Originally Posted by
PEI_3721
[left]A challenge for Boeing via investigators is to identify how ‘AoA’ came to dominate these accidents, whilst apparently using the same / similar AoA probe on older 737s.
Some confusion for Ppruners is not knowing where the FDR AoA value is taken from -
IF it's same as NG, then according to the AMM (CH27 - 27-32-00) the SMYD (Stall Management Yaw Damper) receives analog AOA inputs from sensor and sends digital AOA to FDR (via FDAU).
Edit: Gums has pointed out that apparently it's not the same as the NG, so probs best to disregard most of my speculation...
FCC is listed as
receiving digital AOA from the on-side ADIRU (
not the SMYD). ADIRU is
not listed as getting digital AOA from SMYD, SMYD is
not listed as
sending digital AOA to ADIRU or to FCC. All of which
could mean that ADIRU also gets the analog inputs from AOA sensor - but I can't find confirmation of this in the manual, only in some non-Boeing diagrams online.
The implication is that something other than the probe / probe output might contribute to the problem - wiring.
Yup, wiring gremlins definitely in the picture (until ruled out) in my opinion. Iff analog sensor outputs are in fact going into ADIRU
and SMYD then we can probably discount (dual simultaneous) ADC failure, and maybe makes a wiring fault more likely near the sensor than near the FCC/SMYD? Wiring (or connector) fault at near the sensor would also fit Lion Air previous flights with different / intermittent errors until sensor replacement which then made it worse.
Peter Lemme at satcom.guru did some analysis of possible wiring failures and results on AOA decoded values without coming up with much, however he didn't consider a short (or partial short) between sin and cos. I reckon this might give a fixed AOA offset over a range of AOA - but I am not sure what range, depends on offset and gain values that I don't know. Also I considered only resistance of the short, throw in capacitance, AC signals and result of phase shift on resolver decoding, and, er, well at that point I realised how long it is since I did analog stuff and how much I have forgotten...
Just spotted this from Reuters (
https://www.reuters.com/article/us-e...-idUSKCN1QZ1R9 ): now saying that ET FDR shows AOA data "very very similar" to Lion Air.
