Maintenance Lapse Identified as Initial Problem Leading to Lion Air Crash [Archive]

climber314

25th Dec 2018, 13:16

Maintenance Lapse Identified as Initial Problem Leading to Lion Air Crash

Crash investigators have concluded preliminarily that improper calibration of an airspeed (AoA?) sensor during maintenance touched off the sequence of events that led to October’s fatal Lion Air jetliner crash in Indonesia, according to people familiar with the details.

The conclusion is subject to further analysis, these people said, but it is the firmest indication so far that a suspected maintenance lapse was the initial misstep that ended with the months-old Boeing (https://quotes.wsj.com/BA) Co. 737 MAX aircraft plunging into the Java Sea, killing all 189 people on board.

Two days before Lion Air Flight 610’s early morning takeoff from Jakarta on Oct. 29, according to these people, mechanics installed but failed to properly calibrate a replacement airspeed sensor called an angle-of-attack indicator.

Based on information downloaded from the flight-data recorder, last month’s interim report revealed a constant 20-degree difference between signals from the angle-of attack sensor on the captain’s side—which had been replaced—and those from the co-pilot’s-side sensor.

Until now, the precise cause of the improper signals from the captain’s-side sensor hasn’t been clear.

People closely tracking the probe said that after U.S. air-safety experts re-enacted the tasks of installing, calibrating and verifying operation of the sensor, they deemed current maintenance instructions appropriate.

https://www.wsj.com/articles/maintenance-lapse-identified-as-initial-problem-leading-to-lion-air-crash-11545739204?mod=e2tw

BluSdUp

25th Dec 2018, 15:50

I, for one would deem the current procedure NOT appropriate , considering the result!

Now , I am sure they can pull out the old card : It hardly ever happens that someone gets it wrong!
Well that was OK until the Max as the misaligned AoA sensor was just a faulty indicator and did not lead to an uncontrollable aircraft via false trim input ( only needing one crew error to open up the last hole in the cheese.)

I am looking forward to see what was actually done, and it would be nice to know if this has been done before and if an aircraft has gotten airborne with same error.

climber314

25th Dec 2018, 16:27

jimtx

25th Dec 2018, 16:52

20 degrees seems like a lot to change, however it is done. I’m sure we’ll see the maintenance procedure from someone here.

CONSO

25th Dec 2018, 18:29

FWIW seems to be confusion in the above posts between AOA and Airspeed. AFIK the AOA ( Angle of attack ) indicator measures the angle of the plane to the mass of air flowing past the plane- and has little to do with airspeed ( the indicator FUNCTION is independent of airspeed) . Whil it is true that for stable flight, the PROPER angle of attack is a function of airspeed and weight and CG and . . . the measurement device is like a windvane whose angle is RELATIVE to the body-airframe .

Sure sounds like somehow, the AOA indicator was installed in the wrong set of holes ( clocked ) relative to the correct set. " If it doesn't FIT - FORCE IT ' assmbly method :mad:

DaveReidUK

25th Dec 2018, 20:07

Sure sounds like somehow, the AOA indicator was installed in the wrong set of holes ( clocked ) relative to the correct set. " If it doesn't FIT - FORCE IT ' assmbly method

According to the Preliminary Report, the AoA values from the suspect sensor were 20 degrees adrift.

If you tried to fit the sensor offset by 20 degrees, none of the attachment screws would pick up on the captive nuts, so that sounds highly unlikely.

tdracer

25th Dec 2018, 20:54

I, for one would deem the current procedure NOT appropriate , considering the result!

BluSd, they are saying there is nothing wrong with the existing procedure - IF IT'S FOLLOWED!!!

The best AMM procedure in the world is worthless if it isn't followed.

DR - I'll be very curious to find out how they messed up the installation, but Murphy's Law says that people can be extraordinarily inventive when it comes to messing things up. I remember us getting back parts that had indexing keys to prevent improper installation where the indexing keys had been very professionally machined off... :ugh:

FlyingStone

25th Dec 2018, 21:00

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.

A lot of FBW aircraft have much more flight control surface movements (including completely limiting pilot authority) based on AoA without flight deck indication of the AoA and seem to be flying around safely. Also, it would be very hard to design an aircraft that will be still very safe with "some poor maintenance and sketchy aviating".

Hopefully they find the CVR and we see in final report what really happened.

Capt Quentin McHale

25th Dec 2018, 22:09

Looking from another angle (no pun intended) a question for the Engineers/Mechanics out there.... Is the AOA sensor calibrated at the factory BEFORE the operator receives it into his spares stock inventory so as to enable a quick sensor change out on the line. For example, disconnect cannon plug, remove old AOA sensor / Install new AOA sensor, reconnect cannon plug, do test (bite check?) and on your way OR install new AOA sensor and then carry out calibration of said sensor.

Rgds McHale.

jimtx

25th Dec 2018, 22:30

Maybe left and right use the same unit and you have to adjust the unit for the correct side?

RatherBeFlying

26th Dec 2018, 03:01

The sensor is mounted on the fuselage; so in the case of a sensor that can be mounted on either side, perhaps needs to calibrated to the angle of incidence between local flow at the fuselage location and wing which will be positive or negative. However being 20° off implies a 10° difference between local flow and AoA:confused:

That kind of difference implies a misalignment when mounting.

Are the sensors generic units with various calibration values for different aircraft and sides?

BluSdUp

26th Dec 2018, 11:29

tdracer
I am all with You on that.
I am wondering how many times , if any, an 737 AOA unit has been installed wrong to this extent, and were it was found out, ie on ground or airborne?
If never, then procedure is good.
The fact that a miss-installed AOA on my 737- 800 is not doing much more then making a lot of rattle and making me work hard for some minutes on the QRH as opposed to the MAX that will demand me to take prompt action on that semi- runaway trim , if not A/c becomes uncontrollable as so tragically demonstrated, SHOULD demand of the Maint Procedure even more rigid adherence and double checking!

I wager the procedure was the same, even with the potential for dramatically worse outcome on a different aircraft.
I have seen this on the Dornier 328 Turboprop when they certified the D328 Jet: Same airframe, lots of cut and paste, small details not applicable ( Propheat ciquitbraker being the nobrainer in QRH)

Anyway
I am looking forward to fly the MAX next year.

weemonkey

26th Dec 2018, 12:30

Maybe left and right use the same unit and you have to adjust the unit for the correct side?

Yes.You just turn it inside out.

BAengineer

26th Dec 2018, 12:44

climber314

26th Dec 2018, 12:59

The NY Times has some fancy new graphic images and a step by step narrative this morning.
It doesn't seem to add anything to the conversation at hand.
https://www.nytimes.com/interactive/2018/12/26/world/asia/lion-air-crash-12-minutes.html?mtrref=t.co

jimtx

26th Dec 2018, 19:02

Yes.You just turn it inside out.

What Ametek says about it: "• Port and Starboard AOA Transducers Are Interchangeable"

climber314

26th Dec 2018, 20:44

MickG0105

26th Dec 2018, 21:32

FWIW I found the following online regarding MCAS:
"This is from a maintenance training manual for technicians/mechanics.
Pilots do not see this. Maintenance do a two week difference course, pilots get a handout."

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/640x475/mcas_51a478cb08f4b788370aa690fcf09efcebabedfa.jpg
Only the F/Os column cutout switch module is affected because it is the only module that interfaces with the FCCs.

What the hell does that mean? Could that have anything to do with the apparent ineffectiveness of the ANU trim commands after control was handed over to the FO?

Separately, with regards to focus on the calibration (or alleged lack thereof) of the replaced left AOA sensor, that aircraft had a history of left-side invalid air data and AOA data problems that preceded the fitting of the replacement sensor. My money has been on a developing problem with the left ADIRU; a Sunwing MAX 8 (https://www.flightglobal.com/news/articles/sunwing-737-max-suffers-spurious-indication-incident-453805/) delivered 6 weeks before PK-LQP had a left ADIRU problem two weeks after the JT610 crash.

