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-   -   Maintenance Lapse Identified as Initial Problem Leading to Lion Air Crash (https://www.pprune.org/tech-log/616624-maintenance-lapse-identified-initial-problem-leading-lion-air-crash.html)

Capt Quentin McHale 27th Dec 2018 06:24

BAengineer,

Many thanks, you are indeed correct.

Upon a bit of further investigation I have been led to believe that the AoA sensors are calibrated at the factory and are interchangeable between L/R and attach directly onto the fuselage with screws and can't be misaligned. The respective sensor vane has a resolver/s attached to it (internally) and sends an electrical signal to the SMYD's (SMYD1 for L(Capt) and SMYD2 for R(F/O)) which alerts the respective SMYD as to what position (L/R) the AoA sensor is in (vane has to move through 180deg when swapped from L to R or R to L positions). The respective SMYD's then recalibrate vane angle and transmit to Capt/FO screens.

There is a ground test function on the SMYD's for AoA sensor test/chk, but as you mentioned NOT mandatory upon installation which I find rather odd. This info pertains to the B738 and I would assume (very dangerous in aviation) that the MAX would be similar if not the same.

Rgds McHale.

jimtx 27th Dec 2018 16:08


Originally Posted by Capt Quentin McHale (Post 10345091)
BAengineer,

Many thanks, you are indeed correct.

Upon a bit of further investigation I have been led to believe that the AoA sensors are calibrated at the factory and are interchangeable between L/R and attach directly onto the fuselage with screws and can't be misaligned. The respective sensor vane has a resolver/s attached to it (internally) and sends an electrical signal to the SMYD's (SMYD1 for L(Capt) and SMYD2 for R(F/O)) which alerts the respective SMYD as to what position (L/R) the AoA sensor is in (vane has to move through 180deg when swapped from L to R or R to L positions). The respective SMYD's then recalibrate vane angle and transmit to Capt/FO screens.

There is a ground test function on the SMYD's for AoA sensor test/chk, but as you mentioned NOT mandatory upon installation which I find rather odd. This info pertains to the B738 and I would assume (very dangerous in aviation) that the MAX would be similar if not the same.

Rgds McHale.

I don't know what became of this NPRM but it would seem that a test should be mandatory: https://www.regulations.gov/document...2012-1041-0001

BAengineer 27th Dec 2018 16:46


Originally Posted by jimtx (Post 10345380)
I don't know what became of this NPRM but it would seem that a test should be mandatory:

That is only relevant to 737 classics - not the NG or MAX.

radken 27th Dec 2018 16:47

FWIW, Comment #21 (MickG0105)...Ref MM page is for -7/-8 series. Does same or similar page for Max 8 read the same? I may be wrong, but hasn’t this thread, and other one suspended, led us to knowledge that MCAS was “new” to 737 family solely by reason of aerodynamic perf differences of Max 8’s? Now we see it was also in -7’s/8’s?

BAengineer 27th Dec 2018 16:54


Originally Posted by Capt Quentin McHale (Post 10345091)

There is a ground test function on the SMYD's for AoA sensor test/chk, but as you mentioned NOT mandatory upon installation which I find rather odd. This info pertains to the B738 and I would assume (very dangerous in aviation) that the MAX would be similar if not the same.

I assume that the reason of the check not being mandatory is that there is no calibration adjustment possible on the aircraft - it either passes or it doesn't. If it doesn't then you replace the sensor again. So if you have confidence that the sensor is correctly calibrated from the shop then I can see why checking it again would not be mandatory.

infrequentflyer789 27th Dec 2018 17:17


Originally Posted by radken (Post 10345394)
FWIW, Comment #21 (MickG0105)...Ref MM page is for -7/-8 series. Does same or similar page for Max 8 read the same? I may be wrong, but hasn’t this thread, and other one suspended, led us to knowledge that MCAS was “new” to 737 family solely by reason of aerodynamic perf differences of Max 8’s? Now we see it was also in -7’s/8’s?

Think you'll find that 737-700/800 = NG, the 737 -7/-8 (without the zeros) = MAX (and -9/-10 also).

I have MM pages for the NG and MCAS isn't there (there is an anti-stall function in NG STS, but very different characteristics to MCAS).

