AF 447 report out
If a technical solution was simple to implement without serious knock-on effects, it would be done - but the fact is that it is an extremely difficult problem to solve.
as I am done with this hamster abuse.
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How would a WoW switch affect the fact that the sensors don't work in the air if certain parameters are exceeded?
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DozyWannabe,
The stall warning system looks at three values of AoA from three independent sensors. It sounds a warning if the highest of the valid AoA values exceeds the threshold defined by the highest of the valid airspeeds. If I understand correctly, the system can loose any two of those values and it will still operate on the single remaining one. In other words, the system is strongly biased towards giving a warning when required, at the detriment of unwanted operation. Yet, when all three AoA values suddenly disappear, the last valid value being 41.5 degrees, the system just goes silent. No warning, not even a message on the ECAM.
The stall warning system looks at three values of AoA from three independent sensors. It sounds a warning if the highest of the valid AoA values exceeds the threshold defined by the highest of the valid airspeeds. If I understand correctly, the system can loose any two of those values and it will still operate on the single remaining one. In other words, the system is strongly biased towards giving a warning when required, at the detriment of unwanted operation. Yet, when all three AoA values suddenly disappear, the last valid value being 41.5 degrees, the system just goes silent. No warning, not even a message on the ECAM.
Against my better judgment:
The decision to accept or reject the signal and be silent at <60 kts is what is being pointed to as a design oversight. This isn't new. To avoid spurious warnings at low speed on T/O or Ldg with a WoW avoids spurious warning. Any other time, weight off wheels, that you approach those values, the warning will have already been going off long before the "threshold" value of 60 kts.
HazelNuts covered the rest.
See also Confiture's point about air flow when falling at 10,000 fpm.
The decision to accept or reject the signal and be silent at <60 kts is what is being pointed to as a design oversight. This isn't new. To avoid spurious warnings at low speed on T/O or Ldg with a WoW avoids spurious warning. Any other time, weight off wheels, that you approach those values, the warning will have already been going off long before the "threshold" value of 60 kts.
HazelNuts covered the rest.
See also Confiture's point about air flow when falling at 10,000 fpm.
Last edited by Lonewolf_50; 21st May 2013 at 21:29.
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@HN39:
I'm sure you're correct. However we're talking about a situation whereby all three airspeed sensors (pitot tubes) are being fouled by disturbed air as a result of extreme AoA.
You're also absolutely right that implementing a warning - either visual through ECAM and/or aural - that AoA is no longer valid would be do-able, but it's then incumbent on the crew to understand what that means.
@LW_50:
I'm not disputing what you're saying - but the <60kts limit was not implemented purely for the purpose of avoiding spurious warnings on the ground, it was also because the data from the vanes is unreliable in the air in that scenario. CONF iture is working on the assumption that the airflow from the descent would be enough to generate a valid reading, but I'd need to know what the specification said to know whether that was true or not.
I'm sure you're correct. However we're talking about a situation whereby all three airspeed sensors (pitot tubes) are being fouled by disturbed air as a result of extreme AoA.
You're also absolutely right that implementing a warning - either visual through ECAM and/or aural - that AoA is no longer valid would be do-able, but it's then incumbent on the crew to understand what that means.
@LW_50:
I'm not disputing what you're saying - but the <60kts limit was not implemented purely for the purpose of avoiding spurious warnings on the ground, it was also because the data from the vanes is unreliable in the air in that scenario. CONF iture is working on the assumption that the airflow from the descent would be enough to generate a valid reading, but I'd need to know what the specification said to know whether that was true or not.
Last edited by DozyWannabe; 21st May 2013 at 21:44.
It makes sense that the vane is unreliable below 60 KTS, but I'm not seeing how you could have a false stall warning at that speed (absent weight on the wheels). Since the aircraft would be stalled, any warning would be accurate.
Unreliable data might mean that the alarm would not always sound below 60KTS even if the aircraft was stalled. Apparently the designers decided a stall warning that would never work below 60KTS was better than one that would sometimes fail below that speed.