CONSO

26th Dec 2018, 21:33

What Ametek says about it: "• Port and Starboard AOA Transducers Are Interchangeable"

Hmmmm- normally the transducer is the device that TRANSforms motion- movement or pressure/vacuum into a electrical signal . But the ' windvane' or aoa ' vane' and mounting brackets would have to be reversible when mounting on opposite sides of aircraft. left side would be O---! versus !---O for right side ( O being transducer mounted on --- axle and ! being vane ( up being forward )

BAengineer

27th Dec 2018, 00:46

I seem to remember that the position (port or stbd) is indicated by pin programming.

Capt Quentin McHale

27th Dec 2018, 05:24

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, 15:08

BAengineer

27th Dec 2018, 15:46

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, 15:47

BAengineer

27th Dec 2018, 15:54

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, 16:17

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, 18:04

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, 19:38

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 (https://www.satcom.guru/2018/12/angle-of-attack-failure-modes.html)
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.org-vbulletin/188x200/screen_2bshot_2b2018_12_02_2bat_2b12_43_53_2bam_c8523285db15 64e0830b86e56e6f574c4865f0b3.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.org-vbulletin/640x455/aoa_2b_2b1_2b_25281_2529_3222b6f47cae71b6fc7cb4ebd08d5ad7573 05251.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 (https://www.analog.com/en/analog-dialogue/articles/precision-rtdc-measures-angular-position-and-velocity.html)
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, 20:31

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, 22:53

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, 22:59

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, 02:02

Capt Quentin McHale

28th Dec 2018, 05:33

CONSO

28th Dec 2018, 14:20

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, 14:21

wiedehopf

28th Dec 2018, 14:32

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, 15:05

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, 15:45

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

climber314

28th Dec 2018, 15:59

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 (https://twitter.com/Satcom_Guru)
13 hours ago
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, 18:08

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....

swh

28th Dec 2018, 18:27

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..

Exactly, the article is a pile of horse manure. No calibration is done, it comes calibrated from the shop and the screw holes line up where it goes.

if you read the full article and not just what is above the source of this information is not the Indonesian investigations, NTSB, or Boeing, they all declined to comment about this.

it comes from joe public who is “following the investigation”. They got their $1000 of click bait out of it.

CONSO

28th Dec 2018, 21:58

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 (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html)

and at bottom of online text emphasis added

SUMMARYAOA is a long-standing subject that is broadly known but one for which the details are not broadly understood. While AOA is a very useful and important parameter in some instances, it is not useful and is potentially misleading in others.

The relationship between AOA and airplane lift and performance is complex, depending on many factors, such as airplane configuration, Mach number, thrust, and CG.
AOA information is most important when approaching stall.
AOA is not accurate enough to be used to optimize cruise performance. Mach number is the critical parameter.
AOA information currently is displayed on Boeing flight decks. The information is used to drive the PLI and speed tape displays.
An independent AOA indicator is being offered as an option for the 737, 767-400, and 777 airplanes. The AOA indicator can be used to assist with unreliable airspeed indications as a result of blocked pitot or static ports and may provide additional situation and configuration awareness to the flight crew.

DaveReidUK

29th Dec 2018, 12:34

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.

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.

wiedehopf

29th Dec 2018, 12:50

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_PK-LQP-NTSC_Preliminary%20Report.pdf)

Before changing the AoA sensor the main symptom is:

Speed and Altitude Flag show on Captain Primary Flight Display (no speed and altitude indication)

Only after changing the AoA sensor the IAS disagree behaviour started:

IAS and ALT Disagree shown after take off

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.

gums

29th Dec 2018, 15:13

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...

infrequentflyer789

29th Dec 2018, 20:22

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.

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.

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.

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...

Machinbird

30th Dec 2018, 02:57

Thank you weidehopf for pointing out the earlier flight maintenance history. It points out an area for investigation.
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?

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.org-vbulletin/584x453/maintaction1_aebcd967de164680d2a98333caf5e8541797bbe1.png
https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/591x953/maintaction2_5e40eb5ad038a35d09fd7e79a8d620dc67a7e9f9.png
https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/581x267/maintaction3_3afebe910d7aa9bab389cbfa304cb5ec6899cfea.png
Comments to follow. I'm, looking at the conductive FOD scenario causing erratic behavior.

Machinbird

31st Dec 2018, 06:00

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:

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?
There is a secondary correction of “Reconnect and clean electrical plug of data module” Is this data module located inside SMYD1?
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.

wiedehopf

31st Dec 2018, 09:43

BluSdUp

31st Dec 2018, 11:14

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

Machinbird

1st Jan 2019, 21:04

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.
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.

wiedehopf

1st Jan 2019, 23:15

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)

CONSO

1st Jan 2019, 23:39

wiedehopf

2nd Jan 2019, 00:02

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?

Machinbird

2nd Jan 2019, 02:10

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.

Digitize the two signals independently, and then digitally convert the result to an angle, or
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.

Machinbird

2nd Jan 2019, 02:28

is there 9 equally spaced screws mounting resolver to vane ?
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.

CONSO

2nd Jan 2019, 04:00

IFixPlanes

2nd Jan 2019, 05:20

https://s15.directupload.net/images/user/190102/temp/he4r4ib5.jpg (https://www.directupload.net/file/u/51580/he4r4ib5_jpg.htm)
1 AOA
2 Alignment Pins
8 Screws

DaveReidUK

2nd Jan 2019, 08:18

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.

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

infrequentflyer789

2nd Jan 2019, 13:14

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... :)

DaveReidUK

2nd Jan 2019, 13:45

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 (http://en.tempo.co/read/923176/knkt-lion-air-airspeed-indicator-damaged-since-3-flights-before/full&view=ok)

gums

2nd Jan 2019, 14:03

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..

FCeng84

2nd Jan 2019, 16:29

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.

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.

CONSO

2nd Jan 2019, 18:24

RE FCeng84 # 63

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.

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:

DaveReidUK

2nd Jan 2019, 19:20

gums

2nd Jan 2019, 19:56

Salute!

according to FCeng

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.

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...

KRUSTY 34

2nd Jan 2019, 19:57

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.

More to the point: If only Boeing had thought not to Murphy-enable the MAX by introducing a system, whereby the aircraft will attempt to destroy itself (along with everyone on board) if the right set of conditions existed.

As far as I know, this particular outcome is unique to the 737 MAX. So, the statement regarding being “lucky” over the last 50 years really doesn’t apply. I’ll even go further and suggest that over the past 50 years it is probable, or even certain that a similar maintenance FUBAR would have occurred, given the the enormous amount of hours flown by the type.

I know this is ”Tech Log”, but the vast discussions surrounding the almost mind bending set of variables, only serves (IMHO) to muddy the waters as to the ultimate cause of this tragedy. An elephant in the room, that many on this, or any other forum, seem unable, or unwilling, to come to grips with?

gums

2nd Jan 2019, 20:47

Salute Krusty!

so....
I know this is ”Tech Log”, but the vast discussions surrounding the almost mind bending set of variables, only serves (IMHO) to muddy the waters as to the ultimate cause of this tragedy. An elephant in the room, that many on this, or any other forum, seem unable, or unwilling, to come to grips with?

And what would that elephant be, Krusty?

More to the point: If only Boeing had thought not to Murphy-enable the MAX by introducing a system, whereby the aircraft will attempt to destroy itself (along with everyone on board) if the right set of conditions existed.

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...

DaveReidUK

2nd Jan 2019, 21:11

I know this is ”Tech Log”, but the vast discussions surrounding the almost mind bending set of variables, only serves (IMHO) to muddy the waters as to the ultimate cause of this tragedy.

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:

safetypee

2nd Jan 2019, 21:31

CONSO, et al; if you are looking for a number, then reconsider the two charts “40% shift in SIN value” and “a mix of positive offset SIN and negative offset to COS” in https://www.satcom.guru/2018/12/angle-of-attack-failure-modes.html
”…about a 20 degree shifted output greater than normal… offsets to SIN and COS can cause shifts in the resolver output. These shifts either maintain nearly a constant value across the angular range ….”

tdracer

2nd Jan 2019, 21:38

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 ...

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".

KRUSTY 34

2nd Jan 2019, 21:44

[QUOTE=gums;10349998]Salute Krusty!

so....

And what would that elephant be, Krusty?

I’m really not sure how to answer that gums!?

DaveReidUK

2nd Jan 2019, 21:57

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 .... What makes you so sure they didn't?

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

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".

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.

