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Iced Pitots
HN39,
Thanks for your characteristically perceptive response to my post #1224.
Quote:
"Sensor Validation points to the possibility of pitot overpressure due to drain blockage, but adds that he doesn’t know by which percentage. Until someone comes up with that information, I’m inclined to think that the effect is quite small."
That's my assumption too (low percentage). My own pilot's-eye view during walk-rounds (until they got too high for close inspection!) was that the holes' exit ends were often contaminated slightly by oxidation and/or burnt de-icing fluid. I doubt their area of cross-section is controllable in everyday service. It makes absolute sense that an unblocked drain-hole bleeds some dynamic pressure, requiring a correction. But if a partial blockage is acceptable, my guess is that a complete one would not in itself make a serious difference.
Quote:
"To my knowledge, stall warnings are not phase advanced (stickpushers sometimes are)".
Maybe not, and my assumption was influenced more by experience on VC10s and OneElevens than what little I've found in Airbus Tech Manuals. We used to get stick-shaker occasionally on the OneEleven in approach turbulence; and pre-stall ignition in cruise turbulence on the VC10, particularly if we were operating with a buffet margin below 1.35g for the weight. We were told these were phase-advanced warnings/protections, but the manuals are not to hand...
You quote the French text from the part of the BEA Report which states that the "two distinct signatures" of speed anomalies:
"Les anomalies de vitesses peuvent se caractériser par deux signatures distinctes :
- chutes intermittentes (pics),
- chute suivie d’un palier (période continue)."
Readers of my post will have noted that it is the word "pics" (translated to "peaks" in the English version) that creates the ambiguity in the English text:
" - intermittent falls (peaks);"
In the English, I'm led to think that, because "peaks" is in brackets, it refers to sharp peaks and sharp troughs of a negative (under-reading) curve. If it had been written "intermittent falls/peaks", that would have suggested alternate under-reading/over-reading.
In the French, the latter interpretation seems even less likely: they would have needed to write something like "chutes ou pics intermittentes". One of the definitions of "pic" in my Petit Larousse is "FIG: Maximum d'intensité atteint par un phénomène...". In this case, I think the maximum intensity is in the negative sense.
Apologies to (any) readers for the descent into semantics, but − unless the BEA author is prepared to clarify it him/herself − it's a necessary one. (The designers of FBW logic necessarily have to avoid possible ambiguities of this kind!)
Chris
Thanks for your characteristically perceptive response to my post #1224.
Quote:
"Sensor Validation points to the possibility of pitot overpressure due to drain blockage, but adds that he doesn’t know by which percentage. Until someone comes up with that information, I’m inclined to think that the effect is quite small."
That's my assumption too (low percentage). My own pilot's-eye view during walk-rounds (until they got too high for close inspection!) was that the holes' exit ends were often contaminated slightly by oxidation and/or burnt de-icing fluid. I doubt their area of cross-section is controllable in everyday service. It makes absolute sense that an unblocked drain-hole bleeds some dynamic pressure, requiring a correction. But if a partial blockage is acceptable, my guess is that a complete one would not in itself make a serious difference.
Quote:
"To my knowledge, stall warnings are not phase advanced (stickpushers sometimes are)".
Maybe not, and my assumption was influenced more by experience on VC10s and OneElevens than what little I've found in Airbus Tech Manuals. We used to get stick-shaker occasionally on the OneEleven in approach turbulence; and pre-stall ignition in cruise turbulence on the VC10, particularly if we were operating with a buffet margin below 1.35g for the weight. We were told these were phase-advanced warnings/protections, but the manuals are not to hand...
You quote the French text from the part of the BEA Report which states that the "two distinct signatures" of speed anomalies:
"Les anomalies de vitesses peuvent se caractériser par deux signatures distinctes :
- chutes intermittentes (pics),
- chute suivie d’un palier (période continue)."
Readers of my post will have noted that it is the word "pics" (translated to "peaks" in the English version) that creates the ambiguity in the English text:
" - intermittent falls (peaks);"
In the English, I'm led to think that, because "peaks" is in brackets, it refers to sharp peaks and sharp troughs of a negative (under-reading) curve. If it had been written "intermittent falls/peaks", that would have suggested alternate under-reading/over-reading.
