AF 447 Thread No. 9
"relaxed static stability" et al
I'll go out on my normal limb and tell all that the FBW system on the 'bus can't provide "simulated" or "apparent" static stability. The jet is either stable or not, and it is not like that little thing I flew long ago which was actually unstable until 0.95 M and depended upon HAL to keep the pointy end forward.
If we are talking about little or no speed stability because HAL is trimming for a gee versus an AoA ( just like the Viper), then you may think you have "neutral" static stability, but you don't. The 'bus has positive static stability throughout a good combination of cee gee versus center of aero pressure, but never has inherent aerodynamic negative static stability.
I agree with those here stating that it would never have been certified within any realm on the charts indicating negative static stability. Even AF447 showed positive static stability at 40 degrees AoA or whatever the exact value was. Push forward and eventually recover. What we DID SEE was an AoA and cee gee that allowed it to remain fairly stable while stalled, courtesy of HAL, which was constantly commanding nose up and trimming the THS to reach the nominal one gee command, hands on or hands off. Only forward stick and maybe manual THS trimming could have enabled a recovery.
God, but I would love to get some of you in my old jet and show you the differences between the older jets and one with a FBW system with all the "limits"/"protections" and fly it to the limits. Knowing a lot more about the 'bus now than two years ago, I would relish the oppo to do the same thing in the beast. As Doze is fond of reminding me, our jet was designed with different goals/missions. Nevertheless, it was the first one with FBW, no mechanical back up, and we all became test pilots for a year or two, even after 4 or 5 years of development and testing at Edwards.
out,
If we are talking about little or no speed stability because HAL is trimming for a gee versus an AoA ( just like the Viper), then you may think you have "neutral" static stability, but you don't. The 'bus has positive static stability throughout a good combination of cee gee versus center of aero pressure, but never has inherent aerodynamic negative static stability.
I agree with those here stating that it would never have been certified within any realm on the charts indicating negative static stability. Even AF447 showed positive static stability at 40 degrees AoA or whatever the exact value was. Push forward and eventually recover. What we DID SEE was an AoA and cee gee that allowed it to remain fairly stable while stalled, courtesy of HAL, which was constantly commanding nose up and trimming the THS to reach the nominal one gee command, hands on or hands off. Only forward stick and maybe manual THS trimming could have enabled a recovery.
God, but I would love to get some of you in my old jet and show you the differences between the older jets and one with a FBW system with all the "limits"/"protections" and fly it to the limits. Knowing a lot more about the 'bus now than two years ago, I would relish the oppo to do the same thing in the beast. As Doze is fond of reminding me, our jet was designed with different goals/missions. Nevertheless, it was the first one with FBW, no mechanical back up, and we all became test pilots for a year or two, even after 4 or 5 years of development and testing at Edwards.
out,
In respect of the FD Bars, they do appear to have been available when 2 ADRs (also 2 IRs) agreed within 20 KTS.
When ALT2'something' latches as a result of multiple 'ADR disagree', I can count down 1 knot at a time and when the difference between the 'median' ADR and the next closest reaches 19 knots, the FD's return and the AP 1+2 inop status is removed, regardless of the magnitude of the remaining ADR discrepancy. The A/P pb can then be engaged, even though ACTUAL speed (CAS) is below ACTUAL VLS. VLS is neither displayed nor indicated on the FMGEC landing phase PERF page. It is dashed and appears not to be computed.
As soon as I take the speed discrepancy back to 20 knots or above, the FD's go away and the A/P pb cannot be engaged.
Either I'm not really in ALT2B, which, on the surface, is problematic to determine, or there is a potential Level D problem, always a possibility, or 20 IS the magic number.
This is what I'm trying to determine.
Last edited by OK465; 20th Jul 2012 at 00:59. Reason: thought consolidation
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C of G
There was quite a lot about C of G about a year ago in Thread 5 #394 ...etc. Some of it IIRC implied that fuel was transferred " in chunks" forward for use in flight whilst keeping the C of G within the normal "in flight limits" by some margin. I would guess that during the transfer of each "chunk" the aircraft would respond to the change of C of G with a change of speed or F/L , normally adjusted by A/T and A/P - but if these are not available then manual control would seem more sensitive.
( On a York hand flown, it could feel as though the passengers were playing cabin football, with a goal at each end of the cabin !)
There was quite a lot about C of G about a year ago in Thread 5 #394 ...etc. Some of it IIRC implied that fuel was transferred " in chunks" forward for use in flight whilst keeping the C of G within the normal "in flight limits" by some margin. I would guess that during the transfer of each "chunk" the aircraft would respond to the change of C of G with a change of speed or F/L , normally adjusted by A/T and A/P - but if these are not available then manual control would seem more sensitive.
