This lack of robustness existed since the early implementation and persisted for decades.[1] The updated flight control system will use both flight control computers and compare their outputs. This switch to a fail-safe two-channel redundant system, with each computer using an independent set of sensors, is a radical change from the architecture used on 737s since the introduction on the older model 737-300 in the 1980s. Up to the MAX in its prior-to-grounding-version, the system alternates between computers after each flight.[140] The two computers' architecture allowed switching in-flight if the operating computer failed, thus increasing availability.