In the French, the latter interpretation seems even less likely: they would have needed to write something like "chutes ou pics intermittentes". One of the definitions of "pic" in my Petit Larousse is "FIG: Maximum d'intensité atteint par un phénomène...". In this case, I think the maximum intensity is in the negative sense.
Apologies to (any) readers for the descent into semantics, but − unless the BEA author is prepared to clarify it him/herself − it's a necessary one. (The designers of FBW logic necessarily have to avoid possible ambiguities of this kind!)
Chris
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takata
The gap in connection is not proved, I agree.
HazelNuts39
append...... One of your points was to say that the VS joints had not failed, hence the vertical failure of the Laterals is affirmed. I disagree. With respect, look at the condition of the female component in a separate picture of this area. Where the Web changes thickness, a most distinct fracture. This is classic shear failure, ie: lateral. No vertical stress can produce that separation; required is a lateral, and more than one. That this failure signs lateral stress short of part separation is most compelling and supportive of in flight failure. Is there also vertical damage? I would say yes. Separated from the a/c, the VS and Rudder will fall straight down with the heavy end down. This area is filled with skin from the aft tail deck. It entered the water and slowed immediately; this also explains the good condition of the sides, and the Rudder. The damage to the Rudder at its lower end is consistent with hydraulic deformation. Will you take another look?
bear
The gap in connection is not proved, I agree.
HazelNuts39
append...... One of your points was to say that the VS joints had not failed, hence the vertical failure of the Laterals is affirmed. I disagree. With respect, look at the condition of the female component in a separate picture of this area. Where the Web changes thickness, a most distinct fracture. This is classic shear failure, ie: lateral. No vertical stress can produce that separation; required is a lateral, and more than one. That this failure signs lateral stress short of part separation is most compelling and supportive of in flight failure. Is there also vertical damage? I would say yes. Separated from the a/c, the VS and Rudder will fall straight down with the heavy end down. This area is filled with skin from the aft tail deck. It entered the water and slowed immediately; this also explains the good condition of the sides, and the Rudder. The damage to the Rudder at its lower end is consistent with hydraulic deformation. Will you take another look?
bear
Last edited by bearfoil; 28th May 2010 at 19:39.
Bearfoil,
the scenario you described here seems not to be supported by any of the debris.
The aftstructures show compression damage. As the Tail produces downforce, it would obviously fail downwards. This would rather cause strain failure of the tail structure.
Moreover the LDCRM has been flattened. Structure above that was bent upwards. So a Belly first impact seems the only logical assumption.
In the scenario you described the HS/tail would fail first in a downward bending.
This would be followed by an extreme pitch down with enourmóus negative g's, tearing off the wings and with some likelyhood the nose.
All those parts would uncontrollably tumble back to mother earth.
Probability that all parts would impact upright is almost below 0.
Authopsy reports had stated that all bodies showed severe spinal compression injuries.
So taking all this together, I would say this scenario is among the highly improbable ones. (I never say impossible, in real life there is no such thing as 'impossible').
the scenario you described here seems not to be supported by any of the debris.
The aftstructures show compression damage. As the Tail produces downforce, it would obviously fail downwards. This would rather cause strain failure of the tail structure.
Moreover the LDCRM has been flattened. Structure above that was bent upwards. So a Belly first impact seems the only logical assumption.
In the scenario you described the HS/tail would fail first in a downward bending.
This would be followed by an extreme pitch down with enourmóus negative g's, tearing off the wings and with some likelyhood the nose.
All those parts would uncontrollably tumble back to mother earth.
Probability that all parts would impact upright is almost below 0.
Authopsy reports had stated that all bodies showed severe spinal compression injuries.
So taking all this together, I would say this scenario is among the highly improbable ones. (I never say impossible, in real life there is no such thing as 'impossible').
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bearfoil;
I've looked at the picture with the label: "Right-hand aft lug: shearing of the fuselage along main frames 86-87". In my post I referred to "lateral failure of the main lugs or a failure in the structure between left and right attachments." What I meant was a failure that would provide room for the V/S spar and the male lugs attached to it to move laterally sufficiently to rupture the pick-ups. That's not shown in this picture. Furthermore, if that had happened, one of each pair of lateral pick-up rod would have failed in tension and the other in compression.
