Behind the Lion Air crash

In 2010, Airbus announced it would introduce a more fuel-efficient version of its popular A320 jetliner. Within months, Boeing announced plans to upgrade its own 737 jets.

In designing the new 737 Max, Boeing tried to avoid a complete overhaul of its systems so that it could persuade regulators at the U.S. Federal Aviation Administration that there would be no need for expensive, time-consuming retraining for pilots.

That decision appears to have left the crew of the Lion Air jet that crashed in Indonesia in October, killing 189 people, without a full understanding of how to respond when faulty data led the flight control system to repeatedly push down the nose of the plane.