United Airlines moving its Boeing 737 MAX jets to short-term storage in Arizona

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13. Let me add to what jimmil wrote:

All of the new testing around any fixes will be based on new safety assessments of any code, sensors, systems and structures which could be involved. When systems need to be single and multiple fault tolerant, the number of conditions in a failure mode analysis to formulate integration and verification plans and procedures grow geometrically.

My understanding is that part of the strategy for maintaining the 737 model with a set of changes and updates was to avoid having to have every commercial pilot go through training and clearance to fly a whole new model of aircraft. The MCAS was one of the items planned to make the MAX work like the previous versions.