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On the surface of it, the accidents to two aircraft of the same type (Boeing 737 MAX), which eventually led to the suspension of airworthiness of the type, was caused by faulty data from one of the angle-of-attack sensors. This in turn led to automatic nose-down trim movements, which could not be countered effectively by the flight crew. Eventually, in both cases, the aircraft became uncontrollable and entered a steep accelerated dive into terrain, killing all people on board on impact.
In the course of the investigation, a new type of flight assistance system known as the Maneuvering Characteristics Augmentation System (MCAS) came to light. It was intended to bring the flight characteristics of the latest (and fourth) generation of Boeing's best-selling 737 airliner, the "MAX", in line with certification criteria. The issue that the system was designed to address was relatively mild. A little software routine was added to an existing computer to add nose-down trim in situations of higher angles of attack, to counteract the nose-up aerodynamic moment of the new, much larger, and forward-mounted engine nacelles.
Apparently the risk assessment for this system was not commensurate with its possible effects on aircraft behaviour and subsequently a very odd (to a safety engineer's eyes) system design was chosen, using a single non-redundant sensor input to initiate movement of the horizontal stabiliser, the largest and most powerful flight control surface. At extreme deflections, the effects of this flight control surface cannot be overcome by the primary flight controls (elevators) or the manual actuation of the trim system. In consequence, the aircraft enters an accelerated nose-down dive, which further increases the control forces required to overcome its effects.
Finally I will take a look at certification processes where a large part of the work and evaluation is not performed by an independent authority (FAA, EASA, ...) but by the manufacturer, and in many cases is then simply signed off by the certification authority. In a deviation from common practice in the past, EASA has announced that it may not follow the FAA (re-) certification, but will require additional analyses and evidence. China, which was the first country to ground the "MAX", will also not simply adopt the FAA paperwork.