CONSO 27th Dec 2018 19:04

JIMTX in post # 22 said ...

what position (L/R) the AoA sensor is in (vane has to move through 180deg when swapped from L to R or R to L positions).
Which to this SLF suggests that some sort of detent must be built in to assure exactly 180 rotation. Could it be that the detent is set allow a plus- minus 20 degree movement of AOA VANE ? And thus if not twisted past the detent, a relatively fixed error may be possible ?

Machinbird 27th Dec 2018 20:38

Smoking Gun?
 
Peter Lemme's Blog on the JT610 accident contains a lot of good information on the 737NG AOA systems which I am going to draw on. AOA Failure Modes
In this blog, he basically shows that any interference with the signal coming directly from the AOA sensor can be expected to create an error that varies with the indicated angle. By overlaying the left and right AOA signals, he shows that the 22 degree error between the sensors does not vary significantly until the aircraft is on the ground. Therefore we should really look elsewhere in the aircraft for the problem. Due to cross wind effects on the ground, you can understand that there might be some variation while on the ground, but there is one other difference visible in the trace while the aircraft is riding over the bumps in the taxiways, the electrical noise level!
The trace from the left AOA forms a fat line with numerous spikes and the trace from the right AOA is smooth. This is the smoking gun that gives significant clues as to what may have happened to cause the problem.
https://cimg8.ibsrv.net/gimg/pprune....4c4865f0b3.png

In the course of a rather long and varied life, I have had occasion to troubleshoot misbehaving electrical circuits that carry signals, and when I see a circuit responding to vibration, I suspect either a "sneak circuit" or an intermittent circuit. In the case of JT610, other data suggests we are dealing with a sneak circuit, but when you get to the circuit board level where data is manipulated, it could also be an intermittent condition. Typically what you do to troubleshoot these problems is to put your meter/scope into the energized circuit and shake things until you see a response. In this case, we see the response, but we do not know just what got shaken. Instead, we have to infer what might be the problem by looking at the system and where it gets its signals and do a bit of mental exercise.

https://cimg2.ibsrv.net/gimg/pprune....d757305251.png
In the top left corner of the Stall Warning System-Stall Management Yaw Damper diagram, we see the signals coming in directly from the AOA resolver.
They do not go through any box to get there. If there were some sort of cross connection in those lines we would see data that varies differently from what we are observing (constant error at all angles while inflight).
This would indicate that the problem is downstream from the wires running to the SMYD. Since Boeing went to a high level of attention to ensure that there is a high level of commonality with prior 737 aircraft, and an ability to interchange between right and left AOA sensors, we should assume that it would be very difficult to mis-install one of these sensors (relatively fool proof). So lets focus our attention down stream on the SMYD box.
I do not have the schematic on this item, so I'm left to improvise by learning what I can about the topic of converting the Sine and Cosine signals inside the SMYD box. One thing I learned is that the signals likely receive conditioning by various circuit elements (amplification and filtering of noise) before being combined into a signal representing an angle. Background-signal conditioning-resolver to digital converter
There is therefore an area inside the SMYD box that can foul up these angle signals. And what does the SMYD box do? Among other things, it activates the stick shaker!

Now what else do we know about this aircraft? Why it was nearly new! You might even say, it was still on its "shakedown" period. Supposing a loose nut, a blob of solder, or a wire fragment was floating around inside the SMYD box and eventually found the place where it could do the most damage.
Someone at Boeing who knows this system will have to look at the contents of SMYD box to have a chance at finding what failures might behave the way we have seen in the JT610 AOA data, I don't have enough info.

I think the AOA probe change is likely a red herring that can easily lead this accident investigation astray. The core problem is that the system was changed in the MAX, and with it, certain systems became critical. It is true that maintenance did not fix the problem, but given the procedures they were given, would they ever have found the problem without resorting to shotgun type trouble shooting? Does the troubleshooting ever lead to a changeout of the SMYD box?

gums 27th Dec 2018 21:31

Salute!

Thanks, 'bird, I am almost "all in" withya on this line of inquiry/reasoning. I.e. not necessarily a poorly installed or faulty sensor, but something in that "box". And recall we had a plane going down in that area a few years back due to a bad solder joint on a card.