( On a York hand flown, it could feel as though the passengers were playing cabin football, with a goal at each end of the cabin !)
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John, yes I meant within aft CG limits if that was what you were asking.The LSS thing isn't familiar to me but it must have something to do with stability and CG. Us yanks use different terms so help us out.
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God, but I would love to get some of you in my old jet
Yes, please. Only FJ I've been in was an RAAF ARDU Mirage in the early 70s and that was good fun.
The LSS thing isn't familiar to me
Apologies. (Longitudinal) Static Stability. Basic thing for (required) positive stability is
(a) maintaining slower than trimmed speed requires the pilot to pull
(b) maintaining faster than trimmed speed requires the pilot to push
Yes, please. Only FJ I've been in was an RAAF ARDU Mirage in the early 70s and that was good fun.
The LSS thing isn't familiar to me
Apologies. (Longitudinal) Static Stability. Basic thing for (required) positive stability is
(a) maintaining slower than trimmed speed requires the pilot to pull
(b) maintaining faster than trimmed speed requires the pilot to push
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That's the way the testing and measuring is approached ..
The typical pilot training thing about strange balls in teacups is useful if explained appropriately but, near invariably, it is a case of the blind leading the blind and the explanations tend to be more fanciful than useful.
Stick force for positive static stability can be thought of in terms of "if I relax the load, the aircraft initially would seek to return to the trim speed".
In terms of balls and cups .. the analogy is "if I let the ball go .. its initial movement would be in the direction of the bottom of the cup".
Either way static stability is to do with the aeroplane's being at a fixed off trim attitude and looking at what it might want to do if you were to let go of the stick/ease off the stick load .. dynamic stability looks at what it actually does.
The typical pilot training thing about strange balls in teacups is useful if explained appropriately but, near invariably, it is a case of the blind leading the blind and the explanations tend to be more fanciful than useful.
Stick force for positive static stability can be thought of in terms of "if I relax the load, the aircraft initially would seek to return to the trim speed".
In terms of balls and cups .. the analogy is "if I let the ball go .. its initial movement would be in the direction of the bottom of the cup".
Either way static stability is to do with the aeroplane's being at a fixed off trim attitude and looking at what it might want to do if you were to let go of the stick/ease off the stick load .. dynamic stability looks at what it actually does.
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@OK465;
Agreed!
But there is one more set of variables (includes CAS > VLS) to be confirmed before the A/P will actually engage, and that is not met. The logic presented by A33Zab clearly shows that with regard to the A/P the CAS/Pitch/Roll condition is the final inhibition - irrespective of FMGEC, other than when Pitch Angle Protection is active in Normal Law.
The A/P pb can then be engaged, even though ACTUAL speed (CAS) is below ACTUAL VLS. VLS is neither displayed nor indicated on the FMGEC landing phase PERF page.
But there is one more set of variables (includes CAS > VLS) to be confirmed before the A/P will actually engage, and that is not met. The logic presented by A33Zab clearly shows that with regard to the A/P the CAS/Pitch/Roll condition is the final inhibition - irrespective of FMGEC, other than when Pitch Angle Protection is active in Normal Law.
Last edited by mm43; 20th Jul 2012 at 22:14. Reason: added PAP criteria
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Originally Posted by Rockhound
Regarding the meaning of the captain's "prends įa", Otelli wrote that it is difficult to know what he was referring to but the BEA investigators favoured the idea that it was the FPV
In a 6 seconds period, the captain said it once and the PNF twice - I cannot figure out what they were talking about ... ?
That's a period of time when the PF was full back stick !
BTW that part of the CVR has changed since the third Interim Report ...
Also, as mentioned earlier by Lyman, why the captain is telling "AP OFF" at time 2:13:53 ?
What is that 'bruit de selecteur' ... an AP that is selected would make such a 'bruit' ...
That BEA is really not curious, at least publicly.
Another point is that total confusion related to the vertical speed indications or possible intermittent lack of it.
The comments made by the crew suggest how that information was erratic when displayed ... How were behaving the altimeters ... ?
Otelli's book ?
I should read it before commenting further but have no intention to pay for that ... Any link for the CVR part ?
Thanks for the chart, Turbine, it's the one I used making my assertion that the jet was well within the cee gee envelope.
To JT, I tink you have to further explain about LSS. So if I am below the trimmed AoA, the plane noses up to get there all by itself if I let go. And vice versa. So trimmed for 80 knots in my Cessna and I am zooming along at 100 knots without re-trimming, then the thing pulls up if I relax the wheel/yoke/stick. U.S. nasal radiators like 'bird are very familiar with this, as it was the way they made their approach to the deck - they trimmed for AoA and used power to control rate of descent.