KRUSTY 34

2nd Jan 2019, 22:02

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:

Agree with you 100% on that score Dave. Quite often the prevention of the last hole lining up though, is the action(s) of the crew. I doubt that anyone here with at least a reasonable amount of experience hasn’t seen this in one form or another. I know I have.

There can can be no doubt that the process leading up to this disaster is complex, perhaps even complicated! I have always believed though that when the automation goes haywire, disconnect and fly attitude + power to = performance. If that hapless crew did that on the day, and they still all died, then WTF!

CONSO

2nd Jan 2019, 22:33

seems to me if one carefully reads the entire link
https://www.satcom.guru/2018/12/angle-of-attack-failure-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:

FCeng84

2nd Jan 2019, 22:44

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...

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.

gums

3rd Jan 2019, 00:02

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:
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.

Thnks again,

Gums...

Bleedtrip

3rd Jan 2019, 01:30

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.

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.

tdracer

3rd Jan 2019, 02:49

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

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.

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).

CONSO

3rd Jan 2019, 02:51

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...

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 - ???

BedakSrewet

3rd Jan 2019, 05:59

Unless I missed it, I have been wondering why the Lion Air Engineers did not consult the ( Jakarta based ) BOEING Tech Rep(s) when they were trouble shooting the problem following the flight from DPS to CGK ?

ATC Watcher

3rd Jan 2019, 07:14

And we probably will never know for sure what happened as CNN this morning reported that the search for the CVR and more debris have been abandoned

IFixPlanes

3rd Jan 2019, 14:59

RE FCeng84 # 63
Interesting - a pic of " typical " AOA Sensor shows 8 holes in post # 58
...
That is not a "typical" AOA Sensor. This pic shows the sensor of an B737MAX Sensor!

DaveReidUK

3rd Jan 2019, 15:37

That is not a "typical" AOA Sensor. This pic shows the sensor of an B737MAX Sensor!

Are you implying that it's different from the sensor on the NG ?

IFixPlanes

3rd Jan 2019, 15:43

No, PN 0861FL1 can be installed on NG and MAX.

DaveReidUK

3rd Jan 2019, 16:49

No, PN 0861FL1 can be installed on NG and MAX.

Thanks, that helps to discount the theory that the wrong part number was fitted to the accident aircraft and had to be forced to fit.

phil gollin

3rd Jan 2019, 17:25

And we probably will never know for sure what happened as CNN this morning reported that the search for the CVR and more debris have been abandoned

IF that is confirmed as a final decision, then I find it unbelievable.

tarkay01

3rd Jan 2019, 17:41

IF that is confirmed as a final decision, then I find it unbelievable.

No, Lion is stopping their search. The Indonesian government is going to step in and resume searching, albeit with arguably less capable equipment.

wiedehopf

3rd Jan 2019, 17:53

IF that is confirmed as a final decision, then I find it unbelievable.

Any money more spent on this search would probably be much better spent in extra sim time for the pilots.
Don't think the CVR will tell you why the AoA sensor was reading wrong.

An improvement for MCAS is already under way at Boeing as far as i understood. Anyway any improvement in that area can be made without finding any further parts.

What do you think further search will provide?

DaveReidUK

3rd Jan 2019, 18:10

Don't think the CVR will tell you why the AoA sensor was reading wrong.

No, of course it won't.

But the CVR will, in all likelihood, help the investigators (and us) to understand what happened differently on the accident flight compared to the previous flights where the same problems were mitigated by the crews' actions.

Personally, I think we stand to learn a lot from that.

underfire

3rd Jan 2019, 23:21

No, PN 0861FL1 can be installed on NG and MAX.

This is correct. UT manufactures only 2 models of the sensor, one wing mounted, Model 0012, and the fuselage mounted Model 0861...

CONSO

3rd Jan 2019, 23:43

This is correct. UT manufactures only 2 models of the sensor, one wing mounted, Model 0012, and the fuselage mounted Model 0861...

Since the sensor shown seems to be for the left side,is there a different dash number for the right side or is the change taken care of by mounting ' upside down ' on right side and acomodated via electrical ?

DaveReidUK

4th Jan 2019, 08:53

Since the sensor shown seems to be for the left side,is there a different dash number for the right side or is the change taken care of by mounting ' upside down ' on right side and acomodated via electrical ?

Given that, as posted earlier, the physical movement limits of the AoA vane are the same in both directions, there is no need for the sensor to be handed. The aircraft is clever enough to understand that clockwise rotation of one vane means the same as anticlockwise rotation of the one on the other side.

The other giveaway is that the Part Description on vendors' and MRO websites doesn't include a "LH" or "RH" qualifier:

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/688x253/737_aoa_sensor_5023a6add37d47dbd171b88056da37d9066ff84f.jpg

MickG0105

4th Jan 2019, 09:43

The Indonesian government is going to step in and resume searching, albeit with arguably less capable equipment.

Less capable equipment? That'd be difficult. The MPV Everest was only fitted with a pair of Triton XLX ROVs and a Forum Comanche ROV. None of those ROVs are particularly useful for search work unless you intend to search visually for a target sitting on the surface of the sea floor. On the basis that the CVR is almost certainly buried in mud the whole Everest exercise was essentially a waste of time and money. What's required is an acoustic sub-bottom profiler.

underfire

4th Jan 2019, 12:02

given the 'keyed' bolt pattern, the unit can be either RH or LH
https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1318x886/screen_2bshot_2b2018_12_02_2bat_2b12_03_26_2bam_c704f7d2b080 755487e89c1477c8a90c899aeade.png

DaveReidUK

4th Jan 2019, 14:38

given the 'keyed' bolt pattern, the unit can be either RH or LH

Correct, though the photo isn't the variant that's fitted to the 737NG/Max.

See my previous post, and this is what it looks like in situ, courtesy of Burkhard Domke:

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/422x305/0861fl1_cebad52bcae57b9ff8a7869cc2c3e51d1b9c0a72.jpg

There are no fewer than four locating pegs, plus what look like non-uniformly-spaced mounting bolts, designed to Murphy-proof the sensor against being fitted out-of-alignment, but which allow it to serve as the LH or RH sensor.

CONSO

4th Jan 2019, 14:45

Given that, as posted earlier, the physical movement limits of the AoA vane are the same in both directions, there is no need for the sensor to be handed. The aircraft is clever enough to understand that clockwise rotation of one vane means the same as anticlockwise rotation of the one on the other side.

The other giveaway is that the Part Description on vendors' and MRO websites doesn't include a "LH" or "RH" qualifier:

https://cimg8.ibsrv.net/gimg/pprune.org-vbulletin/688x253/737_aoa_sensor_5023a6add37d47dbd171b88056da37d9066ff84f.jpg

translation- must be an electrical function built in somewhere in software or wiring ? or having a connector pin open/blocked differently on each side. ....

underfire

4th Jan 2019, 15:31

Correct, though the photo isn't the variant that's fitted to the 737NG/Max.

The AOA sensor you showed is from Rosemont. The AOA sensor on the MAX is supplied by United Technologies.

This looks like its installed on a battleship, not a max!

https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/422x305/0861fl1_cebad52bcae57b9ff8a7869cc2c3e51d1b9c0a72.jpg

EDIT: Even though the UT AoA sensor is advertised as mounted on the LionAir MAX, either there are multiple options, or multiple manufacturers...

Spirit of Renton MAX is the rosewood you mentioned Dave

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/841x996/max_aoa_copy_75bef0a1d9efd02c7f422304d183944c0bca8741.jpg

DaveReidUK

4th Jan 2019, 15:45

The AOA sensor you showed is from Rosemont. The AOA sensor on the MAX is supplied by United Technologies.

Rosemount Aerospace Inc is a division of what was United Technologies, now Collins Aerospace.

