted by Chris Scott
A slightly earlier, late-1987 edition of the Airbus FCOM Flight Controls chapter (1.09.10 P8, REV 03, SEQ 001) omits any specific alpha figures; instead providing the following figures of the relationship between the alphas at the protection stages and the alpha at Vs.
All configurations at low speed:
alpha-prot >= alpha (1.13 Vs)
alpha-floor = alpha (1.1 Vs)
alpha-max = alpha (1.06 Vs)
All configurations at low speed:
alpha-prot >= alpha (1.13 Vs)
alpha-floor = alpha (1.1 Vs)
alpha-max = alpha (1.06 Vs)
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Hi CONF iture,
Maybe the system can rapidly achieve aMax during +ve delta g, because it is able to avoid exceeding aMax by simply reducing the rate of rotation.
Whereas at Habsheim, there was only the last 2.5° (if 17.5° is correct) of rotation available, and would only approach the limit gradually.
Absolutely no hesitation to go rapidly to alpha max.
Why no such intention in Habsheim ?
Why no such intention in Habsheim ?
Whereas at Habsheim, there was only the last 2.5° (if 17.5° is correct) of rotation available, and would only approach the limit gradually.