Finally, I've no experience in looking at fractures and, frankly, don't see what you apparently see.
HN39
I've looked at the picture with the label: "Right-hand aft lug: shearing of the fuselage along main frames 86-87". In my post I referred to "lateral failure of the main lugs or a failure in the structure between left and right attachments." What I meant was a failure that would provide room for the V/S spar and the male lugs attached to it to move laterally sufficiently to rupture the pick-ups. That's not shown in this picture. Furthermore, if that had happened, one of each pair of lateral pick-up rod would have failed in tension and the other in compression.
Finally, I've no experience in looking at fractures and, frankly, don't see what you apparently see.
HN39
Last edited by HazelNuts39; 28th May 2010 at 20:19. Reason: word missing
A couple of days back there was a discussion about buoyancy of bodies in the water.
One thing which was not mentioned in this context (or at least I failed to notice it) was the fact that the authopsies mentioned that the cause of death was blunt force, no drowning.
This means that the lungs probably were not filled with water (at least not fully). The usual cases where bodies are in the water are those where people drowned. This makes a significant difference with regard to buoyancy.
This fact combined with the fact that it was in rather salty water makes me wonder if the bodies were really submerged in the first days. Personally, I doubt it for the reasons mentioned.
One thing which was not mentioned in this context (or at least I failed to notice it) was the fact that the authopsies mentioned that the cause of death was blunt force, no drowning.
This means that the lungs probably were not filled with water (at least not fully). The usual cases where bodies are in the water are those where people drowned. This makes a significant difference with regard to buoyancy.
This fact combined with the fact that it was in rather salty water makes me wonder if the bodies were really submerged in the first days. Personally, I doubt it for the reasons mentioned.
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HazelNuts39
In vertical failure, the rods are both in compression, acting as a jack would when bearing a Landing gear in the shop. The stress is assumed to be roughly equal (symmetrical), so these rods would be actually lessening the Stress on the lugs when so challenged, as opposed to their lateral design function, in which the rods alternate compression, tension, when the surface of the VS and Rudder deviate from centered aspect. Consider that as designed, they alternate pushing and pulling in each control input. Both serve to stress the eyebrow (mounting rail) away from the stressed face, and toward the unstressed side. This rail is surface mounted to the skin of the fuselage. One can be misled by entertaining one cycle of this stress at a time. The fracture one sees on the female lug shows the result of overstressing the entire bedding of the system. Its expression as a failure in the body of the component rather than one or both of its lugs merely shows that the system has worked, but exceeded its design load. The crack in each component is the result of the entire tail surface "racking", the rods have succeeded, but the design has failed. If one assumes skin cannot rack, think of a high wing aircraft with dyhedral that has flipped onto its back, (The 330 skin is riveted), every rivet will show a very small amount of slip. The added slip of each rivet contributes to several inches over several feet. The difference in additive skin movement between what occurred and what was necessary is the dimension of the fracture at maximum load, that load that occurred past the last successful defense of the lugs, laterally.
henra
Regarding the "Spinal Compression" results, I believe it was not all the victims, but 43, and these in concert with pelvic fracture and thoracic bone fractures.I too dont believe the recovered victims sank, except as a result of wave action, and then only briefly.
bear
In vertical failure, the rods are both in compression, acting as a jack would when bearing a Landing gear in the shop. The stress is assumed to be roughly equal (symmetrical), so these rods would be actually lessening the Stress on the lugs when so challenged, as opposed to their lateral design function, in which the rods alternate compression, tension, when the surface of the VS and Rudder deviate from centered aspect. Consider that as designed, they alternate pushing and pulling in each control input. Both serve to stress the eyebrow (mounting rail) away from the stressed face, and toward the unstressed side. This rail is surface mounted to the skin of the fuselage. One can be misled by entertaining one cycle of this stress at a time. The fracture one sees on the female lug shows the result of overstressing the entire bedding of the system. Its expression as a failure in the body of the component rather than one or both of its lugs merely shows that the system has worked, but exceeded its design load. The crack in each component is the result of the entire tail surface "racking", the rods have succeeded, but the design has failed. If one assumes skin cannot rack, think of a high wing aircraft with dyhedral that has flipped onto its back, (The 330 skin is riveted), every rivet will show a very small amount of slip. The added slip of each rivet contributes to several inches over several feet. The difference in additive skin movement between what occurred and what was necessary is the dimension of the fracture at maximum load, that load that occurred past the last successful defense of the lugs, laterally.
