Old 2nd Jan 2019, 02:10
  #55 (permalink)  
Join Date: Jul 2009
Location: Not far from a big Lake
Age: 77
Posts: 1,457
Wiedehopf, we are badly handicapped by not having access to schematics of the SMYD box, its associated systems, and the Interactive Fault Isolation Manual to help better understand the maintenance codes. All we have going for us is a bit of theory. Some activity like Boeing that holds the actual data package for the aircraft, or the manufacturer of the SMYD boxes are the only ones likely to have sufficient data to figure this out in detail.

If we look at the Sine and Cosine signals from the AOA resolver, these are analog signals and have to be put together to make a piece of angle data. At some point, apparently in close proximity to the SMYD box, these signals are digitized and sent by the aircraft data bus throughout the aircraft.
There are two ways to put these signals together that I can immediately think of.
  1. Digitize the two signals independently, and then digitally convert the result to an angle, or
  2. Combine both Sine and Cosine signals to an analog angle signal, and then digitize that.
There are purpose built circuits for digitizing resolver data, but they may be newer in design than the currently utilized SMYD method.

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

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

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