Originally Posted by
DaveReidUK
The AOA DISAGREE annunciation worked as intended on the aircraft of operators who had opted for the PFD AoA indicator (a notable example being American Airlines), but was non-functional on aircraft without that option (i.e. most other airlines' aircraft).
Why in god’s name was this offered as an optional extra?
The second vane was there. The wiring to the FCC was there, the software which compared the two values and outputted an annunciation was there, and it would be presumably just as easy to provide it for
all customer options, rather than just some.
We will never know whether such an annunciation on the accident aircraft would have ‘fast-tracked’ the diagnosis of the problem especially on ET where by that time it was known that MCAS existed and it was known that MCAS was triggered by data from an AoA vane.