I also note the noisy data that was on the flight data recorder which you pointed out over a month ago. Almost like there is a bad connection or poor solder joint or ....... I assume the FDR got its AoA plot downstream of the "box" and not directly from the actual vane, but your diagram doesn't show where else AoA data goes from the sensor. I also doubt that the FDR decodes the synchro/resolver analog signals, huh?

Gums sends...

Machinbird 27th Dec 2018 23:53

Gums,
Just thinking like a systems engineer, without concrete knowledge of the 738 systems, but I think you will find that the AOA data is digitized inside the SMYD box and sent through a data bus to the FDR and ADIRU and wherever else needed in the aircraft.

k3k3 27th Dec 2018 23:59

When the first NATO E-3A aircraft were delivered a couple of them had recurring AOA problems, left and right indicators consistently had the same disparity. After much head scratching it was found that during manufacture, the template for drilling the holes for securing the mounting rings for the interchangeable transducers had not be turned over when going from the left to the right of the nose, with the results we found. The men from Boeing came with their template, drilled the holes in the right place and all was good.

But this was 35 years ago.

KRUSTY 34 28th Dec 2018 03:02


Originally Posted by climber314 (Post 10344335)
Not sure why AoA display is an option on the 737 MAX if a critical flight control surface is controlled by a (single) AoA sensor.
Maybe Boeing should update the QRH for BOTH Runaway Stabilizer AND AoA Disagree?
Seems like Boeing rushed this "PATCH" and didn't think this through completely.
In Boeing's defense it took some poor maintenance and sketchy aviating for this issue to manifest.

“In Boeing’s defense”!

Are you even listening to yourself?

Capt Quentin McHale 28th Dec 2018 06:33

CONSO,

The AoA sensor vane is aerodynamically shaped and can swing through 360deg without a problem (on the ground) and you often see the vanes on a parked aircraft drooping vertically or with a good breeze blowing, 180deg in reverse and anywhere in between. This is NOT a problem and is merely the nature of the beast. The SMYD computers ignore this because the aircraft is in "ground mode" signalled through squat switches, no engines running, park brake on, airspeed <30kts etc, etc, etc.

When the aircraft is starting to accelerate down the runway, the airflow around the nose of the aircraft will reposition the vanes to the required position. I don't know the exact requirements/signals for the SMYD's to start paying attention to AoA vane position/angle (transmitted by internal resolvers/synchro's) but it could be when (don't quote me) BOTH engines are started and >80% N1, park brake off, all cabin doors closed, airspeed >30kts etc, etc, etc. Hope this helps.

Rgds McHale.

CONSO 28th Dec 2018 15:20


Originally Posted by Capt Quentin McHale (Post 10345718)
CONSO,

The AoA sensor vane is aerodynamically shaped and can swing through 360deg without a problem (on the ground) and you often see the vanes on a parked aircraft drooping vertically or with a good breeze blowing, 180deg in reverse and anywhere in between. This is NOT a problem and is merely the nature of the beast. The SMYD computers ignore this because the aircraft is in "ground mode" signalled through squat switches, no engines running, park brake on, airspeed <30kts etc, etc, etc.

When the aircraft is starting to accelerate down the runway, the airflow around the nose of the aircraft will reposition the vanes to the required position. I don't know the exact requirements/signals for the SMYD's to start paying attention to AoA vane position/angle (transmitted by internal resolvers/synchro's) but it could be when (don't quote me) BOTH engines are started and >80% N1, park brake off, all cabin doors closed, airspeed >30kts etc, etc, etc. Hope this helps.

Rgds McHale.

Thanks - theat seems to leave ( per the 4 previous posts ) some sort of electrical-issue since if installed with wrong tgemplate- or force fit, the problem would have been happening from day one.

But IMO that does NOT excuse Boeing from allowing a single point failure (AOA system ) from being able to fubar a major flight control system- with NO mention of the MCAS.

thus the ' genie unzipping and urinating on the pillars of science ' comment by Ernie Gann "

CONSO 28th Dec 2018 15:21


Originally Posted by Capt Quentin McHale (Post 10345718)
CONSO,

The AoA sensor vane is aerodynamically shaped and can swing through 360deg without a problem (on the ground) and you often see the vanes on a parked aircraft drooping vertically or with a good breeze blowing, 180deg in reverse and anywhere in between. This is NOT a problem and is merely the nature of the beast. The SMYD computers ignore this because the aircraft is in "ground mode" signalled through squat switches, no engines running, park brake on, airspeed <30kts etc, etc, etc.

