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Old 29th Nov 2018, 22:32
  #68 (permalink)  
wiedehopf
 
Join Date: Jul 2014
Location: Germany
Posts: 339
Originally Posted by Lonewolf_50 View Post
As also mentioned elsewhere, multiple previous sectors were flown using manual trim wheel, apparently, to handle the defect that never got quite fixed.
Could you point to the page where "multiple" previous sectors were flown using the trim wheel?
Looking at FR24 data the typical altitude excursions associated with the MCAS trim and stickshaker are only present on the accident and previous flight DPS to Jakarta.

Reading pages 7, 8 and 9 it appears to me the plane intermittently didn't show any speed or altitude on the captains side.
If i understand correctly this could happen if the left AoA signal was missing as static pressure which is needed for speed and altitude gets corrected by AoA.
So intermittently failing AoA sensor might have been the problem.

Now in Denpasar the AoA sensor was replaced. This is the most likely point for the introduction of the 20 degree offset.
A stuck sensor that just transmits one value might happen as a failure. But tracking a 20 degree offset is very unlikely to occur without some change being made.

Possible reasons for the 20 degree offset i can imagine are:
Wrong part number
Faulty manufacturing
Very creative attachment to the plane
AoA vane bent after installation and test


To quote the report:
For troubleshooting due to repetitive problem perform replaced 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.
Some people said the AoA sensor needs to be tested with a special jig to orient it while someone in the cockpit is checking the output value.
Now either that test was not performed or they made a mistake here.
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