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Old 30th Sep 2016, 19:03   #1141 (permalink)
 
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Uplinker,
Take care not to extent your experience too far. Detecting unusual speed tape behaviour depends on having a datum; what is 'unusual', when to act, the dividing line between normal and abnormal.

EFIS sensor failures are usually clear cut - blank display / red flag. Another form of failure is the state of 'no computed data', where the sensor is working but not outputting valid data.
In all of these cases the manufacturer has to undertake a risk assessment and only then as necessary provide a crew drill for rectification or risk mitigation.

However, in the event of unusual system behaviours without alerts or warning (very very unusual, i.e. low risk) then crew intervention might be required; but given system reliability and certification requirements it would be more likely that the situation has been miss assessed;- "what have I misunderstood or not seen".

Most abnormal drills only apply to the situation as defined by the manufacturer; do not attempt to fit a procedure to a situation.
Infrequently pilots will encounter situations where their assessments are inaccurate and thus choose an inappropriate procedure; a situation and reasoning which you eruditely recalled from your simulator experience.

The lesson to be learnt is to reassess a situation, particularly if the chosen drill is ineffective.
But how long to wait, how much time is available, what else to check, ... such is the pilots burden, experience, knowledge, judgement.

But all is not lost; with diligent observation pilots can store knowledge of what is 'normal' (a datum), which can help in the very rare situations.
We would be unfortunate to encounter one of these in a career ... except those, in one form or another, which do exist, more often generated by ourselves; and because of this they involve mind numbing surprise and reluctance to change.
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Old 1st Oct 2016, 12:51   #1142 (permalink)
 
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Pax so free to ignore. OG, re the stall warning, I think the point about latching the warning is that the system alarmed on valid data, the data then went invalid (as it happens because of the scenario which generated the alarm) and the system logic gave priority to the validity limit. So the system discounted it's previous diagnosis which was based on data it judged valid and just shut up. The logic designers missed this in my view; at the least some warning that the system was off line could be generated ie you might be stalled but I can't tell, best check yourselves!

Edit: mind you if the environment is such that you don't register the aural alarm, as witnessed above, perhaps other design changes are needed first. Perhaps the mitigation claimed in the safety case by raising an aural alarm needs to be factored if other alarms are sounding concurrently.

BTW, wasn't Uplinked case a bit more insidious than AF ? They had the a/p drop out, not the slowly developing situation in Ups case. Mind you, the captain absenting himself could have been a clue that something was afoot

Last edited by Mr Optimistic; 1st Oct 2016 at 13:11.
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Old 1st Oct 2016, 17:45   #1143 (permalink)
 
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Quote:
Originally Posted by RetiredF4
nice to see you guys back
I second that!

No, really. I am glad that due to lack of fresh Airbus accidents, most of you are reduced to flogging the largely decomposed horse of AF447 threads with same old unrealistic theories and not quite believable "look what happened to me" stories.

Point about "I didn't hear stall warning so it should be made stronger and work below 60 kt" was covered in Final report, 1.16.2 and Appendix 7 to Interim2. 40-odd recorded events of pitot blockage and degradation to alternate law on 330/40ies ended uneventfully. Except AF447. Whoever got stall warning, heard it and pushed. Except that one crew. BEA was unable to explain what made AF447 case unique, or rather very rare, as similar stuff of pulling till impact in stall happened on DHC-8 and MD-80 before and A320 after the crash.

With 40 other similar cases on the type ending without scratch (except maybe TAM), it would be very brave to stake one's reputation on the claim "Aeroplane did it!" without protection of PPRuNeanonimity.
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Old 1st Oct 2016, 18:49   #1144 (permalink)
 
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Clandestino
I agree with you.
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Old 1st Oct 2016, 20:24   #1145 (permalink)
 
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Pax again. I am surprised by this. The system is crew+ aircraft and it failed. From this rare accident, in the context of the very low probability targets used by the industry, I do not know why the conversation has ended here.
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Old 1st Oct 2016, 23:07   #1146 (permalink)
 
