A long detailed discussion of AOA sensors by Boeing- note that 737 is not mentioned- except in the document dont know how old this is but pdf document shows date of year 2000

Aero 12 - Angle of Attack


and at bottom of online text emphasis added

Are you sure about that ?

I haven't seen any FDR traces for the two flights where problems were reported that occurred prior to the AoA sensor replacement. Nor AFAIK has there been any report on the condition of the removed sensor.

IMHO it's entirely possible that neither sensor was faulty and both were correctly installed, i.e. the fault was farther downstream and replacing the sensor made no difference.

No i'm not sure. I'm reading between the lines.
On the preliminary report on pages 7, 8 and 9 the maintenance log is described.
(Preliminary Report: http://jacdec.de/database/2018-10-29...y%20Report.pdf)

Before changing the AoA sensor the main symptom is:
Only after changing the AoA sensor the IAS disagree behaviour started:
Also i'm quite certain if the wrong AoA with stick shaker activation would have been present prior to the Denpasar-Jakarta leg it would have been mentioned in the report.

Could also be maintenance screwed up the SMYD1 somehow i don't know. But it seems certain the really dangerous fault was introduced with that maintenance option.

Of course the report could have left out that the other flights had an AoA disagree as well but somehow i doubt that. Also yes the maintenance log could be wrong, but no speed indication vs airspeed disagree indication, i trust them to get at least that much right. Especially because there were multiple occasions of that problem.

Salute!

One thing I would have hated in that left seat would have been the stoopid shaker remaining activated no matter what we did.

The trace we see from the previous flight clearly shows the shaker active until some "event" upon landing ( no flow chart, so I guess it was WoW), and it was clearly not operating as part of the MCAS implementation. The shaker remained active regardless of flap position, time after takroff and turning off the electrical pitch trim system ( those guys flew using the manual trim wheel, and I guess the captain let the FO do most stuff and just "tolerated" the shaker when he used his control). So now we see that the shaker is driven independently of MCAS. This had to be confusing, even if you knew all about MCAS. Especially if you had turned of the stab trim to get rid of the annoying nose down trim that Hal was commanding over and over and over and "went manual" ( feel the Force, Luke)

The maintenance log shows the wrenchbenders trying to cure the airspeed gripes, and one of the computer codes pointed to the AoA. Had they left that sucker alone we would not be here on this forum, huh? And why didn't the crew gripe about the incessant shaker?

Something bad happened from the AoA sensor along the path to the MCAS and the shaker. A good flow chart/block diagram would really help here, but I am not sure the current generation believes in those things anymore. ANother process was our "murder boards" where we crackerjack pilot/engineers briefed our system or proposal using said charts/diagrams, and then explaining each and every interface and results from failures all along the various paths Hal or the lowly carbon-based lifeforms could take.

Gums sends...

No FDR but Maint log in prelim report shows the following AOA errors before sensor replacement:

AOA SIGNAL FAIL (34-21023)
34-21107 (AIR DATA SIGNAL INVALID)
34-21123 (AOA SIGNAL OUT OF RANGE)

We also know the problems were intermittent because I think it's already been shown in these threads that there was at least one uneventful sector in between.

Agree - I think far more likely one fault elsewhere than two faulty sensors. However, replacing the sensor did make a difference: an intermittent signal corruption / loss changed into a constant signal corruption...

Thank you weidehopf for pointing out the earlier flight maintenance history. It points out an area for investigation.
IMO the AOA maintenance had nothing to do with the stick shaker problem, but something else done at the same time probably did trigger the AOA signal problem. I agree that the prior crew needed to do a better job explaining the seriousness of the problem to maintenance.
I also have to fault Boeing for failing to tell pilots about their new MCAS system which handicapped the prior crew (and probably maintenance as well) in explaining what their problem was. The aircraft should never have been released for a revenue flight without a test flight by an experienced crew.
The hardest problems to diagnose are intermittent ones. Computers are able to aid the diagnosis of problems by recording what the symptoms were, but system complexity often prevents actual diagnosis, and only suggests a path toward correction.
Lets dig into the maintenance history a bit:


https://cimg5.ibsrv.net/gimg/pprune....541797bbe1.png
https://cimg6.ibsrv.net/gimg/pprune....dc67a7e9f9.png
https://cimg0.ibsrv.net/gimg/pprune....ec6899cfea.png
Comments to follow. I'm, looking at the conductive FOD scenario causing erratic behavior.