For the 'bus drivers here. you could try this in the sim, but the control laws would fight you. Normal law is not heavily biased for AoA, and the auto-throttle would thwart you. But I guess ya gotta play the cards that you are dealt.
To JT, I tink you have to further explain about LSS. So if I am below the trimmed AoA, the plane noses up to get there all by itself if I let go. And vice versa. So trimmed for 80 knots in my Cessna and I am zooming along at 100 knots without re-trimming, then the thing pulls up if I relax the wheel/yoke/stick. U.S. nasal radiators like 'bird are very familiar with this, as it was the way they made their approach to the deck - they trimmed for AoA and used power to control rate of descent.
For the 'bus drivers here. you could try this in the sim, but the control laws would fight you. Normal law is not heavily biased for AoA, and the auto-throttle would thwart you. But I guess ya gotta play the cards that you are dealt.
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So if I am below the trimmed AoA, (ie motoring along the aerial freeway faster than trim speed) the plane noses up to get (back) there (somewhere near trim speed - within a reasonable tolerance) all by itself if I let go (strictly, if you ease off the stick load gradually). And vice versa. So trimmed for 80 knots in my Cessna and I am zooming along at 100 knots without re-trimming (but holding a pretty decent push stick force as you descend - test normally would be for a fixed thrust setting), then the thing pulls up if I relax the wheel/yoke/stick. U.S. nasal radiators like 'bird are very familiar with this, as it was the way they made their approach to the deck - they trimmed for AoA and used power to control rate of descent.
[My red additions above for additional wordiology confusion].
The current test procedure for FAR 25 is given in the FTG AC 25-7B starting at page 109.
Similarly, for Part 23, the FTG is at AC 23-8C starting at page 78. (be warned - 14MB)
[My red additions above for additional wordiology confusion].
The current test procedure for FAR 25 is given in the FTG AC 25-7B starting at page 109.
Similarly, for Part 23, the FTG is at AC 23-8C starting at page 78. (be warned - 14MB)
Imho some of you are on the wrong path concerning the stability issue.
Fact is, that BEA brings in the "neutral stability" issue and they wouldnīt have done it for no reason.
This neutral stability is not a built in by design into the airframe or caused by shift of CG (trimtank), it is done by managing the position of aerodynamic pressure by the computers with the elevators in the short term and the THS in the long term. Therefore i did use the term "natural" and "artificial".
Gums described it in his words.
While the new generation of combat jets are "natural unstable" and are made stable by the computers, it is quite different in the A330. It is natural stable with positive static stability (See the graph posted by TurbineD) even when computers change CG a bit by transfering fuel to or from the trimtank. With the automatic THS trim besides the elevators the computers have a formidable tool to change the aerodynamic pressure, hence the airframe is kept in neutral stability all the time.
In a conventional aircraft it is the same situation when you have the aircraft perfectly trimmed, with one big difference. The trimmed conventional aircraft is speed stable and will return to equilibrium if disturbed out of that trim condition, the discussed A330 is not speed stable but flightpath stable within limits defined by the built in flight envelope protections.
At least BEA mentions this neutral stability, wether as excuse for the problems the crew faced, as a hint to the manufacturer that the reversion sequence and the documentation needs a look up, or as a hint to better training and understanding of the resulting flight behaviour when vital inputs are lost? Or any other reason we can think of.
As described, here with the A330 itīs vice versa. The aircraft has a natural positive stability, it needs the computers to keep it in the neutral region.
Imho the "Nil Box Assistance" is not the problem, as the aircraft would then (with manual THS trim) be a natural positiv stable airframe.
The problem here is, that with all protections except load factor protection lost the aircraft was kept in this neutral stability by the computers and the crew could not cope with the responsiblity to keep the aircraft within the flight envelope.
See RRīs post below.
Exactly
Imho not necessarily, as explained before. The aft CG helps in saving fuel, but is not necessary for creating the neutral stability. The continuous autotrim is creating this permanent neutral stability with the help of the box.
The CG doesnīt change except for the fuel in the fin, the location of the center of aerodynamic pressure is constantly changing and keeps the aircraft in neutral stability until the degradation drops into direct law. Then you might have an aircraft completely out of trim, but it is basically now an aircraft with positive static stability. As in direct law only the trim wheel is available for longitudonal control, you wind it where it needs to be and the aircraft would be no longer flightpath stable, but speed stable.