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Quote from Mr Optimistic:
"...I think the point about latching the warning is that the system alarmed on valid data, the data then went invalid (as it happens because of the scenario which generated the alarm) and the system logic gave priority to the validity limit. So the system discounted it's previous diagnosis which was based on data it judged valid and just shut up. "

Yes, that's a fair description. Owain's suggestion that the stall warning should be latched on until the system receives a valid non-stall AoA from the AoA sensor is a good one, IMO. That clearly was not the case in AF447 but - assuming he is quoting the relevant Airbus documentation correctly, without significant omission - Confit rightly points out that the Airbus documentation can be interpreted as promising that it would. Quote from AI documentation via CONF_iture:
"...a permanent aural alert (“STALL, STALL” synthetic voice then cricket) is triggered until a correct angle-of-attack is recovered."

I would define that as the first aspect of Airbus FBW stall-warning logic that might be reconsidered. The second area that concerns me was also applicable to the stall-warning systems on most of the six swept-wing transport types I've flown. As I commented recently here in post 1110, the warning signals operate at a constant intensity regardless of how much the stall AoA is being exceeded. When the warning results from intermittent exceedances, perhaps with phase-advanced warnings in turbulence or wind-shear, the pilot may infer that the degree of exceedance, while unacceptable and demanding immediate recovery-action, is low. In the case of AF447, however, the intermittent nature of the warnings persisted (for reasons that, with the benefit of hindsight, are well understood) despite the stall becoming deeper and deeper. This could happen again.

However, moving away from the specific example of AF447: if a crew was suddenly provided with an AoA indicator on each PFD that appeared only when the stall warning was activated, and if that AoA clearly showed the degree of AoA exceedance, any subsequent removal of it due to sensor invalidation - announced with a suitable failure caption - would be far from ideal. But at least the pilot would have been given a good idea of the depth of the stall during the period of validity of the system.

The inclusion of an AoA indicator would demand initial and recurrent training in the simulator, of course, during which recoveries from low-altitude shallow stalls and high-altitude stalls, both shallow and deep, would be practised in Alternate or Direct Law. Simulators would need to be improved in the high AoA, high altitude regimes, assisted to some extent by data from AF 447.

The third area where I would suggest change relates to handling practice. On revenue flights airlines have increasingly discouraged, if not forbidden, flight without AP and FD, as well as the use of manual thrust, particularly at high altitudes. Pilots are expected to spend many thousands of mind-numbing (on long-haul) hours simply monitoring AP and A/THR, only to perform safely on the once-in-a-career occasion that one or both of them fail.

Although it would have to be in Normal Law, regular practice of handling cruise flight, including step climbs and descents, at least puts pilots in a stronger position to handle abnormalities such as befell AF447. Easy to read that a pitch change of one degree represents a VS change of about 800 ft/min at Mach 0.8; quite another to be in regular practice at doing it. Thus, the management of pitch & thrust become second nature, instrument scan is improved, and the panic factor in failure situations significantly reduced.
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Old 2nd Oct 2016, 09:08   #1147 (permalink)
 
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Quote:
I would define that as the first aspect of Airbus FBW stall-warning logic that might be reconsidered.
AI has already considered the problem and is trialling new FWC software:
See post #1100

"STALL WARNING enhancement: Stall warning will work when:
Undetected erroneous computation of pitot
Pitot out of the airflow
Pitot obstructed by ice or any foreign material at any speed (function now possible below 60 kts)"
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Old 2nd Oct 2016, 15:27   #1148 (permalink)
 
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STALL WARNING enhancement

Quote:
"STALL WARNING enhancement: Stall warning will work when:
Undetected erroneous computation of pitot
Pitot out of the airflow
Pitot obstructed by ice or any foreign material at any speed (function now possible below 60 kts)"
Actually the logic isn't changed, the AoA sensor information is fed trough the IRU part of ADIRU i.s.o. the Air Data part, resulting in the AoA data remaining available to FWC in case of Airspeed < 60 kts.

This kind of system updates depend on the modification status of the particulair aircraft/fleet.

AFAIK If you didn't opt for de BUSS enhancement you won't have the quoted "STALL WARNING enhancement"

Besides this there are more updates since the event.
  • BUSS(before event) and further enhancement >Reversible BUSS.
  • Hybrid FPA.
  • Visible Red 'STALL STALL' warning on PFD.

