In addition to not including inputs from two AOA sensors in its original design, Boeing did not flight test AOA sensor malfunctions and how MCAS software would respond, according to several sources.

A former Boeing pilot who tested the 737 Max, who requested anonymity due to fears of negative repercussions, told CNN "I don't think we appreciated the ramifications of a... failure of an AOA probe."

Another source familiar with the 737 MAX testing said the failure of an AOA sensor was not flight tested, but rather "analyzed in the design and certification" of the aircraft, and it was determined trained pilots would have been able to handle the failure.

A second former Boeing test pilot was surprised to learn that the company had relied on a single sensor, as opposed to a redundant system, to perform such a vital function in the first place. "I would be very curious to know what their logic was on that...and what drove them to think that was a suitable solution," said the pilot, who also spoke on the condition of anonymity.