Unreliable data might mean that the alarm would not always sound below 60KTS even if the aircraft was stalled. Apparently the designers decided a stall warning that would never work below 60KTS was better than one that would sometimes fail below that speed.
Last edited by Chu Chu; 21st May 2013 at 23:02.
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Apparently the designers decided a stall warning that would never work below 60KTS was better than one that would sometimes fail below that speed.
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Since no aircraft certified for airliners can fly below 60 knots I can see why stall warning is inhibited. If the crew can't figure that out it is their problem. We need smarter pilots. I don't want to point any fingers but sometimes pilots do not act as pilots, they just react to stall warnings and don't think.
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Originally Posted by DOZY
The AoA readings became invalid because the aircraft had been pulled into an attitude so extreme that the sensors could no longer keep up.
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To clarify that statement I just wanted to say AF447 pulled up into a full stall for no reason. They were probably taught the Airbus wouldn't stall but it did. It had different laws and what they did was very stupid but they did what they were told to do by some instructor probably. I have been told very stupid procedures by instructors that never fly an airplane so ignore them. Be careful about what they tell you. They just teach, they don't fly.
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In terms of the horizontal component of travel yes - I was more concerned with the vertical. The aircraft was oriented such that the vertical airflow was fouling the pitot tubes - I suspect it would also have been sufficient to give false AoA readings.
Of course, this is all speculative as we don't have the spec to hand. I also think we're splitting hairs at this point, because we can't prove one way or another - and the last thing I want to do is give the hamster wheel another push.
Of course, this is all speculative as we don't have the spec to hand. I also think we're splitting hairs at this point, because we can't prove one way or another - and the last thing I want to do is give the hamster wheel another push.
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Quote:
Apparently the designers decided a stall warning that would never work below 60KTS was better than one that would sometimes fail below that speed.
Dozy, I'll agree with that statement, however the whole loss of stall warning below 60 knots was a conceptual design error.
Airspeed and AOA are each independent measures of aircraft performance. To force AOA to become dependent on airspeed is an error.
If you will recall, I ran some experiments on a surplus airline type AOA vane sensor, and the thing came alive below 20 mph.
There is no reason to suspect that there was inadequate dynamic pressure to activate the vanes on AF447, nor is there reason to suspect that the location of the sensors while at that high AOA would subject them to flow reversal or turbulence. The worst that could happen would be that the AOA would exceed the range of the sensor, but at least they would be at the limit in the proper direction.
This shut down of the stall warning system was not without consequence. It deprived the captain of an essential piece of information when he attempted to size up the situation upon arrival back on the flight deck (Remember? Stall warning shut down almost co-incident with his arrival on the flight deck).
The question I have now is, has the conceptual design error been quietly corrected?
Apparently the designers decided a stall warning that would never work below 60KTS was better than one that would sometimes fail below that speed.
Originally Posted by Dozy
I don't think the designers made a conscious decision in that regard - it's so far outside the flight envelope that it probably wasn't considered at the time.
Airspeed and AOA are each independent measures of aircraft performance. To force AOA to become dependent on airspeed is an error.
If you will recall, I ran some experiments on a surplus airline type AOA vane sensor, and the thing came alive below 20 mph.
There is no reason to suspect that there was inadequate dynamic pressure to activate the vanes on AF447, nor is there reason to suspect that the location of the sensors while at that high AOA would subject them to flow reversal or turbulence. The worst that could happen would be that the AOA would exceed the range of the sensor, but at least they would be at the limit in the proper direction.
This shut down of the stall warning system was not without consequence. It deprived the captain of an essential piece of information when he attempted to size up the situation upon arrival back on the flight deck (Remember? Stall warning shut down almost co-incident with his arrival on the flight deck).
The question I have now is, has the conceptual design error been quietly corrected?