henra
Regarding the "Spinal Compression" results, I believe it was not all the victims, but 43, and these in concert with pelvic fracture and thoracic bone fractures.I too dont believe the recovered victims sank, except as a result of wave action, and then only briefly.
bear
Last edited by bearfoil; 28th May 2010 at 21:32.
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Originally posted by auv-ee ...
The spec sheet (Dukane model DK120 and DK100 Underwater acoustic locating beacons) shows a different tolerance for the frequency (1kHz/37.5kHz =2.7%) than for the pulse width and repetition rate, each at 10% (each listed as 9ms and 0.9sec from a likely, but unstated, nominal of 10ms and 1 sec, respectively). This means that they are not derived from the same time base.
The spec sheet (Dukane model DK120 and DK100 Underwater acoustic locating beacons) shows a different tolerance for the frequency (1kHz/37.5kHz =2.7%) than for the pulse width and repetition rate, each at 10% (each listed as 9ms and 0.9sec from a likely, but unstated, nominal of 10ms and 1 sec, respectively). This means that they are not derived from the same time base.
It appears the 37.5kHz piezo-ceramic transducer looks after itself, and the only requirement is for the 10msec duration switching pulse every second, and that would be derived from a separate time base.
Note the 1800 - 3600m detection range. That would imply that in deeper water, the Emeraude would need to go deep[?] to stand a chance of detecting a signal, whereas the Towed Pinger Locaters are designed to do that.
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The other way would have been for Emeraude to weight down the tail end of the towed array and to cruise much much slower.
More pings per metre. That's the name of the game in sonar location.
More pings per metre. That's the name of the game in sonar location.
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All the evidence examined to date clearly shows the fuselage must have impacted the water intact in a rapid vertical descent with relative slow travel in the horizontal plane. The bodies recovered in evidencing compression injuries, being well clothed and well preserved, that there was no fire or explosion, that the radome, a fairly fragile structure, was recovered, bear witness to this.
The possibility of electrical activity at the FL existed, but the presence of SCW at FL 350 has not been considered very probable and would necessarily have been limited to small quantities. However there was convective activity along the planned route. I do not therefore believe that pitot icing was a factor. It is I believe more likely that the aircraft in its manoevers to avoid CBs, inadvertently entered an area of intense wheather and possibly lost a part or the whole of one or both wings, resulting in a spiralling flat spin to the ocean below. This I believe is a far more likely scenario which may also explain the lack of any further transmissions from the aircraft and the location of debris recovered.
I wait with some trepidation to see what the TV moguls make of it. My guess would be that it would be the usual sensationalism to scare the travelling public just at the right moment in time, the start of the holiday season in Europe.
The possibility of electrical activity at the FL existed, but the presence of SCW at FL 350 has not been considered very probable and would necessarily have been limited to small quantities. However there was convective activity along the planned route. I do not therefore believe that pitot icing was a factor. It is I believe more likely that the aircraft in its manoevers to avoid CBs, inadvertently entered an area of intense wheather and possibly lost a part or the whole of one or both wings, resulting in a spiralling flat spin to the ocean below. This I believe is a far more likely scenario which may also explain the lack of any further transmissions from the aircraft and the location of debris recovered.
I wait with some trepidation to see what the TV moguls make of it. My guess would be that it would be the usual sensationalism to scare the travelling public just at the right moment in time, the start of the holiday season in Europe.
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originally posted by Low Flier ...
The other way would have been for Emeraude to weight down the tail end of the towed array and to cruise much much slower.