Air Data Systems - Collins Aerospace (https://utcaerospacesystems.com/product_gallery/air-data-systems/)

IFixPlanes

4th Jan 2019, 15:46

Here is the "keyed" bolt pattern of the 737MAX AOA:
https://s15.directupload.net/images/user/190104/temp/owqlap6h.jpg (https://www.directupload.net/file/u/51580/owqlap6h_jpg.htm)
You can see that is is symmetrical and due to the fact that the alignment bushings are at 0 and 180° it can be installed on both side simply by a 180° turn.
@ underfire:
The MAX also use the PN 0861FL1 (Rosemont) - check the IPC (if you can...)

underfire

4th Jan 2019, 15:53

okay, got it! Thanks!

on a similar note, what is that crazy airspeed sensor mounted on a window port??
https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/780x439/170214151919_boeing_737_max_9_window_780x439_33b739c408403d5 b7036c57d19fed0ad4615050c.jpg

wiedehopf

4th Jan 2019, 16:44

Maybe similar to the combo AoA probe/ Pitot tube combinations that's on the A350?
https://www.flickr.com/photos/carlos_f/22484032131/

DaveReidUK

4th Jan 2019, 17:27

on a similar note, what is that crazy airspeed sensor mounted on a window port??

Looks like the 737 MAX 9 prototype.

Given that it's going to an airline eventually (ironically to Lion Air, albeit their Thai offshoot), it makes sense to mount a test probe on a window blank rather than cutting holes in the fuselage skin.

tjernagel

4th Jan 2019, 17:55

Wodrick

4th Jan 2019, 18:09

Looks like maybe an OAT probe, maybe not

tdracer

4th Jan 2019, 18:21

Looks like maybe an OAT probe, maybe not

Certainly looks to me to be a Total Air Temperature (TAT) probe - obviously fitted for flight test proposes. Airspeed probes nearly always have a round or oval inlet, for some reason the rectangular inlet is preferred for temp.
Just speculating - perhaps a different design probe, using flight test to verify it's performance vs. the normal TAT probe. Or maybe unheated, so that they can measure the effect of probe heat on the normal probe when not in icing or similar conditions.

tjernagel

4th Jan 2019, 18:21

It's a flight test airplane... Its used to verify the "real" systems during the flight test program.

gums

4th Jan 2019, 22:16

Salute tjernagel!

The crash was not a simple loss of AoA that the crew was using to fly or a vital input to a fly-by-wire system such as the Airbus 320 and following versions. As much as it is touted, the 777, the early F-18 and such were nowhere near FBW than the Airbus, shuttle, or the Viper I flew long ago.

It's a flight test airplane... Its used to verify the "real" systems during the flight test program.

Correct, but who did the crew verification of the "real" MCAS during the flight test and have one test point where the AoA was off the wall and the plane kept commanding nose down? Not "runaway trim", because you could beep the trim and the system stops commanding nose down for 5 or 6 seconds, then whaaahoo.

I will try to return to my cave for awhile, but a point or two.
- I cannot imagine a new system in a new mod of an old plane that has the potential to make the plane go nose down without pilot consent and without some kinda warning light that the new system is active. Stick shaker? Yes. Some kinda restricttion on commanding further nose up? Sure. But commanding nose down over and over without some kinda signal that something was awry? I don't wanna fly that beast as a pilot or SLF.
- I cannot imagine the system above not being made very public to all pilots and carriers using the plane. GASP!! Without the AoA aspect as a contributing cause to this crash, how come all pilots flying the type did not know a new system was installed and what its purpose was and how it worked. Sheesh.

Gums sends...

Bleedtrip

4th Jan 2019, 22:49

Salute tjernagel!

The crash was not a simple loss of AoA that the crew was using to fly or a vital input to a fly-by-wire system such as the Airbus 320 and following versions. As much as it is touted, the 777, the early F-18 and such were nowhere near FBW than the Airbus, shuttle, or the Viper I flew long ago.

Correct, but who did the crew verification of the "real" MCAS during the flight test and have one test point where the AoA was off the wall and the plane kept commanding nose down? Not "runaway trim", because you could beep the trim and the system stops commanding nose down for 5 or 6 seconds, then whaaahoo.

I will try to return to my cave for awhile, but a point or two.
- I cannot imagine a new system in a new mod of an old plane that has the potential to make the plane go nose down without pilot consent and some kinda warning light that the new system is active. Stick shaker? Yes. Some kinda restricttion on commanding further nose up? Sure. But commanding nose down over and over without some kinda signal that something was awry? I don't wanna fly that beast as a pilot or SLF.
- I cannot imagine the system above not being made very public to all pilots and carriers using the plane. GASP!! Without the AoA aspect as a contributing cause to this crash, how come all pilots flying the type did not know a new system was installed and what its purpose was and how it worked. Sheesh.

Gums sends...

Newer 737 NG's already has a stall protection system that nosed the plane down during a stall.
The difference between the MAX and NG is that the NG will cut out all nose down stab trim if the control column was pulled back far enough. The MAX mcas fcc output locks out the column cutout switch so this can't be done unless the control stand switches are flipped.
So would it make sense that someone who lacked the mcas training but had sufficient NG training should have believed something is wrong with the stab trim when it continues to nose down with the control column pulled back? Just because you can bump it back with manual trim means nothing, you are effectively in a tug of war with the fcc.

Bleedtrip

4th Jan 2019, 22:56

What the hell does that mean? Could that have anything to do with the apparent ineffectiveness of the ANU trim commands after control was handed over to the FO?

Only the FO column cutout module interfaces with the fccs. The mcas engage coil is contained within it.

FCeng84

4th Jan 2019, 22:59

Gums - I for one appreciate your well reasoned and clearly presented thoughts. I hope you don't spend too much time in that cave or that if it does it includes a means to connect to PPRUNE often to share your insights.

Your last post included a reference that has long been a touchy one for me. FWB literally means Fly-By-Wire. We most often presume that to be an electrical wire, but if you extend "wire" to also include mechanical cable then our commercial fleets have been FBW for a long time! My real point, however, is that FBW and Augmented are not one and the same. If I take a mechanical linkage from pilot stick to elevator and replace it with an electrical sensor that measures stick position, a wire that transmits that stick position to an elevator actuator, and an elevator actuator that responds to electrical inputs I have FBW, but I do not have an augmented control system. The augmentation part is where the system uses other inputs besides pilot stick position to determine where to put the elevator.

The 777 is a good example of both simple FBW and complex control augmentation. For it's pitch axis the 777 in its full-up Normal Mode provides considerable augmentation to increase stability, compensate for configuration and thrust changes, and include envelope protection to aid the pilot in keeping angle-of-attack and speed within normal ranges. In the lateral axis the 777 essentially provides proportional gearing from the pilot's wheel to the various wing surfaces used to command roll. 777 also has Bank Angle Protection that works by directly moving the pilot's wheel, but the linkage from wheel to wing surface does not involve any augmentation. In its back up (Secondary/Direct) modes the 777 essentially provides direct pilot controller to surface gearing in all axes.

Those of you who strapped themselves into birds like the Viper were the real pioneers of fully augmented flight with lots of trust in the systems and hopefully an ejection seat that you never had to call upon. Building on that foundation we now see more and more augmentation being used in commercial transport control systems. There is certainly room and need for sound judgement as to how those systems are designed, validated, verified, trained, and maintained. One of the biggest challenges is determining what the limits should be as to how much we are willing to employ augmentation to make up for less than desirable open loop handling characteristics.

Back down off my soap box for now,

FCeng84

gums

5th Jan 2019, 02:45

Salute FCeng!
Taking a rest now, but I have to clarify my definition of FBW for you and others to ponder. I take a harsh position concerning "augmentation" versus pure FBW. The Airbus inplementation is the closest system to what I flew, and even it has a control cable or two for one control surface. It also is a gee command, albeit biased by pitch and roll attitude. Still the closest system to the F-16 and shuttle.

Well before the Viper prototype we had the A-7D "control aug" component of our flight control system that used transducers in the stick control grip to talk to the autopilot. It did this when we did not have autopilot engaged, so-called "manual", but used the autopilot connections to move ailerons and elevator. On my FCF flights I would hold the stick between by legs and simply twist/tilt the grip to bank and pitch. It was functionally FBW, but not really.... heh he. What it did was allow very small, sensitive inputs to the pitch and roll control surfaces using minimal actual movement of the stick. It also used "inertial" sensors that I never bothered to locate, and assumed were in the autopilot hardware. The result was control surface movement when you had your hand completely off the stick! You could tell if the guy in front had "control aug" on because the ailerons and elevator would be twitching on every bump in the taxiway. This characteristic was very evident in the Viper, and you can even see it in the AB 320series. For we "light" types, it made for super close formation flying and very nice ILS approaches following the HUD and/or ADI steering bars. From what I have learned the past two months about the 737 is that even tho the pilots call it flying "manual" the plane still has Hal "helping", as in the STS system. And now the MCAS.