Whereas in ALt law2 the aircraft was still kept in neutral stability by the box and was flightpath stable, but the necessary protections to make such a system safe (including the backup system "aircrew" ) were not working, thus leading to the flight-envelope excursion.
True for a conventional aircraft, ts different here.
I hope Owain Glyndwr can cut in and correct where iīm wrong or to add where necessary.
Fact is, that BEA brings in the "neutral stability" issue and they wouldnīt have done it for no reason.
This neutral stability is not a built in by design into the airframe or caused by shift of CG (trimtank), it is done by managing the position of aerodynamic pressure by the computers with the elevators in the short term and the THS in the long term. Therefore i did use the term "natural" and "artificial".
Gums described it in his words.
Gums
If we are talking about little or no speed stability because HAL is trimming for a gee versus an AoA ( just like the Viper), then you may think you have "neutral" static stability, but you don't. The 'bus has positive static stability throughout a good combination of cee gee versus center of aero pressure, but never has inherent aerodynamic negative static stability.
If we are talking about little or no speed stability because HAL is trimming for a gee versus an AoA ( just like the Viper), then you may think you have "neutral" static stability, but you don't. The 'bus has positive static stability throughout a good combination of cee gee versus center of aero pressure, but never has inherent aerodynamic negative static stability.
In a conventional aircraft it is the same situation when you have the aircraft perfectly trimmed, with one big difference. The trimmed conventional aircraft is speed stable and will return to equilibrium if disturbed out of that trim condition, the discussed A330 is not speed stable but flightpath stable within limits defined by the built in flight envelope protections.
JT
However, this latter consideration may be tied in with the present mishap - inextricably. Are we not all concerned with the possibility that this crash may have been involved with events so out of left field (but only because of presumptions regarding training and competence standards) that the original certification program may not have considered them at all ?
However, this latter consideration may be tied in with the present mishap - inextricably. Are we not all concerned with the possibility that this crash may have been involved with events so out of left field (but only because of presumptions regarding training and competence standards) that the original certification program may not have considered them at all ?
JT
neutral stability means the CG is at it's aft limit area to save fuel but it needs a computer to keep it stable
neutral stability means the CG is at it's aft limit area to save fuel but it needs a computer to keep it stable
JT
However, some of the posts appear to be suggesting that the degradation can extend to nil box assistance and a statically unstable aeroplane.. that's my problem at the moment.
However, some of the posts appear to be suggesting that the degradation can extend to nil box assistance and a statically unstable aeroplane.. that's my problem at the moment.
The problem here is, that with all protections except load factor protection lost the aircraft was kept in this neutral stability by the computers and the crew could not cope with the responsiblity to keep the aircraft within the flight envelope.
See RRīs post below.
rudderrudderrat
The aircraft is only statically neutral with the aid of the FBW computers. In Direct Law (stick deflection proportional to control surface deflection) they are (beautifully) dynamically stable.
The aircraft is only statically neutral with the aid of the FBW computers. In Direct Law (stick deflection proportional to control surface deflection) they are (beautifully) dynamically stable.
JT
This is the bit with which I'm having difficulty although my reading of the various links (and thanks to those who PMd/emailed other suggestions) may answer my questions ..
Neutral static stability infers a more aft CG than "normal". The box then can give the pilot an impression of stability (and, at the end of the day, it is this impression that is required).
This is the bit with which I'm having difficulty although my reading of the various links (and thanks to those who PMd/emailed other suggestions) may answer my questions ..
Neutral static stability infers a more aft CG than "normal". The box then can give the pilot an impression of stability (and, at the end of the day, it is this impression that is required).
If, now, the box departs controlled operation .. the aircraft CG is still where it was (until it can be moved) and the pilot potentially now is stuck with neutral static stability .. OK if you recognise it and know what to do .. but, otherwise, a recipe for rapid disaster ...
Whereas in ALt law2 the aircraft was still kept in neutral stability by the box and was flightpath stable, but the necessary protections to make such a system safe (including the backup system "aircrew" ) were not working, thus leading to the flight-envelope excursion.
Bubbers44
John, I agree, neutral stability means the CG is at it's aft limit area to save fuel but it needs a computer to keep it stable. A pilot would have a hard time to keep it at a constant altitude.
John, I agree, neutral stability means the CG is at it's aft limit area to save fuel but it needs a computer to keep it stable. A pilot would have a hard time to keep it at a constant altitude.
I hope Owain Glyndwr can cut in and correct where iīm wrong or to add where necessary.
Last edited by RetiredF4; 20th Jul 2012 at 10:54.
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Hi JT,
Ref your post 594
I've confused you with the wrong terms.