Related to the Blocked AoA sensor issues:
  • Separate AoA sensor monitoring.
  • Replacement of all UTAS sensors by Thales sensors.
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Old 2nd Oct 2016, 15:56   #1149 (permalink)
 
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Sorry Goldenrivett, I don't know why I didn't pay attention to the post #1100 initially.
So I can see things have been moving on the software side and our concerns were well justified after all ...
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Old 2nd Oct 2016, 16:34   #1150 (permalink)
 
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Clandestino, I invite you to visit the thread about the Egyptian A320 that recently fell from the sky between Crete and Cairo. Plenty of discussion of AB features there, though with the various tidbits and clues, at least UAS isn't in the list of factors felt likely so far.
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Old 2nd Oct 2016, 19:29   #1151 (permalink)
 
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Hi Goldenrivett,

Apologies for overlooking your post #1100, and thanks for the heads-up on mods. Must admit, however, that - assuming your quote is verbatim - the AI information on the logic changes could be more specific and more informative.

Quote from A33Zab:
"Actually the logic isn't changed, the AoA sensor information is fed trough the IRU part of ADIRU i.s.o. the Air Data part, resulting in the AoA data remaining available to FWC in case of Airspeed < 60 kts."

Thanks for that, from which I infer that the minimum figure of 60 kt (sensed) IAS for validity still applies to all AoA data except that used for the stall warning?
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Old 3rd Oct 2016, 08:41   #1152 (permalink)
 
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A33Zab
Quote:
Actually the logic isn't changed, the AoA sensor information is fed trough the IRU part of ADIRU i.s.o. the Air Data part, resulting in the AoA data remaining available to FWC in case of Airspeed < 60 kts.
When BUSS was installed it needed switching off all three ADRs and with that Stall warning would have disappeared, that is why they brought in a mod that gets AOA data through IRs, that's all. Whatever was happening through ADR now happens through IR. I do not think it has any connection to invalidity below 60kts. Yes you need BUSS for this MOD. The FPA has been modified through
MOI: 153528/P12909 (A320FAM)
Where with all ADRs loss baro-inertial vertical parameters are replaced by hybrid GPIRS parameters. As a result Bird is available and should be used in Unreliable speed situation.
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Old 3rd Oct 2016, 21:36   #1153 (permalink)
 
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Quote from A33Zab (my emphasis):
"Besides this there are more updates since the event.
BUSS(before event) and further enhancement >Reversible BUSS.
Hybrid FPA.
Visible Red 'STALL STALL' warning on PFD."

The problem with a simple "STALL, STALL" warning on the PFD is that it is qualitative, but not quantitative. How is the pilot to decide what degree of pitch-down to apply?

In the past on swept-wing jets, we have been advised on the receipt of a stall warning to pitch down a relatively small amount AND apply TOGA thrust. While that technique has recently been rightly criticised (not least on this thread and its predecessors), it would probably suffice in most cases if the first activation of stall warning has happened immediately the stall AoA is exceeded - as would normally be the case on final approach. But in the event of UAS, the stall warning may not occur initially because of the consequent invalidation of AoA data. In that case the first stall warning activation may not take place until the a/c is already in a well-developed or even deep stall.

That is why I'm suggesting consideration be given to the provision of an AoA indicator that would appear on the PFD only in the unique situation of stall-warning activation. The degree of exceedance of stall AoA could be approximated in a clear graphic on the indicator, giving the pilot a good cue as to what kind of pitch-down may be required to start the recovery. I say "approximated" because some data needed to calculate the precise stall AoA may be unavailable or unreliable, but the stall warning system itself must be using a credible value.

After the initial pitch-down, a secondary stall could be more easily avoided in the subsequent pull-up if the AoA indicator remained displayed until the AoA (and perhaps pitch-attitude) returned to within normal ranges.

