Boeing suffers new 737 Max issue that could delay return

[Major Nikon]

2. It's not a case of only one being active

Boeing made software that compared the two AOA sensors optional, so some aircraft have it and some don't.

Jets made at least since the 80's typically have two AOA sensors that operate the stick shaker and stick pusher in the event of an impending stall. So this part of the technology is nothing new and has been around for a very long time when processors were far less powerful. If you look at the MD-80 series (a design which goes back to 1980), you'll see a sensor underneath each cockpit side window that has a red circle around it. That is the AOA sensor tied to the stall protect system.