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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 29th Mar 2019, 14:19
  #2721 (permalink)  
 
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Originally Posted by VicMel
The software in the two ADRIUs is different, it has to allow for the fact that the two vanes are on opposite sides of the cockpit, so I believe they probably use different correction tables, or the algorithm to apply to them is subtly different, possibly something as simple as using a wrong sign.
Yes, the L and R AoA sensors on the 737 are interchangeable, so for a given AoA they will generate rotational signals in the opposite sense to each other. At the very least, the ADIRUs will have to resolve that.

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Old 29th Mar 2019, 14:30
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Originally Posted by Luc Lion
Originally Posted by VicMel
...
2) Looking at a picture of the AoA module, I am presuming that the A to D convertor is in the body, and it is of the ‘rotary’ kind, i.e. the A to D converts the rotated position of the vane shaft into a digital signal. Most of the A to Ds I worked on were typically 12 bit parallel output, I suspect that more modern ones would be a serial output. Either way the signal would be very noisy, responding to every vibration that the vane felt....
VicMel,
it looks as though, at least for the NG, the AOA sensor provides an analog output to both the ADIRU and the SMYD box.
I couldn't find a diagram of the ADIRU Analog interfaces, but here is the SMYD Analog Interfaces diagram.
The three AOA sensor wires are labeled "SIN", "COS", "COM", whose meanings are obvious.
[img]https://3.bp.%62%6C%6F%67%73%70%6F%74.com/-qCwEtMS3-yU/W-xF4QyEZLI/AAAAAAAAFCo/mpLqB83Yau4ph1pPKUjgqSKBFmPoYOWugCLcBGAs/s640/Screen%2BShot%2B2018-11-14%2Bat%2B7.56.25%2BAM.png
Helping Luc with his diagram...
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Old 29th Mar 2019, 14:32
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Originally Posted by VicMel
3) The offset is not due to a ‘stuck bit’. The offset in AoA as the aircraft first started to move is about 12 deg, increases to about 17 deg and then settles on about 22 deg. A stuck bit would be 22.5, or 11.25, or 5.625; to get 17 deg would take two ‘stuck’ bits. Besides which serial buses of themselves (e.g. ARINC 429) do not have ‘stuck’ bits, they have multi bit corruption, usually recognised as a ‘bad message’ and rejected.
4) The AoA digital signal goes into the applicable ADIRU for further processing. Part of this processing *should have been* to reject any invalid input!! Another part will be to smooth the data to get rid of the noise, but this is the same software in both ADIRUs, so would not give an offset.

5) ‘Light bulb’ moment -
The software in the two ADRIUs is different, it has to allow for the fact that the two vanes are on opposite sides of the cockpit, so I believe they probably use different correction tables, or the algorithm to apply to them is subtly different, possibly something as simple as using a wrong sign.
I know which bit of code I would be diving into now!
Hmm!

The AoA disagree is pretty close to 22 degrees from half way down the runway onwards. I suspect that at low speeds, say during taxi, the AoA vane does not respond to the airflow accurately.

I still quite like the One Bad Bit explanation although I agree that it could be a software issue. As you clearly will know it is quite common for software (in general, not in necessarily in aviation) to set a memory Bit to store a status. Perhaps the wrong bit got changed?

Do these computers have hardware memory protection such that one "process" cannot interfere with another? I suspect that they will not have such protection but I am not sure. I read somewhere that they were 80286 based . The 286 does not have integrated hardware memory protection although I suspect that an addition Memory Management Unit might be able to provide it.
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Old 29th Mar 2019, 14:38
  #2724 (permalink)  
 
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Originally Posted by DaveReidUK
Yes, the L and R AoA sensors on the 737 are interchangeable, so for a given AoA they will generate rotational signals in the opposite sense to each other. At the very least, the ADIRUs will have to resolve that.
I have no knowledge of this position system. But could it be so simple as to swap the cos and sin wires and then the output would be in the other direction? So no need for different software in the ADIRU?
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Old 29th Mar 2019, 15:01
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Originally Posted by SteinarN
But could it be so simple as to swap the cos and sin wires and then the output would be in the other direction?
That sounds neither simple nor reliable.

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Old 29th Mar 2019, 15:14
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SIN and COS tend to be a resolver type of angle “encoding”. A very robust passive type of design. You need an excitation AC signal and you get two AC signals back, one with the SIN of your angle and the other with the COS of your angle signal. Typical those signals are differential and isolated from GND, because they come out of a transformer style winding. That makes them robust against noise on the harness. A possible scenario, where you might get wrong or offset values if there is a short between harness lines either the output or the excitation line which is not detected. That might tell us, why exchanging the sensor did not improve the problem.

For Boeing it should be easy to make some tests on the ground to determine what harness failure can create a 22 degree error in their “normal” flight orientation.
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Old 29th Mar 2019, 15:55
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Originally Posted by DaveReidUK
Yes, the L and R AoA sensors on the 737 are interchangeable, so for a given AoA they will generate rotational signals in the opposite sense to each other. At the very least, the ADIRUs will have to resolve that.
In which case the software in the ADRIUs *has* to be different, one side has to add the correction, the other has to subtract it. I can well believe from bitter experience that the software team did not know enough about the hardware subtlety to appreciate that the ADIRU software has to have a L and a R version. This would explain why testing and peer reviewing did not spot anything wrong, the software met its spec!

