Fatal airline accidents are mercifully rare, so when two aircraft of the same new model crash in succession, killing a total of 346 people, it is an exceptional tragedy. That is the case with Boeing’s new 737 Max, a single-aisle jet that is an update of its best selling model. It was grounded last week by regulators around the world — belatedly including the US Federal Aviation Administration.

It is too early to tell the causes, but both a Lion Air crash off Indonesia in October and the Ethiopian Airlines accident last week involved Boeing 737 Max 8s encountering difficulty shortly after take-off. An initial investigation into the Lion Air crash found that the onboard flight control software had repeatedly countermanded the pilot’s commands and pushed the aircraft’s nose down as they tried to climb.

The accidents raise troubling questions about whether Boeing and the FAA allowed the 737 Max to be introduced into service without fully educating pilots about new software that was meant to prevent a stall as they climbed too steeply, or making the pilots retrain. With hindsight, they also look complacent in letting the aircraft keep flying after the Lion Air crash.