Other than some "x" planes and the captured saucer at Groom Lake, we had the first true FBW in the YF-16, although I beleive at the time the X-15 was pure FBW, and the shuttle was being developed with pure FBW. I cannot emphasize enuf, and have repeated over and over, there were zero mechanical connections to any control surface, period. Constant hydraulic pressure was supplied to the integrated servo actuators and electrical commands moved the small valves in the ISA's. If the electrical connections were gone, then we only had the nylon letdown. A few years later we saw the Enterprise glide down to the strip at Edwards, and it had zero mechanical connections of any kind to the control surfaces. We also saw a few test articles that did away with the hydraulic connection to the ISA's, and had pure electric jets. I dunno what they did with that saucer over near Wendover.

With respect to the point about "augmentation", it looks like we are defining terms and design features. My community used the term FBW meaning no mechanical connections., only electrical signals to the actuators regardless of them being electrically powered or using hydraulic pistons. The only FBW mode I have seen in any plane that has negligible "augmentation" is the "direct" law found in the ;bus, where stick position is turned into control surface position with minimal or little regard for dynamic pressure ('q") or other variables. It is entirely possible to have a functional FBW system with a surface movement corresponding to a stick position or pressure using a fixed "gain" - e.g. 30 degrees of deflection moves the aileron 10 degrees. No problem, and you see this every day with home hobby drones and high perforemance RC planes. However, that implementation is relatively coarse and can easily be "augmented" by bringing into play body rates, dynamic pressure, Aoa, gee and attitude. As with the 'bus, you can get a plane to feel really good and easy to put where you want it to go. The Viper was like that, and the interaction of all those variables came into play. For example, left/right pressure ( mainly pressure, but stick moves less than a eight of an inch) commands roll rate, not aileron deflection. Hal looks at dynamic pressure and AoA and ......, then commands the ailerons, rudder and the independent horizontal stabs to move at "x" rate to "y" degrees in order to result in commanded roll rate. The damned thing is like a video game except for the 9 gees and bouncing your head off the canopy when rolling too sharply.

I personally liked the "augmentation" in the Sluf, and I cannot imagine what a nightmare the Viper would have been without variable gains and rate functions, not to mention AoA inputs.

Thanks for all the tech stuff, FCeng, and we'll see each other later for sure..

Gums sends..

KRUSTY 34

5th Jan 2019, 08:02

My problem with this entire accident is bad AOA or not, the airplane should not have nosed into the ocean. This was not a fatal discrepancy. If the horizontal stab is continuing to trim nose down disconnect it and manually trim the stab.

Nailed it IMHO tjernagel.

The fundamental question must be, did the crew fail to disconnect the trim and operate it manually? If not, then why not?

DaveReidUK

5th Jan 2019, 08:24

The fundamental question must be, did the crew fail to disconnect the trim and operate it manually? If not, then why not?

Yet another reason why it's essential that the CVR is found.

weemonkey

5th Jan 2019, 21:10

keesje

14th Mar 2019, 14:21

:ooh:
:yuk:

Lessons learned ?

scifi

14th Mar 2019, 15:42

If you look at the FDR trace of the trim inputs by the captain, he made 25 separate attempts to trim the aircraft, only to be countered by the airplane trimming down ten seconds later each time, for 26 times. There are similar patterns in this most recent crash, but fewer of them, as the aircraft was lower. Both crews must have tried all they knew, but to no effect.

Machinbird

14th Mar 2019, 16:05

Lessons learned ?
No aircraft is perfect. They are a collection of engineering compromises.
Really know your aircraft, including all its warts and kluges!
Beg borrow or steal as much stick time as you can.
Know the type's accident and incident history and the lessons learned.
Play the what if, game.
I would expect you to be able to lay your hand on any switch or lever in your cockpit-while blindfolded, and to know under what conditions and how you would use that switch/lever.
When the warts become exposed, it will be up to you to save the day.
Then the rest of us will find out if you are/were a glorified bus driver or an aviator.

It is beginning to look like the Ethiopian crew did not learn the lessons from the preceding Indonesian 737 Max crash. Why?

blind pew

14th Mar 2019, 20:46

Great post and have seen both sides. Often it's about keeping quiet and protecting the image whether the airline, manufacturer, regulator or mates.
often based on ignorance.
From losing a best mate with a known problem which was kept secret on an aircraft that had been in service for a decade to two aircraft I transitioned onto in the first months of commercial service.
It includes software problems, structural deficiencies and the books being wrong.
The average line pilot working his socks off has to rely on the training department and they on the manufacturer/authority.
Trident, DC 9 50, MD 80, 747 classic, DC 10, Fokker 100..all of which I had some deficiencies which some crews knew nothing about...flown all ...

CONSO

17th Mar 2019, 02:18

To get this december thread back up re lionair and MCAS and AOA issues

Time to get the whiz bang software types to read A few Ernie Gann novels re gotchas that almost trapped him due to bad sparkplugs, rigging, etc. for object lessonsFate Is the Hunter: A Pilot's Memoir

The High and the Mighty

in the days before microsoft simulators, computers,

Double07

31st Mar 2019, 18:03

In 2006 a Boeing 747 taking off from London Heathrow with over 400 people onboard had both stick shakers begin to operate as the aircraft started to rotate. Once in the air, the stick shakers continued to operate, and the pilot’s and F/O’s instruments showed a continuous disagreement in the IAS air speed. The pilots turned back to land at Heathrow, where maintenance engineers diagnosed the problem as an ADIRU problem and replaced the RHS ADIRU. When the pilots tried to take off again, the stick shakers began to operate in exactly the same way, so they aborted the takeoff and taxied to an airport parking area, where they shut down the plane. Data from the flight data recorder did not show anything unusual in any of the sensors or the electronics. The next day, while the engineers were conducting a simulated flight test on the ground by forcing air past the pitot sensors, it was found that the right hand stick shaker was activated even when the AoA vane remained in the horizontal position. The RHS AoA sensor was then replaced and the system was re-tested, showing that the problem was fixed. The aircraft was returned to service, but a ten days later several diagnostic messages started to appear indicating a different type of intermittent “AoA vane” failure on the RHS. When the RHS AoA sensor was replaced again, the problem was fixed again.

The first failed sensor was returned to the manufacturer’s facility in Seattle Washington, where it failed during a test. The problem was found to be a loose main drive gear on the vane shaft that connected the vane shaft to two inductive-type resolvers and an oil-filled damper. The loose drive gear was caused by an improperly torqued set screw that allowed the vane to rotate freely 360° around the main shaft. This caused angle readings that were offset randomly from the zero position, which correlated with the continuous abnormal activation of the stick shakers. A counter-weight was also found to be loose and had a free play of about ±2° rotation.

The second failed AoA sensor was tested at the facility of its different manufacturer in the UK. It also failed its test. The problem was found to be an “open spot” in the older potentiometer-type resolver, which could have been caused by a contaminant particle acting between the brush-type angle contactor and the continuous resistor coil. This “open spot” almost certainly accounted for the intermittent failures reported by the aircraft’s diagnostic messages.

What this incident report shows is that a vane-type AoA sensor is more complicated than just a vane with a resolver on the same shaft. The additional gearing required between the vane and the resolver can be the cause of random offsets in either direction if a gear can slip on its supporting shaft. The offsets can remain constant until sufficient force is placed on the vane to cause the gear to slip on its shaft, which causes a new offset value in the sensor output. This failure mechanism may explain the 22° offset in the LHS AoA reading on the 737 MAX Lion Air JT610 flight of 29 October 2018, which caused the stick shaker to activate and remain activated for the entire flight. While this reported anomalous sensor behavior does not prove that the AoA sensor was at fault in the Lion Air incident, it does provide a working hypothesis that may be tested further.

For further information, see AAIB Bulletin 8/2008, titled G-VHOT, EW/C2006/12/01.