Yes, I realise this would increase the airlines' training costs...
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Old 4th Oct 2016, 18:01   #1154 (permalink)
 
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Chris your argument for a dedicated AoA display makes several assumptions.
First that attitude is proportional to the indicated AoA; this might not be true for large values, or a range of configurations, wt, cg, and in this instance gross mis trim.
Might there be a difference between a recovery using the alpha gauge only (military pilots?) vs the AoA value having to be interpreted to cue an attitude change; the latter being important re roll attitude.

Second, it is assumed that the crew will 'see' the indicator, comprehend its meaning, and act accordingly. EFIS aircraft already have forms of AoA displays, e.g. min speed and barbers pole, providing the ASI is working. Any new display could add workload in an emergency situation.
Where new displays have been introduced to minimise risk after a primary system failure; they are located in similar (logical) task related positions as those in normal operation.

A key aspect in this and similar accidents is that crews were apparently unaware of the situation and 'puzzled' by indications and cues already available; thus an important quality of any new display / warning is to trigger a change in perception, together with high quality information, - not necessarily about what is happening, but what to do.
This is a demanding design task, particularly as in the 'highlight' accidents, the crews put the aircraft into the stall, thus there was a strong belief that their awareness and actions were correct.

Also, with the thread continuing focus on recovering from an extreme situation, the problems of awareness preceding the event and methods of stall avoidance are being overlooked. How did the crews get into these situations, what was the basis of their awareness.
This accident involved a complex interaction and degradation of several systems, hence the concepts of BUSS and FPV using existing display location for 'failure' conditions, which appear to be have been chosen to provided a degree of normality for stall avoidance and recovery, not an extreme 'one off' accident.

A further area for consideration is whether protected aircraft generate crew dependance on the protections, vs indicators and crew action for non protected aircraft; which crew would have the better awareness. If this is the case then crews in protected could suffer the greater surprise, adding complexity and difficulty in comprehending the situation when the protections degrade.
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Old 5th Oct 2016, 10:22   #1155 (permalink)
 
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Some good points, safetypee, and well put.

My personal experience combined with what we know about human performance factors suggests that the brain will ignore hearing in a crisis and therefore the audio "stall stall" is too subtle and might not be even noticed.

A visual warning on the PFD might work, but if the PFD is already confusing the pilot, then it might only cause more confusion. As a civilian pilot, I have never flown with an AoA gauge, so cannot comment. All the Cessnas, Pipers, Shorts, Dashs, and BAe146s that I flew previous to the Airbus have some sort of shaking felt through the controls as a stall is approached - either actual shaking of the horizontal stabiliser felt through the yoke, or an artificial stick shaker.

As a previous electronics engineer, it would seem to me a simple matter to make the Airbus side-stick vibrate in order to warn of an impending stall. A vibrating element such as the type used in mobile phones could be fitted into the side stick - in place of that oval panel on the top, and a small electronic driver circuit the size of a matchbox to convert the already present stall warning into pulsing the vibrating element on and off could be constructed for a few pounds. The wires feeding the vibrating element would route through the same channel in the side-stick as the wires to the PTT switch. So it would be a very simple retro fit.

Then, no matter how much the pilot's hearing had shut down, and no matter how confusing the PFD might be, the pulsed vibrating of the side-stick in their hand would warn of a stall, and should provide an instinctive un-stall reaction.

Some might say that I should not be thinking of modifying the aircraft, but this problem has killed people, so something should be done.
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Old 5th Oct 2016, 16:18   #1156 (permalink)
 
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There is always something to learn from an accident, providing we identify relevant issues.
In complex accidents it is difficult to identify clear safety benefits of technical solutions, particularly those events involving errant human behaviour; engineering arguments often revert to 'more training' or draft another SOP.
The industry needs a measure of the effectiveness of technical interventions being able to influence human behaviour; this cannot be absolute, only a judgement.

Stick shakers have demonstrated a high level of effectiveness. The 146 was particularly good because the stick was mounted on the 'floating' cockpit floor, so there was an enhanced effect which alerted pilots when the autos were engaged (although IIRC the 146 autos disengaged at Vss).
It would be interesting to investigate the effectiveness of floor or seat mounted shakers in sidestick aircraft for improving awareness and stall avoidance.

