Netflix has a new documentary on Boeing called Downfall, centered around the two crashes of the Boeing 737 Max planes. It is worth your time. But I wanted to talk about a lesson from the debacle: the importance of Goodhart’s law. So why did the two Boeing 737 Max planes drop out of mid-air and kill several hundred people? A reasonable answer is that the angle of attack sensor failed, which caused the plane to misbehave. But sensors fail all the time; why did that prove fatal? We can use Systems thinking and apply Iceberg analysis to get to the root cause.