Old 31st Dec 2018, 06:00
  #48 (permalink)  
Machinbird
 
Join Date: Jul 2009
Location: Not far from a big Lake
Age: 77
Posts: 1,457
Maintenance Decryption-Part 1

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

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

A few questions for the maintenance folks here:
  1. Does Reset a circuit breaker mean to push a tripped breaker back in or does it mean to cycle it, or does it mean either of those possibilities?
  2. There is a secondary correction of “Reconnect and clean electrical plug of data module” Is this data module located inside SMYD1?
  3. Is this the device that digitizes the AOA data?
I’m looking for actions that would potentially relocate conductive FOD inside SMYD or its internal / associated components.
If you have intimate knowledge of these systems, feel free to look ahead in the maintenance actions previously posted above to see if you recognize something that would move something associated with processing the AOA signal.
I strongly suspect that something else was moved around at the same time the AOA probe was replaced. That electrical disturbance (noise) seen on the Left AOA signal should not be glossed over. You can even see some extra spikes in the Left AOA data during the accident flight that are not matched by the Right AOA signal.
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