Thinking further afield, some notable stall accidents involved generic issues related to the operators training. Colgan had recently focussed on tail stall (requiring the crew to pull up), which was not applicable to the type. The A330 required refresher training for loss of airspeed, where memory items required an initial high nose attitude, which did not apply in the cruise.
As discussed previously, closer monitoring of training practices might help, so too an understanding of why regulators believed that these training programmes were necessary.

Much of what has been discussed and done for safety improvement focuses on a single event which rarely applies across the industry or for future 'unforeseeable' events; we tend to fix the last problem and overlook generic items because it is difficult to agree on the judgement of their projected effectiveness.
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Old 5th Oct 2016, 21:10   #1157 (permalink)
 
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Hi safetypee,
Thanks for your thoughtful responses and criticisms. Having been accustomed to them on five jet transports until I flew the A320, I agree that some form of stick-shaker should be considered. Just a few points to clarify.

Quote:
"Chris your argument for a dedicated AoA display makes several assumptions.
First that attitude is proportional to the indicated AoA..."


Presumably you are referring to the amount of pitch-down required for initial recovery? In fact I was careful not to suggest that if the AoA indicator showed the AoA was x degrees above the stall the pilot would simply need to reduce the pitch-attitude by the same amount. This is what I wrote:
"The degree of exceedance of stall AoA could be approximated in a clear graphic on the indicator, giving the pilot a good cue as to what kind of pitch-down may be required to start the recovery."

Clearly I'm not prescribing a technique for stall recovery. But I am suggesting that an AoA indicator would provide: (a) information on the degree of stall, particularly important if the initial warning has been delayed by UAS; and (b) a tool to be used during the recovery to help the pilot avoid a second stall during the pull-up.

On your second point: yes, I am assuming that the pilot "will 'see' the indicator, comprehend its meaning, and act accordingly." In any case, if the pilot isn't looking at the PFD, (s)he has little chance of effecting a recovery... The appearance there of a large AoA indicator, maybe accompanied by a flashing "STALL" caption, should grab the attention. The design would be as simple as possible consistent with showing clearly the degree of exceedance.

The problem with using the existing minimum-speed marker-bugs on the ASI (such as Alpha Max or Vsw) is the enormous variation of Vs with normal G. Any bug allowing for that would go down and up like a yo-yo during the recovery. And with UAS, IAS is not a usable tool.

Perhaps one of our ex-military guys will comment on the practicalities of using an AoA gauge for stall recovery.

Quote:
"Much of what has been discussed and done for safety improvement focuses on a single event which rarely applies across the industry or for future 'unforeseeable' events; we tend to fix the last problem and overlook generic items because it is difficult to agree on the judgement of their projected effectiveness."

That's generally true, but this being an AF447 thread we are bound to consider the combination of a UAS followed by a stall - in which the first stall warning may be inhibited while the stall develops steadily.
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Old 6th Oct 2016, 09:05   #1158 (permalink)
 
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@ Chris Scott, safetypee

Interesting discussion but don't forget

JAR 25.207 Stall warning

(a)
(b) The warning may be furnished either through the inherent aerodynamic qualities of the aeroplane or by a device that will give clearly distinguishable indications under expected conditions of flight. However, a visual warning device that requires attention of the crew within the cockpit is not acceptable by itself. ........................
(c)
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Old 6th Oct 2016, 11:07   #1159 (permalink)
 
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Thanks Owain,

From which I infer that the current Airbus FBW provision to comply with the JAR is the aural element.

A stick shaker or vibrator of some kind seems worthy of consideration, but would have no tactile effect unless and until the pilot took hold of the side-stick. So the initial warning would remain aural. (I'm not sure that a chair-shaker would be either acceptable or efficacious!)

safetypee writes of the BAe 146, and the stick-shaker/vibrator would not be greatly different from traditional types with which I'm familiar (VC10, B707, BAC 1-11, A310, DC10), except that the sidestick is so much smaller than their control-column/wheel combination.

Perhaps the stick vibration could be made to increase if the pilot pulls up...
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Old 6th Oct 2016, 11:35   #1160 (permalink)
 
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Slightly expanding; and I ask from a human performance perspective. Do you think the outcome would have been the same in VMC daylight?
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