The other way would have been for Emeraude to weight down the tail end of the towed array and to cruise much much slower.
mm43
Last edited by mm43; 29th May 2010 at 20:09. Reason: clanger! changed 32.5 to 37.5kHz
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In post #1216 Bearfoil wrote:-
Why would the Port Outer Spoiler (the recovered one) have been deployed?
My understanding is that the outer spoilers can only be deployed following both MLG on the deck when landing or in a RTO above 72KT. Both outer spoilers are not used for speed brake or roll.
The Speed Brake section in A340 - A330 Control: Flight Laws gives a fairly concise description.
mm43
The spoilers are part of the upper wing surface. The recovered Spoiler shows evidence of collapse in a deployed position.
My understanding is that the outer spoilers can only be deployed following both MLG on the deck when landing or in a RTO above 72KT. Both outer spoilers are not used for speed brake or roll.
The Speed Brake section in A340 - A330 Control: Flight Laws gives a fairly concise description.
mm43
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Originally Posted by Machinbird
Don't worry about how this hypothetical pressure build up could happen just yet. The key questions I am hoping to answer are,
(1) Would this scenario likely proceed to a sad conclusion?
(2) Is Pilotaydin's sim experience technically flawed in its description?
(3) If the sim experience is technically valid, how could you break the sequence of events?
(1) Would this scenario likely proceed to a sad conclusion?
(2) Is Pilotaydin's sim experience technically flawed in its description?
(3) If the sim experience is technically valid, how could you break the sequence of events?
(3) To prevent such scenario to fully develop, only a binome of 007 agents would deactivate the high speed protection as soon as triggered by degrading the system to alternate law which could be done by switching OFF two of the ADRs.
As I didn’t get the opportunity to practice any of this in a simulator, I cannot guaranty the reasoning behind.
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Bearfoil wrote:-
Yes, and also the Port Outer which was found on 13 June at 1321z in position 04°42.1'N 029°55.6'W, or 42NM approx N by E of TASIL. Both were reported by the BEA to have suffered the same fate.
I have an image of the top surface somewhere, and will post it when I find it.
EDIT :: Pic #1 and #2 show the spoiler on the surface when found and the last two shots are quite clear in showing the underside damage.
mm43
My recall is that the spoiler recovered was the Port, Inner.
I have an image of the top surface somewhere, and will post it when I find it.
EDIT :: Pic #1 and #2 show the spoiler on the surface when found and the last two shots are quite clear in showing the underside damage.
mm43
Last edited by mm43; 29th May 2010 at 05:17. Reason: additional spoiler pics added
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Originally Posted by mm43
I think they are effectively quoting from the TSO C121 spec. The following snippet from their manual would indicate something different:-
Originally Posted by mm43
It appears the 37.5kHz piezo-ceramic transducer looks after itself, and the only requirement is for the 10msec duration switching pulse every second, and that would be derived from a separate time base.
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Originally posted by auv-ee
Ok. We've done this to death, and I suspect you are right with one or two earlier stages to improve the Q. But it potentially shows that with the loose spec, that the likelihood of two pingers on the same aircraft chirping at the same frequency is almost zero.
mm43
A piezo transducer, coupled to water, has a Q of about 3 to 5, which is not really narrow enough to control the frequency within +/-2.7%, as called for by the spec. Thus there must be one or more time bases to generate the 37.5kHz, 10ms and 1second timing.
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The Genesis (i.e. up to that point of sudden autopilot kick-out in the cruise)
Chris Scott said:
I can understand that, in the absence of auto-throttles, that a progressively ice-accumulating pitot head would cause an “indicated” airspeed drop-off – and that the crew would be alerted and would be likely to intervene. Whether they’d interpret it as an actual loss of performance is another thing. With a subtle loss of indicated airspeed, the first inclination would be to start thinking engine icing and airframe ice excrescences gradually (and cumulatively) causing increased drag/perf loss.
