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

climber314 25th Dec 2018 13:16

Maintenance Lapse Identified as Initial Problem Leading to Lion Air Crash
 
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 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/mainten...39204?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

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.

jimtx 25th Dec 2018 16:52

How many turns of the screw?
 
20 degrees seems like a lot to change, however it is done. Im sure well 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


Originally Posted by CONSO (Post 10344384)
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


Originally Posted by BluSdUp (Post 10344318)
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


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

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


Originally Posted by jimtx (Post 10344463)
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


Originally Posted by Capt Quentin McHale (Post 10344455)
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.

On the 737NG you just have to swap the sensor and thats it job done. It is recommended that you you do the calibration but not mandatory.

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/...ml?mtrref=t.co

jimtx 26th Dec 2018 19:02


Originally Posted by weemonkey (Post 10344744)
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

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....fcebabedfa.jpg

MickG0105 26th Dec 2018 21:32


Originally Posted by climber314 (Post 10344956)
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....fcebabedfa.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 delivered 6 weeks before PK-LQP had a left ADIRU problem two weeks after the JT610 crash.

CONSO 26th Dec 2018 21:33


Originally Posted by jimtx (Post 10344922)
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.


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