Dozy,
I agree with you about the designers --there was no reason for them to foresee a need for a stall warning below 60KTS. And if they had, presumably they would have chosen a technology that was reliable below that speed.
But if you're saying the stall warning should be suppressed below 60KTS because the airspeed data might be wrong, I don't agree. No matter what the pitot tubes are reading, the airspeed is either over 60KTS, in which case the AOA sensor is reading correctly, or it's below 60KTS, in which case the aircraft is stalled regardless of what the AOA sensor says.
I agree with you about the designers --there was no reason for them to foresee a need for a stall warning below 60KTS. And if they had, presumably they would have chosen a technology that was reliable below that speed.
But if you're saying the stall warning should be suppressed below 60KTS because the airspeed data might be wrong, I don't agree. No matter what the pitot tubes are reading, the airspeed is either over 60KTS, in which case the AOA sensor is reading correctly, or it's below 60KTS, in which case the aircraft is stalled regardless of what the AOA sensor says.
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Originally Posted by Dozy
If it's an error in design, then why did both major western manufacturers implement similar behaviour?
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It is a ADIRU issue, no airliner is supposed to be capable of flying! at 60 kt or less.
ADIRU ARINC Air data output is set to AoA NCD(No Computed Data) to the subsystems (e.g. FWC) at 60 kt or less and air speed output NCD below 30 kt or less.
IIRC Boeing T7 ADIRU sets NCD <30Kt and SAARU <50 kt-
ADIRU ARINC Air data output is set to AoA NCD(No Computed Data) to the subsystems (e.g. FWC) at 60 kt or less and air speed output NCD below 30 kt or less.
IIRC Boeing T7 ADIRU sets NCD <30Kt and SAARU <50 kt-
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The hamster wheel is running again. The speed of 60 kts is irrelevant. NO large airliner be it Boeing or Airbus will be "flying" at 60kts. As Professional Pilots we should know that and NEVER get the aircraft into that configuration airborne. Stop attempting to put blame where it does not belong.
The stall warning was ignored for almost a minute when the "PF", if you can call him that, zoomed and held the aircraft into the stall, well above max altitude.
Do not see any of you Airbus decriers over on the Lion Air thread or the Nightmare liner thread, wonder why that is?
The stall warning was ignored for almost a minute when the "PF", if you can call him that, zoomed and held the aircraft into the stall, well above max altitude.
Do not see any of you Airbus decriers over on the Lion Air thread or the Nightmare liner thread, wonder why that is?
Last edited by iceman50; 22nd May 2013 at 02:02.
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Originally Posted by A33Zab 
It is a ADIRU issue, no airliner is supposed to be capable of flying! at 60 kt or less.
ADIRU ARINC Air data output is set to AoA NCD(No Computed Data) to the subsystems (e.g. FWC) at 60 kt or less and air speed output NCD below 30 kt or less.
IIRC Boeing T7 ADIRU sets NCD <30Kt and SAARU <50 kt-
It is a ADIRU issue, no airliner is supposed to be capable of flying! at 60 kt or less.
ADIRU ARINC Air data output is set to AoA NCD(No Computed Data) to the subsystems (e.g. FWC) at 60 kt or less and air speed output NCD below 30 kt or less.
IIRC Boeing T7 ADIRU sets NCD <30Kt and SAARU <50 kt-
That shows that the ADIRU design is the source of the conceptual problem.
The same loss of stall warning would apparently occur in a T7 should one somehow be taken to ridiculously high AOA and probably also to the 787.
Let's attempt to discuss why the ADIRU designers determined that it was appropriate to disable AOA at very low airspeeds. Perhaps over on the tech thread.
As we know, at least one aircraft was ill served by this design decision.
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I think we all agree the PF was totally unqualified and the second pilot didn't do anything about it and let him do it. The captain came up too late to fix the situation so everybody died. End of story. Kind of sad isn't it? I don't think it would happen on our airline. I'm not saying US airliners are better just saying we wouldn't let a pilot do anything this stupid and not intervene.