Maintenance Decryption-Part 1
 

On October 26th on a flight from Tianjin Binhai China to Manado Indonesia, the Captains PFD had speed and altitude flags which caused speed and altitude not to be displayed and caused the maintenance light to illuminate after landing.Maintenance used the Onboard Mainteance Function (OMF) to identify a code 27-31000. Maintenance used the Interactive Fault Isolation Manual for that code and did a self test on the SMYD1 box which showed no errors. As a result, they erased the maintenance message, and cleared the aircraft for flight.Between October 26th and 27th the aircraft flew from Manado to Denpasar, apparently without a gripe.

Then, on October 27th, on a flight from Denpasar to Manado, the Captains Speed and altitude flag problem reoccurred together with Speed Trim fail and Mach Trim fail warning lights. This time, there was a message, “Stall Warning System Left” and the OMF spit out a code 27-32-00-810 thru 816.
A self test on SMYD1 failed.While trouble shooting the Speed Trim Fail and Mach Trim fail warnings, there were another set of IFIM codes indicating ADR invalid and AOA signal fail.Corrective action was to reset circuit breakers on ADIRU L DC and AC, and ADIRU L (self-test?) carried out and performed system test on SMYD1-result pass.

A few questions for the maintenance folks here:

1. Does Reset a circuit breaker mean to push a tripped breaker back in or does it mean to cycle it, or does it mean either of those possibilities?
2. There is a secondary correction of “Reconnect and clean electrical plug of data module” Is this data module located inside SMYD1?
3. Is this the device that digitizes the AOA data?

I’m looking for actions that would potentially relocate conductive FOD inside SMYD or its internal / associated components.
If you have intimate knowledge of these systems, feel free to look ahead in the maintenance actions previously posted above to see if you recognize something that would move something associated with processing the AOA signal.
I strongly suspect that something else was moved around at the same time the AOA probe was replaced. That electrical disturbance (noise) seen on the Left AOA signal should not be glossed over. You can even see some extra spikes in the Left AOA data during the accident flight that are not matched by the Right AOA signal.

Identifying the fluctuation in the AoA signal as electrical noise does not make sense to me.

Being only present on the ground it is much more likely it is some form of imbalance situation in the AoA vane.
Normally you want the AoA vane combined with the internal rotating parts of the sensor to be balanced so it doesn't react too much to bumps in the runway or turbulence in the air.
So i find a mechanical problem with the AoA vane like a broken off part combined with a bend much more likely.

Also no one has explained so far why electrical noise would never be visible during flight.

Gums
With regards to tolerating the false stick shaker I would time permitting pull the CB and kill it!
Smart: maybe not?

Wiedehopf, I gather that you have not had to troubleshoot intermittent electrical problems before-fair enough.
Consider an out of balance AOA sensor as a potential cause. This would create a constant torque on the sensor in the increasing AOA direction (assuming vane heavy). As we go faster in the air, the restoring force is increasing and the error would decrease, but this is not what we are observing-we are seeing a constant error between sides.
With regard to a bent vane causing the error, we would be assuming gross incompetence on the part of a number of people, both maintainers and whomever in the aircrew preflighted the aircraft for the last two flights. A bent vane with a 20 degree offset should be as obvious as the nose on your face.
Why would we be seeing Left AOA noise on the ground but not in the air? Consider the difference in vibration environments. The SMYD boxes are mounted in the nose avionics bay, correct? The front end of the airplane experiences much more jiggling on the ground than does the area of the aircraft closer to the CG and main gear. (Watch any Youtube video of a transport crew making al landing, and there should be no doubt in your mind when the aircraft has touched down just from the bouncing around the crew gets when they touch down.)
Finally, consider the intermittency of the problem. Different presentations (or lack of presentation in 1 case) of the problem over the course of 5 flights. During maintenance, the problem would often fail to present itself, and only leave its calling cards. This level of complexity shouts intermittent electrical problem, loud and clear.

While i agree that an intermittent electrical problem may well have been present, the coincidental 20 degree offset connected to the replacement of the sensor is quite stunning.
So the intermittent electrical fault might have nothing to do with the offset at all.
Also a constant exact offset of 20 degrees does not fit at all with an intermittent problem.

Your supposed electrical noise could be wind as well that is coming from one side while taxiing. Maybe it is gusty and therefore producing the "noise" you see.
The temporal resolution of the FDR readout is far too bad to conclude that that is electrical noise.

The imbalance in the sensor could be such that it is apparent when taxiing over rough taxiways creating the "noise" but is irrelevant as soon as the airspeed picks up on the takeoff roll. Aerodynamic forces far outweigh any inbalance at those speeds.
Such imbalance could be created by violently attaching the sensor. Or bending/breaking the vane.