A correction table would also explain the different offsets. I would expect the table to be a 'look-up' table using air speed as the selector. As the airspeed went up it would reach a point where the next value in the table has to be used, so there would be a step change in the correction. So, I would expect the first value in the correction table to be about 6 deg (to give a 12 deg offset), the next value to be about 8 deg and the final value to be about 11 deg.
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Old 29th Mar 2019, 16:27
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Originally Posted by EDLB
For Boeing it should be easy to make some tests on the ground to determine what harness failure can create a 22 degree error in their “normal” flight orientation.
Wonder if it might indeed tie back to wiring. In the "Go Go Go" article quoted earlier, it was claimed that the wiring blueprints had been rushed and lacked the usual amount of detail regarding the fitting of the wiring and and specific tools to be used. Especially if it is shown that L and R AoA was offset in the same way in both crashes, this has to be a red flag. Surely AoA disagree is not commonplace on other 737s.
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Old 29th Mar 2019, 16:37
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VicMel, what you write does not match the FDR data on AOA values that were shown in the interim report of the Lion Air accident.
It doesn't fit how SIN/COS resolvers work either.

There is a good analysis of what type of resolver, mechanical or electronic failure could match such an almost constant 22° offset on the site of Peter Lemme.
It is worth a read. It's here:
https://www.satcom.guru/2018/12/angl...ure-modes.html

I made a quick check of what would produce a constant 34% positive parasite signal on the sinus entry combined with a 16% negative parasite signal on the cosinus entry.
It gives the series (in degrees)
Real Angle - Reported Angle - Delta
-2 19.97512 21.97512
-1 21.00953 22.00953
0 22.03623 22.03623
1 23.05537 22.05537
2 24.06711 22.06711
3 25.0716 22.0716
4 26.06899 22.06899
5 27.05944 22.05944
6 28.04309 22.04309
7 29.02008 22.02008
8 29.99056 21.99056

I have no idea on what could cause such parasite signals.
Could be inductive from a 400Hz current (like the excitation current), or could be a DC parasite signal in the entry of the analog to digital converter.

Don't misunderstand me ; I do not state that this is what caused the AOA failure.
Just looking at failures that give similar symptoms.

Last edited by Luc Lion; 29th Mar 2019 at 16:57.
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Old 29th Mar 2019, 16:41
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Originally Posted by VicMel
In which case the software in the ADRIUs *has* to be different, one side has to add the correction, the other has to subtract it. I can well believe from bitter experience that the software team did not know enough about the hardware subtlety to appreciate that the ADIRU software has to have a L and a R version. This would explain why testing and peer reviewing did not spot anything wrong, the software met its spec!

A correction table would also explain the different offsets. I would expect the table to be a 'look-up' table using air speed as the selector. As the airspeed went up it would reach a point where the next value in the table has to be used, so there would be a step change in the correction. So, I would expect the first value in the correction table to be about 6 deg (to give a 12 deg offset), the next value to be about 8 deg and the final value to be about 11 deg.
The basic (left/right) tables have to be correct since in normal operation the AOA values do match. The tables are likely fine grained and probably interpolated so doubt there is a visible step in correction. Only the tables would change for L/R, the rest of the code would be common.

I agree that a 'stuck bit' does not seem to fit the data since there would be discontinuities as the data crossed the point where the stuck bit should change, especially true since there are 2 A to D converters, one each for SIN and COS. Easily ruled out if we had access to the raw data.

Mention has been made of the AOA analog signals being connected to 2 boxes, this opens a possibility that an electrical fault in one box could cause an offset for both.
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Old 29th Mar 2019, 17:01
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Whilst all of you Tech ‘bit’ people provide valuable information and possible scenarios, could you please consider why ‘failures’ appear to be very rare and so far only relate to two aircraft / three vanes.

How something fails does not necessarily explain why (when) it failed.
Random, probabilistic, bit count, world clock ?
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Old 29th Mar 2019, 17:21
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I see the WSJ information as a little misleading. While it apparently states that "...the MCAS system activated prior to its dive..." it can not be assumed that it was the cause of an initial upset.
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Old 29th Mar 2019, 20:10
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Originally Posted by DaveReidUK
That sounds neither simple nor reliable.
Swapping Sin and Cos could be done in the wiring harness.

Jon
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Old 29th Mar 2019, 22:36
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Originally Posted by jmelson
Swapping Sin and Cos could be done in the wiring harness.
Indeed it could.

Except that it would be pointless - swapping Sin and Cos doesn't turn a positive angle into a negative one.

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Old 29th Mar 2019, 22:48
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It transforms any angle alpha in (90° - alpha).
So 0° becomes 90°.
Not sure this is convenient...
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Old 29th Mar 2019, 22:55
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But both of these aircraft were a few months old...a harness problem should have manifested iteself on the first flight, no?

Last edited by Australopithecus; 29th Mar 2019 at 22:55. Reason: spelling
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Old 29th Mar 2019, 23:05
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An interesting article from Vox.com https://www.vox.com/business-and-fin...ndal-explained
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Old 29th Mar 2019, 23:13
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Originally Posted by Australopithecus
But both of these aircraft were a few months old...a harness problem should have manifested iteself on the first flight, no?
A misWIRED harness would have shown up immediately, but an intermittent problem would usually not show up right away.
It IS quite interesting that two planes may have had similar problems after just a few months of operation. Certainly a mystery, and Boeing and several CAA's are REALLY wanting to know what the heck happened!

I'd love to know more about exactly how these sensors work, and how the signals get into digital form in the computers.
It is looking like they MIGHT just be bare resolvers in the sensor.

Jon
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Old 29th Mar 2019, 23:13
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Originally Posted by SteinarN
I have no knowledge of this position system
me either, but I was wondering if its possible to physically (or even manual adjustment wise) install this AoA sensor 22° out of alignment??

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Old 29th Mar 2019, 23:39
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Originally Posted by Dee Vee
me either, but I was wondering if its possible to physically (or even manual adjustment wise) install this AoA sensor 22° out of alignment.
Discussed at length in one of the Lion Air threads.

No, it's not possible, by design.

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