When the aircraft is starting to accelerate down the runway, the airflow around the nose of the aircraft will reposition the vanes to the required position. I don't know the exact requirements/signals for the SMYD's to start paying attention to AoA vane position/angle (transmitted by internal resolvers/synchro's) but it could be when (don't quote me) BOTH engines are started and >80% N1, park brake off, all cabin doors closed, airspeed >30kts etc, etc, etc. Hope this helps.

Rgds McHale.

Thanks - theat seems to leave ( per the 4 previous posts ) some sort of electrical-issue since if installed with wrong tgemplate- or force fit, the problem would have been happening from day one.

But IMO that does NOT excuse Boeing from allowing a single point failure (AOA system ) from being able to fubar a major flight control system- with NO mention of the MCAS.

thus the ' genie unzipping and urinating on the pillars of science ' comment by Ernie Gann "

wiedehopf 28th Dec 2018 15:32


Originally Posted by CONSO (Post 10345986)
Thanks - theat seems to leave ( per the 4 previous posts ) some sort of electrical-issue since if installed with wrong tgemplate- or force fit, the problem would have been happening from day one.

The problem at first was an intermittent AoA signal. That meant no airspeed and altitude for the captain sometimes because AoA is included in the calculation.
After the change of the AoA sensor in Denpasar the signal was not intermittent but offset by 20 degrees on the previous and the accident flight.

So it could still very well be either that the sensor itself was faulty and not properly checked at the factory or the installation was done "creatively".

gums 28th Dec 2018 16:05

Salute!

Thanks, Cpt McHale. Guess those puppies move about a lot, although the right vane seems to be more highly dampened.

I had noticed a change in the left AoA a minute or two before T/O roll and figured it was some kinda configuration change. Upon looking at the heading trace, the vane prolly changed when the jet "jinked" on the taxiway or ramp when heading to the rwy. Then the thing apparently moves again as the plane lines up. From there we see a near constant bias from the other AoA vane. And so...

@ 'bird and others with lots more maintenance experience that I ------ If we assume that the FDR does not get the raw analog signals from the AoA vanes that we see on the data, but from a "box" or module in a "box", then I wonder if there is a dip switch change or "plug" that must be inserted/removed when the AoA vane is replaced. This would tell the "box" using the raw analog signals to bias the digital data according to "right" AoA or "left" AoA mounting.. In other words, the "universal" vanes can be placed on either side without having different mounting hole spacing. I like that idea for $$$, standardization of parts, and so forth. But it is a procedure that must be followed and then verified
.
Jez wondering...

Gums

IFixPlanes 28th Dec 2018 16:45

B737 MAX AOA sensors have an endstop at 100° each swing direction.

climber314 28th Dec 2018 16:59

According to the CONSO/Lemme diagram there are PINS on the SMYD which I assume could be used to "orient" the device.
Lemme also appears to pursuing another line of thought about this discrepancy in recent twitter posts.

Peter Lemme‏ @Satcom_Guru
A bent vane or mass imbalance are the likely culprits. Mass imbalance might be harder to notice. Mass imbalance would reflect something broken, which might correlate to loss of damping. AoA vane supplier should have failure mode and effects or other analysis to confirm. #JT610

gums 28th Dec 2018 19:08

Salute Wiede
I agree with you about the decoding in the SMYD or two of them, if left vane to one and right to another. From there, looks like AoA value goes to the two ADIRU and FCC components. Since the pilot control had the shaker and from what others here have presented, I have to go with two of the SYMD's or a single one maintains the data from left for left ADIRU and right for the co-pilot side.

Unlike my trusty Viper, only one AoA can cause serious problems in the 737. Our system had the basic two conical AoA probes and then a serious looking hemispherical proble that used 5 holes to produce sideslip, AoA, static and dynamic pressures for use. We used a middle value prorocol from the two mechanical cones and one pneumatic sensor.

Maybe FC eng can find a better "flow chart" or "block diagram" like we old farts used back in FORTRAN IV and sliderule days, heh heh.

Gums sends....


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