Cost savings came from not retraining pilots and not requiring airports to be redesigned, and so on. Boeing needed to use a larger engine for the new plane bec

Cost savings came from not retraining pilots and not requiring airports to be redesigned, and so on. Boeing needed to use a larger engine for the new plane because larger, hotter engines burn less fuel. When the engine is made larger the wings and fuselage have to raised to accommodate the larger engine, which means jet bridges at airports have to be modified to accommodate the new plane. Boeing wanted to avoid this, so they designed the airframe such that the overall geometry of the plane is still compatible with existing infrastructure at airports, but this meant that the airframe was unstable---it has a tendency to tilt up when accelerating. Boeing compensated for this hardware flaw with a software fix. The two serious mistakes Boeing made are (1) designing a multilayer system where bottom layers rely on upper layers for safe operation (i.e., airframe relies on software for stability) and (2) lack of redundant sensors.