This story from the New York Times expounds in detail the nature of the regulatory approval processes from the FAA the tacitly endorsed Boeing’s failure to disclose a new system to pilots during transition training. It appears that a failure of that system, and a lack of training in how to shut it off, were major contributing factors to the crash of a Lion Air Boeing 737MAX that killed 189 people.
The focus of the investigators is the MCAS, the maneuvering characteristics augmentation system, which can push the nose down to compensate for a potential aerodynamic stall of the airplane in certain circumstances. It appears that the system malfunctioned, and since pilots were not trained on that system, were unaware of its characteristics or how to shut it off. Boeing’s position is that the Lion Air pilots should have known how to handle the emergency, even not knowing about the modification in the MAX model that was not in previous models.