Double07

31st Mar 2019, 18:21

One may ask how the error in the captain's LHS AoA sensor on flights JT043 and JT610 originated. If the error was it present when the AoA sensor was installed wouldn’t the installation test procedure have detected this error?
In response, I believe it may be possible that the testing process used after the installation of the new AoA sensor prior to the JT043 flight may have been the cause of the 22.5° offset. My reasoning is as follows.

It is known that the range of rotation of the Rosemount 0861FL AoA sensor on the B737NG and B737MAX aircraft is ±110°. The captain’s LHS sensor is the same 0861FL sensor flipped over 180° about the horizontal axis.

However, the report quoted above implies that the vane of the AoA sensor has no such end stops, because it states that if the main gear on the vane shaft becomes loose, then the vane can be rotated completely around 360°. Therefore, the end stops limiting the range of rotation of the AoA sensor must be elsewhere inside the sensor housing. Now, a review of the specifications of many types of resolvers on the internet shows that all resolvers have limitations on the ±angles they can be rotated through. This implies that the resolvers have stops inside them to prevent them from being rotated beyond their maximum angles of rotation. This implies that the maximum angle of rotation of the AoA sensor vane is limited by the stops in the resolver, and not by any stops on the vane or the shaft on which the vane is installed. Therefore, if one applies too much force to the AoA sensor vane to pin it against one of the stops, it is possible that this force can cause a slipping of the main gear on the vane shaft, causing the vane to be offset from the resolver while the resolver continues to read the same value because it is up against the stop inside the resolver. Therefore, an offset can be created between the vane angle and the resolver output angle.

Now, we know from the maintenance records for the aircraft of flights JT043 and JT610 that an installation test was done after replacing the AoA sensor before flight JT043 because the maintenance engineer noted on 27 October 2018 that: “For troubleshooting due to repetitive problem perform replaced the angle of attack sensor in accordance with Aircraft Maintenance Manual (AMM) Task 34-21-05-000-001 and task 34-21-05-400-801 carried out. Installation test and heater system test result good”.

But the Aircraft Maintenance Manual actually specifies TWO types of reference checks that can be performed: 1) A recommended test using a test fixture similar to the one shown below. (Notice that it uses the two tooling holes on the AoA sensor to register the correct angular position). A maintenance technician outside the aircraft sets the AoA sensor vane to the angles 0°, -10°, and °, respectively, and either the same technician, or perhaps a different technician, checks the output of the ADIRU to see if the same angles are provided to the SMYD display. 2) In the absence of a test fixture, a quick check can be done by setting the AoA sensor vane to the angles 0°, -100°, and +100°, the latter of which are the end stops of the vane travel. The output of the ADIRU is again checked to see if the same angles are provided to the SMYD display.

The first reference check cannot cause an offset in the vane-to-resolver output angle. However, the second reference check CAN cause an offset in the vane-to-resolver output angle if the technician setting vane angle applies too much force while setting the vane against the end stop. Specifically, if the last angle to be tested is +100°, then the AoA sensor output will be offset in the positive direction as observed in the JT043 and JT610 flights. This offset will not be observed during the test because the resolver output remains pinned at its +100° output value. Only if the last angle to be tested is different from the +100° end stop setting will an offset be observed in the AoA output during the test.

One further observation. Several posters have commented that the captain’s LHS AoA sensor that had an offset of 22° on flights JT043 and JT610 appeared to have a higher random noise on it than the F/O’s RHS AoA sensor. This may be the result of a defective viscous damper inside the captain’s LHS AoA sensor. This may indicate that the replacement LHS AoA sensor installed on flights JT043 and JT610 was, in fact, a reworked AoA sensor, which may explain why investigators want to review the procedures at the AoA sensor rework facility in Florida as well as the AoA production facility in Minneapolis. And if the sensor was a reworked sensor, perhaps the gearing between the vane and the resolver was not torqued high enough to prevent offsets from being induced by pressure on the vane against the end stops.

Loose rivets

31st Mar 2019, 23:45

Double07, You mention the Rosemount 0861FL AoA sensor in your post and so one would assume specific/realistic scenarios. On a much earlier post in R&N, there was a link to a basic design which had a <20V AC reference voltage in and (variable) magnetically coupled windings that were centred on zero. What it did say was there was an accurate signal up to ~35 degress, but this got less accurate at ~45. I think this was the limit of the output - wherever the physical rotation stopped.

Oil damping of such a mechanism could I suppose be total, the windings submerged. I'd like to know.

Apropos the F16. I defy any true lover of aviation to stop reading this maiden flight. Sphincter-cringing.

Featured Articles - Electric Jet - How the F-16 Became the World?s First Fly-By-Wire Combat Aircraft (http://www.f-16.net/articles_article13.html)

ecto1

1st Apr 2019, 00:02

I've had a disturbing idea:

IF:

- Resolver coils inside the vane are at 45 degrees with the horizontal (Forming an X , not a +)
- Software block do not stop AOA calculation if a plausibility check is failed (sin^2 + cos^2 = Vmax^2) and does a funny average (AOA= (arcsin(sin) + arccos(cos))/2) instead of AOA=atan(sin/cos)

THEN:

(real nose AOA= resolver angle - 45 deg)

A short SIN to GND or COS to GND inside any computer (stall management for instance) or connector or wiring loom would cause the reading from the vane to have an offset of 22.5 degrees when the real AOA is mostly zero.

And if it's high resistance short, as it is often the case with FOD or chaffing, it may very well pass the installation test because the coil is sending a lot of amps to the short, but burn/short the coil or something after a while (say taxi to runway). That would also explain lack of dampening (something fusing/melting/overheating inside the vane) after the flight. That would also explain why at least two different computers sensed too high AOA, which clearly points to faulty sensors, without having to believe that two sensors failed in a row without any external help, which is an impossible coincidence. That will very well match with the conductive FOD inside computer hypothesis mentioned some posts ago (A/C Cb tripping, invalid AOA, fault SMYD computer...)

question: FDR traces show nose AOA or wing AOA (=nose AOA / 2 from what I read around here)?

I really hope it's not that simple.

Double07

1st Apr 2019, 00:23

Loose Rivets, You asked, "Oil damping of such a mechanism could I suppose be total, the windings submerged. I'd like to know".

The oil damper (or more accurately the viscous damper) in these AoA sensors is a small cylinder filled with oil that slows down the rotation of a shaft extending from the inside of the cylinder to the outside. The outside part of the shaft holds a gear that meshes with a gear on the shaft that holds the AoA sensor vane. Therefore, the entire AoA sensor is not filled with oil, but only the cylinder of the viscous damper..

By the way, sometimes the oil leaks out of this cylindrical damper, allowing the gears and the AoA sensor vane to rotate more freely. This can cause the AoA sensor output signal to have a random noise superimposed on it.

Double07

1st Apr 2019, 00:37

Ecto1, You asked, "Question: FDR traces show nose AOA or wing AOA (=nose AOA / 2 from what I read around here?".

I believe that the FDR traces show the wing AOA, and that the calibration is wing AOA is approximately 0.5 x nose AOA. I do not understand the rest of your post enough to comment any more on it.

Lonewolf_50

1st Apr 2019, 00:53

I was interested to read something that I was pondering over but could not put into words, and then someone sort of did (https://www.pprune.org/showthread.php?p=10435128):
I take bets that it has something to do with the signal wiring form the AoA vane to the flight computer (ADIRU) like shorting out one half of the SIN or COS symmetric signal and creating with that something around a 45 degree/2 offset. If the Ethopian airline FDR does show a similar problem, then there is some latent harness, connector or ADIRU problem which will show up in the other 737 MAX made in a similar timeframe. So if that establishes, the investigation might look into some of the grounded planes build in similar timeframe. I have experienced weird wiring anomalies in a number of aircraft, and a few avionics systems, that frustrate the maintenance troubleshooters for days and weeks.
I've also learned about how "certain batches of finished work" can be recalled.

If the AoA unit itself is an industry standard piece, it may be that the signal (upon arriving at the computer brain) has gone wrong. The trace that shows one AoA signal going high and one steady on makes me wonder if there isn't signal contamination ... a condition which can be a real bugger to isolate on the ground.
Just a thought.
For an automotive defect that was a real pain to trouble shoot ...
My sister in law's ford excursion (big V 8 engine) would not and could not keep the AC on. but the problem wasn't in the AC system.
One of the coils was bad, so only 7 cylinders were firing. The computer brain in the car thus cut out the AC automatically as due to the engine running roughly/badly, and all of the signals not lining up in nice lines when arriving at that little brain. It's almost as though Ford's version of HAL was saying ...