And yes i find gross incompetence and negligence far more likely than an electrical error producing a 20 degree offset.
But i will entertain the idea gladly and ask you how an electrical problem could introduce such a fault?

The SMYD has two inputs for the AoA sensor, labeled sinus and cosinus.

This for me points to a resolver.
https://en.wikipedia.org/wiki/Resolver_(electrical)

Now i really have no idea how an intermittent or basically any electrical problem would introduce the 20 degree offset.
Do you have an idea?
(I'm happy to be corrected on the points made, it's about the puzzle less about being right in my opinion)

RE WEIDEHOPF - is it possible to mount the resolver to the vane incorrectly, eg a 20 degree rotation due to mis- clocking ? or is there some sort of indexing-? neet a detailed assembly diagram or such for the AOA sensor- resolver ... is there 9 equally spaced screws mounting resolver to vane ?

I don't have detailed information on the sensor. But i would presume the resolver is normally not detached from the vane by line mechanics, instead my guess would be that the unit is only opened by the manufacturer.

Maybe it has been said before but i'm curious, is the AoA sensor used on the MAX the same as on the 737 NG?

Wiedehopf, we are badly handicapped by not having access to schematics of the SMYD box, its associated systems, and the Interactive Fault Isolation Manual to help better understand the maintenance codes. All we have going for us is a bit of theory. Some activity like Boeing that holds the actual data package for the aircraft, or the manufacturer of the SMYD boxes are the only ones likely to have sufficient data to figure this out in detail.

If we look at the Sine and Cosine signals from the AOA resolver, these are analog signals and have to be put together to make a piece of angle data. At some point, apparently in close proximity to the SMYD box, these signals are digitized and sent by the aircraft data bus throughout the aircraft.
There are two ways to put these signals together that I can immediately think of.

1. Digitize the two signals independently, and then digitally convert the result to an angle, or
2. Combine both Sine and Cosine signals to an analog angle signal, and then digitize that.

There are purpose built circuits for digitizing resolver data, but they may be newer in design than the currently utilized SMYD method.

In the analog state, any voltage applied to the combined angle analog signal will create an error that has the potential to change the indicated angle. Once the data is fully digitized, the potential for inducing errors is much reduced.
Electronic circuits are very dense, and any conductive FOD has the potential to lodge among the circuit traces (unless potted) and create sneak circuits.

If Boeing can nail down where in the system this proposed FOD probably landed, they could likely duplicate the error by inserting a BB onto the circuit board and rolling it around-
There is also a second potential mode of failure to intermittently lose an internal reference voltage due to a failed component or improperly soldered connection.
Remember, this was a brand new aircraft and it could have experienced an "infantile" failure of a circuit component.
At our level, we are not going to be able to positively solve this question. but we can look for indications in the maintenance record that indicate other maintenance actions could have disturbed SMYD components simultaneous with the changeout of the AOA probe.
The difficulty with this is proper understanding of technical abbreviations, current maintenance word usage, and IFIM codes/workpackages.

Personal background: I first flew using AOA over 50 years ago, and have had plentiful opportunity to observe AOA vane behavior in an aircraft carrier flight deck environment, so I think I understand the subject of cross winds and jet blast impingement fairly well. I have also worked in aircraft maintenance in a number of capacities. There is still a fair bit for this old dog to learn though.

Conso, either find a picture of the nose of a NG/MAX 737 and count the screw holes or go to the airport and make your own picture. I doubt if it has more than 6.
They can limit the interchangeability of probes by playing with the screw hole spacing, the mounting plate size, the electrical connectors, etc. There is an indexing pin involved as well, and that can control proper installation too.
With 9 holes you you would have a 40 degree error. That doesn't fit the scenario.

reason im picked 9 was due to 180 degree change right to left - and i was talking mounting the electric transducer internal to the vane and not the external mount. still cannot find details of AOA sensor which was **PROBABLY** made by Rockwell- Collins

https://s15.directupload.net/images/...p/he4r4ib5.jpg
1 AOA
2 Alignment Pins
8 Screws

Not according to the KNKT, who stated that problems had been encountered on four consecutive sectors.

KNKT also shows the four sectors from the maint log (in the prelim report and posted in full further up the thread):

26 October 2018 :: Tianjin Binhai to Manado
27 October 2018 :: Denpasar to Manado
27 October 2018 :: Manado to Denpasar
28 October 2018 :: Denpasar to Jakarta

Methinks that Occam's razor comes down on the side of "four consecutive" being a translation error, as opposed to teleportation of a 737 from Manado to Denpasar - I could be wrong though... :)

I'm quoting from the statement made by the KNKT on 5th November, where they identified problems encountered on four consecutive flights (including the accident flight):

(Denpasar)-Manado
Manado-Denpasar
Denpasar-Jakarta
Jakarta-Pangkal Pinang

KNKT: Lion Air Airspeed Indicator Damaged since 3 Flights Before

Salute!
and Happy New Year!

