Is it common for automated control systems to use non-redundant sensors? While reading an article about the Lion Air Crash with a Boing 737 Max 8, I was very surprised by the following paragraph:


  In designing the 737 Max, Boeing decided to feed M.C.A.S. with data
  from only one of the two angle of attack sensors at a time, depending
  on which of two, redundant flight control computers — one on the
  captain’s side, one on the first officer’s side — happened to be
  active on that flight.


My naive assumption was that any system that is capable of pointing an airplane towards the ground would certainly have redundant sensor input, and switch off if there is serious disagreement between the sensors.

Is it common to have non-redundant sensors for this kind of purpose in commercial airplanes?
In general, yes. Except in certain configurations, namely autoland, autoflight functions are usually fed with one set of sensors.

This matches the fact that pilots flight instruments are fed from a single source as well. Therefore, the autopilot behavior and instruments should have a coherent and predictable response.

The main reason is fault isolation. If the flight crew identifies a fault with the captain's instruments, then disabling instrumentation and autoflight functions on the captain's side will guarantee the faulty data will not influence the copilot's instrumentation and controls.

If you have the system try to "intelligently" vote, you run the risk of not being able to isolate a faulty sensor, especially in 2 faulty of 3 sensor situations. Another classic problem is how to prevent the voting system from becoming a single point of failure.

Instruments are accompanied by a comparator, which detects significant differences between redundant readings and causes an appropriate warning. Most systems treat the warning as binary: the flight crew manually compares all readings and using human logic, decides which are discrepant.