You want the AC? I can't let you do that, Danielle ...

GordonR_Cape

1st Apr 2019, 15:08

Look up the boeing document " OPERATIONAL USE OF ANGLE OF ATTACK ON MODERN COMMERCIAL JET AIRPLANES
By john Casxhman - which seems to say that commercial aircraft use the relationship of AOA to the ' horizontal' centerline of the fuselage " and do necessary conversions in then FCC to accomodate wings, etc

Posting the link for you, since being a newcomer to this site, you are not yet permitted to post links: Aero 12 - Angle of Attack (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html)

Double07

1st Apr 2019, 17:05

It has now been confirmed that the replacement AoA sensor that had a 22° offset on Lion Air flights JT043 and JT610 was a reworked sensor. On 28 November 2018, investigator Nurcahyo Utomo of the Indonesian National Transportation Safety Commission (KNKT) told reporters that the AoA unit that was installed in the ill-fated airplane had previously been fixed by Boeing in Florida. He stated that the team will also visit the Florida maintenance facility to check the procedure that was used to fix the AoA. Reference: Tempco.Co article entitled “Lion Air Crash; KNKT to Further Study Recovered AoA Unit”, dated 28 November 2018.

DaveReidUK

1st Apr 2019, 17:42

Double07

2nd Apr 2019, 14:43

Unless I've missed it, I haven't seen any report on whether there was a confirmed fault found with the sensor that was removed. Does anyone know?

You are correct. There has been no report to date on whether there was a confirmed fault found with the sensor that was removed.

In two previous posts above I tried to present evidence that the sensor MAY have been at fault after many other posters have been unable to explain how a large angular offset of 22° can be produced by the electronics downstream from the sensor. My post confirming that the replacement sensor was a reworked sensor was merely additional evidence that the sensor MAY be at fault. I was careless, however, in my choice of words. What I should have said was: “It has now been confirmed that the replacement AoA sensor on Lion Air flights JT043 and JT610 was a reworked sensor. This sensor was on the same LHS side as the 22° offset in the AoA angle recorded by the instrument recorder, and may have been the cause of the offset rather than the electronics.“ Even with this additional evidence, it is still not confirmed that the sensor was at fault. This is why the KNKT investigation is visiting the Florida maintenance facility to see if they can confirm that the sensor was at fault.

infrequentflyer789

2nd Apr 2019, 16:52

Unless I've missed it, I haven't seen any report on whether there was a confirmed fault found with the sensor that was removed. Does anyone know?

NYT article (https://www.nytimes.com/2019/04/02/world/asia/boeing-max-8-lion-air.html)as of today is the first I have seen:

After the crash, the replaced angle of attack sensor was shipped to Minnesota, home of Rosemount Aerospace, the Boeing subcontractor that made it, Mr. Nurcahyo said. He and other Indonesian investigators went to Minneapolis in December. The sensor, he said, was deemed defective.

No more details and that is the only report, Indonesian leak, maybe a pinch of salt needed.

Also doesn't explain how a defective sensor was replaced and still the a/c is getting dud data on that side. Either there are a whole batch of defective sensors out there (so we should be seeing failure on NGs as well) or there were two problems co-occurring.

wiedehopf

2nd Apr 2019, 17:07

NYT article (https://www.nytimes.com/2019/04/02/world/asia/boeing-max-8-lion-air.html)as of today is the first I have seen:
No more details and that is the only report, Indonesian leak, maybe a pinch of salt needed.

Also doesn't explain how a defective sensor was replaced and still the a/c is getting dud data on that side. Either there are a whole batch of defective sensors out there (so we should be seeing failure on NGs as well) or there were two problems co-occurring.

The sensor that was removed could have been defective in a way that doesn't produce a wrong AoA.
Actually the maintenance log indicates that before the AoA sensor replacement there was no "Airspeed disagree" or anything else that would indicate an "AoA disagree" but rather air data missing intermittently for the captain.

So to me it seems very likely that during that replacement the offset error/ AoA disagree was introduced.
But it's not at all clear if the new sensor was the problem or the act of replacing it introduced an cabling or other error down th eline.

jimjim1

2nd Apr 2019, 18:00

NYT article (https://www.nytimes.com/2019/04/02/world/asia/boeing-max-8-lion-air.html)as of today is the first I have seen:

No more details and that is the only report, Indonesian leak, maybe a pinch of salt needed.

Leak? More a reported statement by "Mr. Nurcahyo, the KNKT head of air accident investigations"

"Mr. Nurcahyo, the KNKT head of air accident investigations, ...

After the crash, the replaced angle of attack sensor was shipped to Minnesota, home of Rosemount Aerospace, the Boeing subcontractor that made it, Mr. Nurcahyo said. He and other Indonesian investigators went to Minneapolis in December. The sensor, he said, was deemed defective."

https://www.nytimes.com/2019/04/02/world/asia/boeing-max-8-lion-air.html

DaveReidUK

2nd Apr 2019, 19:50

"After the crash, the replaced angle of attack sensor was shipped to Minnesota, home of Rosemount Aerospace, the Boeing subcontractor that made it, Mr. Nurcahyo said. He and other Indonesian investigators went to Minneapolis in December. The sensor, he said, was deemed defective."

That's a somewhat ambiguous statement. Clearly the sensor was deemed (or at least suspected) to be defective at the time it was removed (otherwise why bother?).

So it's not clear whether the comment refers to that assessment, or to the result of subsequent examination by the manufacturer.

Double07

2nd Apr 2019, 20:36

The sensor that was removed could have been defective in a way that doesn't produce a wrong AoA.
Actually the maintenance log indicates that before the AoA sensor replacement there was no "Airspeed disagree" or anything else that would indicate an "AoA disagree" but rather air data missing intermittently for the captain.

So to me it seems very likely that during that replacement the offset error/ AoA disagree was introduced.
But it's not at all clear if the new sensor was the problem or the act of replacing it introduced an cabling or other error down th eline.

My two posts above explained how a large offset angle could be induced in the AoA sensor during the testing process after sensor replacement. This would have been made more likely by a failure during a rework process to tighten a set screw to a required torque in an internal gear of the AoA sensor. This led me to postulate that the replacement sensor may have been a reworked sensor, which was subsequently confirmed by an actual report. This is all consistent with the latest report that the sensor failed a test at the manufacturer in Minneapolis. This does not mean that the original sensor as manufactured by Rosemount was defective. It merely means that a shoddy rework process in a repair facility may have left the sensor susceptible to change during the installation testing process. One of my original posts above describes how this may have happened.

DaveReidUK

2nd Apr 2019, 22:39

My two posts above explained how a large offset angle could be induced in the AoA sensor during the testing process after sensor replacement. This would have been made more likely by a failure during a rework process to tighten a set screw to a required torque in an internal gear of the AoA sensor. This led me to postulate that the replacement sensor may have been a reworked sensor, which was subsequently confirmed by an actual report. This is all consistent with the latest report that the sensor failed a test at the manufacturer in Minneapolis. This does not mean that the original sensor as manufactured by Rosemount was defective. It merely means that a shoddy rework process in a repair facility may have left the sensor susceptible to change during the installation testing process. One of my original posts above describes how this may have happened.

All of the above are theoretically possible. None have actually been demonstrated to be the case.

Loose rivets

2nd Apr 2019, 23:30

Double07, thanks. While it at first it may seem unlikely there could be an oil filled unit, I was mindful of oil immersed transformer windings which have stood the test of time. It also would have allowed lubrication of far fewer parts. Dash-pots (if that term is allowed for oil as well as air) are historically prone to leakage, so I'm surprised at the added complexity.

I used to reach with my hat to test the vanes and was impressed with the exquisite smoothness of the rotation.