@ 'bird, et al..... re: digitizing the analogs. Even way back in the early 90's most of our analog signals were digitized by single purpose chips. A very few really "special" systems would have the A2D conversion circuits as part of a much more complex chip/ckt board versus separate components. In a very few cases dealing with analog sensors that had no A2D data used by more than one subsystem ( think guidance seekers, we had to "capture" the signals before they got to the "user" box in order to properly troubleshoot.

The previous gripes seem to all concern airspeed disagreement and such, and the AoA faults only showed up when maintenance codes were examined. As I understand it, some of the systems use AoA to "condition" the airspeed indications and such. Someplace between those values used for displays/STS and the AoA source should bear the brunt of investigation besides a "simple" mounting error. Considering that the speed gripes were there before the AoA vane change, I tend to rule out the sensor itself and its internal electronics.

Is it the sensor? Is it a BB rolling around on the ckt board? Is it a dip switch setting that lets the A2D processing account for left versus right sensor? And so forth.

So I like looking at the SMYD for the problem. Hell. just yank the thing out and replace! Not possible now, but we may see a similar sequence in another plane and we could keep this one in mind if a gripe shows up two times in a row.

Gums sends..

AOA vane angle is converted to body angle
 

When considering the potential for AOA sensor installation to have played a role in this event, it is important to recognize the difference between AOA vane angle and AOA body angle. The local flow that the vane measures is distorted by the fuselage. As a result, the vane angle changes much more than one degree for each degree change in airplane AOA. Within the AOA signal processing logic is conversion of the vane angle back to the corresponding body angle. This conversion has a gain factor of just about 0.5 to get from vane to body.

The data that we have seen for the Lion Air event shows an AOA bias of approximately 20 degrees body between left and right signals. That could result from the difference in vane angles being approximately 40 degrees.

RE FCeng84 # 63

Interesting - a pic of " typical " AOA Sensor shows 8 holes in post # 58

If 9 holes then 360/9 = 40 degrees but if 8 holes then 360/8 = 45degrees
assuming a plus or minus travel gives either 20 degrees or 22.5 degrees. which IMHO gets close to the constant 20 degree offset noted due to fit it or force it miss- installation ( clocking) . :ugh:

If only Boeing had thought to Murphy-proof the AoA sensor, say by incorporating a locating pin that ensured it could only be fitted in one orientation ...

I guess we've just been lucky that before this event nobody has fitted one incorrectly in the 50-year history of the 737.

Salute!

according to FCeng

Well, FCeng says that the vane deflection requires a "0.5 gain factor" someplace in the sftwe or A2D chip/device, wherever that is.. Is the "gain" two degrees of wing chord to free stream flow? Is it sensed AoA times two or divided by two? Hard to believe the AoA difference between the sensor and the wing chord remains constant when airflow about the nose doesn't seem to be at a constant angle when the plane changes the actual AoA, so a linear "correction" seems too easy. Who does that signal processing? And most important....and according to latest diagram we have of the "system"....

Our diagram shows analog AoA is sent from the SMYD and thence to the stick shaker. But it has been stated that AoA is also used to "correct" airspeed and maybe that value is used for the STS versus raw pitot dynamic pressure. Where do the ADIRU's , cockpit displays and FCC boxes and the MCAS get AoA? From the SMYD? . There must be plenty of room for mischief here. Ya think?

Gums sends...

Salute Krusty!

so....
And what would that elephant be, Krusty?

There are libel and slander laws that come into play, although I feel many of us here could make assertions and render opinions without facing the law, but sometimes we need to leave our escape door open. And BTW, Krusty, I am more in your camp than nit-picking maintenance procedures, simple mis-alignment, BB's on the ckt boards, etc. Nevertheless, I am exercising caution by looking for something that can be corrected easier than the management decisions of a major aerospace company.

Gums sends...

The waters were already muddied before we started this discussion :O. We can take it as read that this isn't one of those very rare accidents that turn out to have a single, straightforward "ultimate cause".

I would expect, when the investigation report finally appears, that there will be both multiple factors listed under "probable cause" and several other "contributory factors" (that's NTSB-speak, but most AIBs make a similar distinction).