However, with no disagreement (at least initially) between the three airspeed/mach indications, in an A330 the compensating autothrottles would simply incrementally pump more go-juice into the equation to maintain the stipulated cruise speed….. would they not? If the pilots had their fuel schematic (instead of engine parameters) up on the screen (often the case I believe, when fuel may be tight, especially in an A330) and with the non-moving Airbus throttles, this minor increase in fuel flow/decreased AoA and slightly lower pitch attitude would probably go unnoticed. That’s the nature of an insidious event such as symmetric ice crystal accumulation in three Thales pitot heads…… until the displayed airspeed split becomes significant enough to trigger a “disagree”. If they were at FL350 in layered CirroCU and CS, it’s not likely to have been more than pebbly turbulent along their radar-chosen best course between build-ups – so they’d have not been “on the qui vive” and expecting trouble (aka settled into mind-numbing cruise ennui, if you will).
The end result (see Belgique pg61/post #1208 at link) of this quietly sinister acceleration would eventually be an aircraft headed, quite non-apparently, towards a Mach crit encounter. At, or even just prior to that juncture, either an airspeed disagree, pitch-trim disparity or (credibly) a mach-induced pitch-down may have caused the autopilot to click out – and then the crew would’ve been in a world of hurt as far as suddenly ascertaining what was what amongst their confusing displayed performance criteria. That’s a recipe for snap decisions, cross-cockpit misunderstandings and a “sucked-in” and fatally incorrect PF choice…… particularly in an environment where very few pilots have handflown a degraded law in a ‘Bus in IMC at night whilst trying to troubleshoot. Bedlam might be an adequate description of the scenario.
A pilot well-drilled in believing his instruments might have trusted the still credible airspeed display on his PFD and unwittingly had a (follow-up) Mach encounter with throttles placed fully forward. I’m not sure to what mach number an A330 would’ve been tested, but I know a few experimental test-pilots who have a healthy regard for flying beyond Mmo/Vne – because there “there be dragons”. It would be an easily achievable no-go area that can hold untold frights... in just this type of set-up. Airbus flight control protections are no longer in play when an aircraft is rolling, pitching and yawing as an aerodynamic direct result of a mach crit encounter. Having myself played those nose-low games in a quite robust jet trainer at great heights, I know that once you leave Mach country in total disarray and get below 20,000ft, things tend to sort themselves out. An airliner isn’t anywhere near as robust and could easily sustain significant structural damage in the first 5000 to 10,000ft plummet…. if the unusual attitude and pilot (or auto-trimmed to well out-of-trim) control response was extreme enough. Which brings me to the next query/point. What could induce a nose-down pitch into an Mmo encounter (which itself involves a reactionary nose-down pitching moment)?
.
During the quiet acceleration into/towards a mach crit encounter, what’s happening with the auto-trim and trimmable horizontal stabilizer (THS)? Is it getting conflicting inputs? (i.e. the computed trim-state from the ADIRU’s accepted and supposedly bona-fide indicated CAS airspeed - versus the actual dynamic pressures resulting from the actual airspeed). If the elevators were deflected and compensating to achieve an out-of-trim height-hold, what could’ve happened pitch-wise when the autopilot jumped out? Would the A330 autopilot have been opposing (i.e. holding) a nose-down pressure on its elevators due to undetected automation conflicts (or would it have been a pitch-up result at A/P disconnect?). Depends how the pitot/static data is being massaged into an autopilot interface - I guess.
We used to be able to declutch the autotrim in a PB20N autopilot and use the varicam trim (similar to THS) on the SP2H to squeeze a few extra knots (about 10 usually) for the cruise. It just achieved an “on the step” optimized attitude (was my diagnosis). It was a genuine increase validated by the navs, rather than an induced IAS position error). However if you forgot, and disengaged the autopilot without first removing the out-of-trim pressure being held by the elevators, you’d get a sudden nasty pitch-down – and some name-calling on the intercom. At coffin corner height in AF447, was this a likely contributing factor to a pursuant Loss of Control?
Any cogent input on the above suppositions?
.
There seems to have been wide agreement here for a day or two that − in level flight, right at the beginning of this AF447 event − whichever ASIs were showing erroneous readings are more likely to have been over-reading the actual IAS/CAS, than under-reading it. Thus, if the crew were misled by the false readings, they might have slowed the aeroplane into a stall near the cruise altitude.