I'd imagine opening one of these units would be akin to working on a watch. I wonder if in the history of their manufacture, it was fully realised by the technicians just how critical the output data was. I recall one of our fleet of six BAC 1-11's used to have the igniters, come at a noticeably lower airspeed than the other five. It was the first indication of AoA being too high. Shake and then Push coming next. Always hand flying to ToC it was sometimes difficult in that aircraft when hot and heavy.

ecto1

3rd Apr 2019, 00:09

JRBarrett

3rd Apr 2019, 03:18

I don't particulary fancy the theory of a mechanical offset because of loose bolt (slipping shaft). People do know how to manufacture square shafts and splines and keyed shafts and safety wire and plenty of other tricks. I don't know how much is one of those vane things, but no less than 3000 dollars for sure. Probably way way more. two cilindrical shafts with a screw tightening one aginst the other is not a proper way to do the job of critical torque transmission without slipping (well, maybe in toys).

In other words, I would not expect any connecting part of a rotary sensor with an all round shaft. Proper way: Splined (if it needs adjusting), keyed, or at least with a D shape.

Also, a mechanical offset does not explain the previous electric gremlins. (FEEL DIFF PRESS, SMYD computer failures and others).

As an AME, I have never had occasion to disassemble an AOA vane, though I have replaced entire AOA sensors. Almost always due to a failure of the heater circuit.

I have replaced RVDT sensors of similar electrical design (with a sin/cos output), used for elevator or aileron position feedback for the autopilot on various aircraft. They have always used a splined shaft. I’ve never seen one with a round shaft held with a set screw. I would think that the internal mechanical connection in an AOA sensor would be similar.

Uplinker

3rd Apr 2019, 09:15

I have experienced weird wiring anomalies in a number of aircraft, and a few avionics systems, that frustrate the maintenance troubleshooters for days and weeks.......

An airship I was once working in (but not piloting - during my television engineering years), had Porsche flat six engines driving the ducted fans. One of the engines had a persistent but intermittent slight misfire. The engineers eventually found that a coaxial cable which carried the ignition signal to that engine had somehow been wired wrong, so the earth was carried by the centre conductor and the signal by the outer screen. A coaxial cable is supposed to have the outer screen earthed so that the signal carried by the inner is screened against external interference.

As somebody has already suggested; one wonders if all the AoA probes on the grounded aircraft will be tested by hand : moving them back and forth, clockwise and anticlockwise by engineers through their whole range to see if there are any bad data spots or signal interruptions?

BluSdUp

3rd Apr 2019, 23:31

What it all boils down to is triple redundancy on all levels critical!
Two engines and the last ultimate mode : Gliding!

Which brings me to training and experience, Gimli 767 crew was **** at Imperial gallon to Kg conversion via Lbs BUT excellent at adapting to glider pilots.
Airtransat , slow to realize fuel leak was the main problem,BUT again great glider pilots.

Point: Engine on fire- useless Shut Down
AOA faulty - Shut down.
BTW . The AOA is of no use to me whatsoever on normal ops with a working Pitot Static system!
3 sensors and a auto shutdown system is clearly needed as standard from what I have read the last 5 months.
Just my 2 cents of common cents
Regards
Cpt B

scifi

4th Apr 2019, 09:42

The eventful flight before the crash flight, had the Jump-Seat pilot save the day by giving an independent assessment of the situation, and finding the correct solution.
Maybe an independent electronic Jump-Seat computer could have done the same job by evaluating the overall picture...
IF Airspeed increasing
and Window not full of sky
and Sense of Negative Gee
and Trimwheels going the wrong way
and Pilots obviously struggling with controls
THEN Advise, Turn off all automatics.

Or maybe the Jump-Seat passenger was just lucky, such as 'What happens if we turn this off..?'

ecto1

4th Apr 2019, 09:57

Scifi, you are 100% right.

That would be the way to go. More intelligent software. 100 times more than 737's, still 100 times less than any os or car.

But that I think is scifi in the aviation industry as we know it.

Lonewolf_50

4th Apr 2019, 13:24

An airship I was once working in (but not piloting - during my television engineering years), had Porsche flat six engines driving the ducted fans. One of the engines had a persistent but intermittent slight misfire. The engineers eventually found that a coaxial cable which carried the ignition signal to that engine had somehow been wired wrong, so the earth was carried by the centre conductor and the signal by the outer screen. A coaxial cable is supposed to have the outer screen earthed so that the signal carried by the inner is screened against external interference.

As somebody has already suggested; one wonders if all the AoA probes on the grounded aircraft will be tested by hand : moving them back and forth, clockwise and anticlockwise by engineers through their whole range to see if there are any bad data spots or signal interruptions?
Thank you. Your post (and a few others) bring me back to something I added to the original Lion Air thread regarding what kind of tech support/field service support Boeing provides to its customers, and what is the cost? The kind of trouble shooting you refer to, and that I was pointing at, is often the kind of digging that a field service rep from the OEM is so good at, and why they exist.
Or did the LOCO airline (Lion Air) go cheap and not buy that? (Did Ethiopian Airlines have a field servicde package, and of what kind?)
The other issue of "why was the AoA cockpit thing an option rather than standard kit" has been done to death in the R&N thread, so I won't go there. The root problem, bad AoA signal getting to the electronic brain, has to be remedied down in detail from AoA probe to the connector at the flight computer.
GIGO.

Double07

24th Apr 2019, 19:02

Originally posted by DaveReidUK (Post 137):

“All of the above are theoretically possible. None have actually been demonstrated to be the case”.

Originally posted by ecto1 (Post 139):

“I don't particularly fancy the theory of a mechanical offset because of loose bolt (slipping shaft). People do know how to manufacture square shafts and splines and keyed shafts and safety wire and plenty of other tricks. I don't know how much is one of those vane things, but no less than 3000 dollars for sure. Probably way way more. Two cylindrical shafts with a screw tightening one against the other is not a proper way to do the job of critical torque transmission without slipping (well, maybe in toys)”.

“In other words, I would not expect any connecting part of a rotary sensor with an all round shaft. Proper way: Splined (if it needs adjusting), keyed, or at least with a D shape”.

Originally posted by JRBarrett (Post 140):

“I have replaced RVDT sensors of similar electrical design (with a sin/cos output), used for elevator or aileron position feedback for the autopilot on various aircraft. They have always used a splined shaft. I’ve never seen one with a round shaft held with a set screw. I would think that the internal mechanical connection in an AOA sensor would be similar”.

In response to the above comments regarding Double07’s posts suggesting that the two B737MAX incidents may have been caused by a defective AoA sensor, please read the recent FAA Emergency Air Worthiness Directive No. 2019-08-51, for the Cirrus SF50 aircraft, dated April 18, 2019. (I can’t post this AD because of my PPRUNE newbie status, but you can search for it on Google).

This emergency AD describes three incidents with the Cirrus Model SF50 aircraft that are astonishingly similar to the incidents with the B737 MAX aircraft. Quoting from the FAA document: “This emergency AD was prompted by Cirrus reporting three incidents on Cirrus Model SF50 airplanes of the stall warning and protection system (SWPS) or Electronic Stability & Protection (ESP) System engaging when not appropriate, with the first incident occurring in November 2018 and the latest in April 2019. The SWPS or ESP systems may engage even when sufficient airspeed and proper angle of attack (AOA) exists for normal flight. The SWPS includes the stall warning alarm, stick shaker, and stick pusher. The ESP includes under speed protection (USP). The SWPS system engaging inappropriately could potentially result in a STALL WARNING crew alert (CAS) message activation, accompanied by an audio alarm and stick shaker activation, followed possibly by either low speed ESP/USP engaging, and/or the stick pusher engaging. The pilot will also observe the dynamic and color-coded (Red) airspeed awareness ranges displaying the stall band, regardless of actual indicated airspeed.”

The AD goes on to state: “Cirrus and Aerosonic (manufacturer of the technical standard order AOA sensor) have identified the probable root cause as an AOA sensor malfunction due to a quality escape in the assembly of the AOA sensor at Aerosonic. Two set screws that secure the potentiometer shaft to the AOA vane shaft may have improper torqueing and no application of thread locker (Loctite) to secure the two set screws. The AOA sensor with this quality escape is labeled with part number 4677-03”.

So, even though the AoA sensors in these two aircraft have different manufacturers and designs, they are sufficiently similar that they can be affected by the same type of defect. And in both cases, this defect can cause an offset in the AoA sensor output leading to a false triggering of the stall warning system.