One finding I don't expect to see is that the engineer replacing the AoA probe either sawed off the locating pin or hammered it through the skin of the 737. :ugh:

What makes you so sure they didn't? Murphy can be extraordinarily creative. In the early days of the PW4000, it used a motor driven actuator for the high pressure fuel shutoff in the fuel control - and for some reason it wasn't the same as the one they used on the JT9D - basically RUN and CUTOFF were reversed. To prevent use of the JT9D actuator, they changed the locating pin location. Turned out that due to the vibration environment on the PW4000, the actuators failed quickly and often (quickly replaced by a solenoid setup that they should have used in the first place).
We got more than one JT9D motor actuator back - with complaints that it didn't work on the PW4000 - with the locating pins neatly machined off so it would fit.
We saw similar issues with throttle resolvers - the locating mechanism to prevent left/right swap being neatly machined off. I think it was Wernher von Braun who said 'It's really hard to make something idiot proof, because the idiots are so creative.'

I can't help but wonder if for some reason they changed the AOA sensor for the MAX (with appropriate Murphy Proofing). When the Lion Air mechanics determined that the NG sensor wouldn't fit the MAX, they "made it fit".

I was being sarcastic - of course the AoA sensor is Murphy-proofed (as several posters have previously confirmed).

While I don't know for a fact, I would be extremely surprised if the NG and MAX sensors were different. Besides, if they were, given that they are serialised rotable components, the investigation would undoubtedly have been able to ascertain by now if the wrong part number had been fitted.

And, while I may be biased from my years spent in airline engineering, I simply don't believe that any LAE, anywhere in the world, would "make a part fit" if it clearly didn't.

seems to me if one carefully reads the entire link
https://www.satcom.guru/2018/12/angl...ure-modes.html

it comes down to Electrical issue unlikely, mechanical issue probable. OK. mis-clocking when attached is a possibility- but perhaps unlikely ? . Seems to leave a clocking between vane and resolver shaft as a possible scenario. A) BUT the probability of two units being factory assembled wrong being used on the same side of the same airplane IMO has way to many zeros behind the decimal point. (but NOT impossible ). B) So how might the shaft be attached to the vane ? C) Or is it one solid piece and the mounting of the internal- attached coil possible to be incorrect - but that gets us back to A) .

Or is occams razor double sided ? Looks like recovery of the AOA sensor and how attached to left side may be the only way to resolve ....:confused:

Gums - 737 Max conversion from AOA vane to AOA body is an approximation that uses a scale factor of slightly more than 0.5 plus a small second order term and a constant offset. This is clearly a simplified fit to a non-linear relationship that was designed to be sufficiently accurate over the range of interest. A vane movement of 2 degrees equates to a free stream AOA change of about 1 degree. The sensor base plate has eight holes and a pair of alignment pins that look from pictures I have seen to be located roughly fore / aft along the horizontal line of symmetry of the circular base plate. I do not know the details about whether or not installation is critical with regard to which pin is forward and which pin is aft. I suspect that the same sensor is designed to work on either side of the plane so there must be something in the wiring or signal processing down stream that differentiates between left and right and sorts out whether clockwise motion is positive or negative AOA change.

Salute FC eng

Thanks for the clarification.
Seems the folks on this thread are looking downstream of the actual sensor/vane to advocate an electrical or signal processing factor versus a poorly installed AoA vane.
as you said:
Thnks again,

Gums...

I'm confused by this. When installing an AOA a reference check is done against the aoa angle and smyd. The quick check uses three points. The upper aoa stop is 100deg, middle is 0 deg and lower stop is -100 deg. These correspond to actual angles as measured at the aoa and as seen on the smyd display. I'm confused as to why a conversion would be needed.

The aoa cannot be installed backwards as it only has 200 degrees of movement. Symd 1 and 2 contain different functionality so I would think the processing of aoa angles is done there with regards to the aoa being in the left or right position.

Sadly, during the course of my career I've seen several examples where they did exactly that - a couple of which I outlined in my previous post (admittedly none of the cases I'm familiar with had bad outcomes, there was a certain amount of luck involved).
I lived in Indonesia for the better part of a year in the mid 1990s - working with what was then IPTN on new aircraft development. I saw things going on there in the manufacturing arena that were down right scary - and had to go to the chief engineer to ground an airplane one time because the lightning protection was so flawed as to be blatantly unsafe (to his credit, once I explained the risk he followed my recommendations).

Seems to this SLF that if it was a simple clockwise versus counterclockwise type change, for the issue a hand, one vane would show 20 degrees pitch up and the other would show pitch down- BUT only one showed a bias in the ' wrong' direction - ???