The aircraft feels like it is neutrally dynamically stable in normal law, and slightly positively stable in ALT law.
The only time we feel Direct Law is after the gear is down. I've never handled it at aft c o g in direct law, but presumably the gear drag couple would help.
Ref your post 594
I've confused you with the wrong terms.
The aircraft feels like it is neutrally dynamically stable in normal law, and slightly positively stable in ALT law.
The only time we feel Direct Law is after the gear is down. I've never handled it at aft c o g in direct law, but presumably the gear drag couple would help.
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Franzl,
I am uncomfortable with your thoughts regarding neutral stability although I have no problem with the Airbus's maintaining a constant flight path hands off.
Indeed, one of our PPRuNe number was in the Regulator's FT chair when the A320 was introduced to AN in Australia and he was quite ecstatic about the aircraft's capabilities at the time of Australian certification.
However, I acknowledge that my FBW knowledge is limited so I guess I will have to dig into the books and read up some more on FBW bits and pieces ...
I am uncomfortable with your thoughts regarding neutral stability although I have no problem with the Airbus's maintaining a constant flight path hands off.
Indeed, one of our PPRuNe number was in the Regulator's FT chair when the A320 was introduced to AN in Australia and he was quite ecstatic about the aircraft's capabilities at the time of Australian certification.
However, I acknowledge that my FBW knowledge is limited so I guess I will have to dig into the books and read up some more on FBW bits and pieces ...
John T,
there are differences between the A320 and the A330, also concerning the reversion of laws.
Iīm not sure wether i should be uncomfortable about the system itself or about the general knowledge about those systems and the asociated training by some operators including their crews.
there are differences between the A320 and the A330, also concerning the reversion of laws.
Iīm not sure wether i should be uncomfortable about the system itself or about the general knowledge about those systems and the asociated training by some operators including their crews.
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Thread Starter
Franzl,
The general dumbing down of things piloting is an extreme worry to most of us. If the systems design folks could guarantee that they get it all correct (which, of course, they can't .. and don't) it might be a different matter .. but, until they can, the folks up front are the last ditch defence and if they don't know much beyond "light A ON .. button B PRESS" we are better off going fishing.
The beancounting fraternity has a lot to answer for I suggest.
[Thanks for the PM offer]
The general dumbing down of things piloting is an extreme worry to most of us. If the systems design folks could guarantee that they get it all correct (which, of course, they can't .. and don't) it might be a different matter .. but, until they can, the folks up front are the last ditch defence and if they don't know much beyond "light A ON .. button B PRESS" we are better off going fishing.
The beancounting fraternity has a lot to answer for I suggest.
[Thanks for the PM offer]
Understanding "speed stable" versus "flight path stable"
If the driving concern is "flight path stable" then I'd gather that the system makes small speed corrections, or small corrections that manifest themselves in small airspeed changes, continuously. These changes would dampen until they reached vitrually zero when we reach desired altitude and mach/speed for a given sector of the flight. This would include automated and minor corrections of the power setting as part of this flight path seeking method.
Do I understand your explanation correctly, or am I simply confusing myself?
To whomever posted flight global article: thanks!
This provides me with a very nice explanation points for some of the laymen I know who are curious about this accident.
If the driving concern is "flight path stable" then I'd gather that the system makes small speed corrections, or small corrections that manifest themselves in small airspeed changes, continuously. These changes would dampen until they reached vitrually zero when we reach desired altitude and mach/speed for a given sector of the flight. This would include automated and minor corrections of the power setting as part of this flight path seeking method.
Do I understand your explanation correctly, or am I simply confusing myself?
To whomever posted flight global article: thanks!
This provides me with a very nice explanation points for some of the laymen I know who are curious about this accident.
Last edited by Lonewolf_50; 20th Jul 2012 at 12:56.
The general dumbing down of things piloting is an extreme worry to most of us. If the systems design folks could guarantee that they get it all correct (which, of course, they can't .. and don't) it might be a different matter ..
This approach standardizes care and produces better outcomes - most of the time. And so risk managers and administrators and legal departments love this stuff.
But (and there is always a but), if the patient gets misdiagnosed and put on the wrong pathway, or if the pathway does not address an uncommon set of circumstances particular to an individual patient, the physician is left with nothing to fall back on.
The physician blames the author of the pathway for not anticipating an unusual scenario, the managers blame the physician for not recognising s/he was out of his depth. The solution is to invariably to revise the guideline - because that is the only component of the system amenable to change.
And so it will be with AF447. Some improvements will be made to the automation. And they will be an improvement. But the underlying problem will be papered over.