Previously, the thinking was that most airline pilots weren't using the AOA information anyway - they had included the feature because some airlines had ex-military staff who were used to having it, but most didn't. So the AOA being wrong wouldn't necessarily be that urgent a problem, if it was only the pilot we were talking about.
Unfortunately, it wasn't only used by a small number of pilots, but also by MCAS. It seems that MCAS kicks in during scenarios that weren't originally intended - i.e. take-offs as well as stall scenarios - and they got two other things wrong:
* MCAS was revised to be more aggressive toward the end of testing;
* MCAS has a bug / design flaw that allowed the system to command an increasingly steep nose-down each time a pilot overrode it.
There were lots of little signs all over the place that there was a problem. Unfortunately, they didn't seem to join the dots, and each seemingly-reasonable decision they made when taken together added up to a dangerous scenario.
Unfortunately, it wasn't only used by a small number of pilots, but also by MCAS. It seems that MCAS kicks in during scenarios that weren't originally intended - i.e. take-offs as well as stall scenarios - and they got two other things wrong:
* MCAS was revised to be more aggressive toward the end of testing;
* MCAS has a bug / design flaw that allowed the system to command an increasingly steep nose-down each time a pilot overrode it.
There were lots of little signs all over the place that there was a problem. Unfortunately, they didn't seem to join the dots, and each seemingly-reasonable decision they made when taken together added up to a dangerous scenario.