Dave re # 1875, et al, AoA disagree alert; Reuters article.
The preliminary accident report indicates that maintenance had knowledge of AoA problems and took appropriate action; page 17 ‘AOA SIGNAL FAIL’ and ‘AOA SIGNAL OUT OF RANGE’ and then changed the vane unit (presumably the left one, due to the associated left bias in other air-data alerts.)
The Reuters argument appears to depend on the supposition that a ‘disagree’ alert can determine which of the two vanes is in error. A disagreement involves the comparison of inputs, i.e. determining a difference (split), but not the validity of any one value, (which one is correct).
The Flight and Maintenance log together with BITE can provide additional information - as reported.
There appears to be no better maintenance action available than that taken; fitting an additional cockpit alert is of little / no value to maintenance.
A further hobby-horse in the article is a pilots display of AoA.
This theme also is taken up with the report of Southwest retrofitting the display option, quoting their reasons as:-
“… guard against any erroneous sensor data that may activate the jet’s controversial stall protection system.”
“… a valuable supplemental crosscheck in the event there is an erroneous AoA signal present” **
The implication in this reasoning again implies that it is possible to determine which value of AoA is correct.
Further misunderstanding comes from the FAA AD checklist, which relates the failed vane to the highest AoA (thus fly with the lowest AoA value). This is only correct for a situation involving MCAS trim operation, highest AoA is incorrect.
It is equally possible to have an AoA disagree alert due to one vane under-reading where the high value is correct (also with disagree alerts in other systems). The crew have no means of knowing which display is correct. This is the same issue as above, ‘disagree’ involves comparison, not validity.
Other instrument systems which have comparator logic have a third system to resolve an errant input; ‘2 out of 3’ logic (airspeed, alt). You cannot expect a dual (single failure exposed) system to provide triple redundancy, which appears necessary for MCAS etc.
**
https://theaircurrent.com/aviation-s...737-max-fleet/