However, with no disagreement (at least initially) between the three airspeed/mach indications, in an A330 the compensating autothrottles would simply incrementally pump more go-juice into the equation to maintain the stipulated cruise speed….. would they not? If the pilots had their fuel schematic (instead of engine parameters) up on the screen (often the case I believe, when fuel may be tight, especially in an A330) and with the non-moving Airbus throttles, this minor increase in fuel flow/decreased AoA and slightly lower pitch attitude would probably go unnoticed. That’s the nature of an insidious event such as symmetric ice crystal accumulation in three Thales pitot heads…… until the displayed airspeed split becomes significant enough to trigger a “disagree”. If they were at FL350 in layered CirroCU and CS, it’s not likely to have been more than pebbly turbulent along their radar-chosen best course between build-ups – so they’d have not been “on the qui vive” and expecting trouble (aka settled into mind-numbing cruise ennui, if you will).
The end result (see Belgique pg61/post #1208 at link) of this quietly sinister acceleration would eventually be an aircraft headed, quite non-apparently, towards a Mach crit encounter. At, or even just prior to that juncture, either an airspeed disagree, pitch-trim disparity or (credibly) a mach-induced pitch-down may have caused the autopilot to click out – and then the crew would’ve been in a world of hurt as far as suddenly ascertaining what was what amongst their confusing displayed performance criteria. That’s a recipe for snap decisions, cross-cockpit misunderstandings and a “sucked-in” and fatally incorrect PF choice…… particularly in an environment where very few pilots have handflown a degraded law in a ‘Bus in IMC at night whilst trying to troubleshoot. Bedlam might be an adequate description of the scenario.
A pilot well-drilled in believing his instruments might have trusted the still credible airspeed display on his PFD and unwittingly had a (follow-up) Mach encounter with throttles placed fully forward. I’m not sure to what mach number an A330 would’ve been tested, but I know a few experimental test-pilots who have a healthy regard for flying beyond Mmo/Vne – because there “there be dragons”. It would be an easily achievable no-go area that can hold untold frights... in just this type of set-up. Airbus flight control protections are no longer in play when an aircraft is rolling, pitching and yawing as an aerodynamic direct result of a mach crit encounter. Having myself played those nose-low games in a quite robust jet trainer at great heights, I know that once you leave Mach country in total disarray and get below 20,000ft, things tend to sort themselves out. An airliner isn’t anywhere near as robust and could easily sustain significant structural damage in the first 5000 to 10,000ft plummet…. if the unusual attitude and pilot (or auto-trimmed to well out-of-trim) control response was extreme enough. Which brings me to the next query/point. What could induce a nose-down pitch into an Mmo encounter (which itself involves a reactionary nose-down pitching moment)?
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During the quiet acceleration into/towards a mach crit encounter, what’s happening with the auto-trim and trimmable horizontal stabilizer (THS)? Is it getting conflicting inputs? (i.e. the computed trim-state from the ADIRU’s accepted and supposedly bona-fide indicated CAS airspeed - versus the actual dynamic pressures resulting from the actual airspeed). If the elevators were deflected and compensating to achieve an out-of-trim height-hold, what could’ve happened pitch-wise when the autopilot jumped out? Would the A330 autopilot have been opposing (i.e. holding) a nose-down pressure on its elevators due to undetected automation conflicts (or would it have been a pitch-up result at A/P disconnect?). Depends how the pitot/static data is being massaged into an autopilot interface - I guess.
We used to be able to declutch the autotrim in a PB20N autopilot and use the varicam trim (similar to THS) on the SP2H to squeeze a few extra knots (about 10 usually) for the cruise. It just achieved an “on the step” optimized attitude (was my diagnosis). It was a genuine increase validated by the navs, rather than an induced IAS position error). However if you forgot, and disengaged the autopilot without first removing the out-of-trim pressure being held by the elevators, you’d get a sudden nasty pitch-down – and some name-calling on the intercom. At coffin corner height in AF447, was this a likely contributing factor to a pursuant Loss of Control?
Any cogent input on the above suppositions?
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Last edited by TheShadow; 29th May 2010 at 09:14.
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Originally Posted by TheShadow;#1256
a progressively ice-accumulating pitot head would cause an “indicated” airspeed drop-off
HN39