Originally Posted by
JDiver
Redundancy was made an optional extra, available separately or together at an additional price. Ultimate greed, IMO.
1) (both) Angle of Attack sensor display in Primary flight display
2) Angle of Attack disagree warning (does not operate unless #1 is installed)
Ethiopian, Lion, United purchased neither. Iirc they saved ~$55,000 per aircraft.
Southwest purchased (2), but weren’t told by Boeing it wouldn’t work without (1).
American, to give credit where it’s due, purchased both for their MAX aircraft.
I believe you're getting 2 similar concepts confused.
The 2 options you mention are for providing AoA info directly to the pilots.
On the other hand, a major flaw (IMHO) of MCAS is that it acted on data only from one of the 2 AoA sensors on the aircraft. I'd imagine the auto-pilot reads from both sensors. MCAS continuously "adjusting" trim for an erroneous readings from a single sensor, with no comparison to the other sensor (and no attempt to validate the data from the sensor it was reading), is the lack of redundancy I think the other poster was referring to. MCAS should have redundant readings irrelevant of the options purchased for pilot indications.
For either of the options to have helped avoid the crashes, the pilots would've needed full training on MCAS including the fact that if the single AoA is giving faulty readings, then MCAS will activate (trim nose down repeatedly) even though it is not needed.
I don't think the optional AoA indicators are common equipment. I've never piloted a plane that had one (granted, I don't fly commercial airliners, but have friends who do). All planes have stall warning systems to indicate you're approaching an aerodynamic stall (your AoA is too high).