Boeing, which had originally planned to develop a new plane from scratch to maximize efficiency, quickly decided to switch course and copy Airbus. It too began developing more fuel-efficient engines and sticking them in old planes: The 737 Max. (3/X)

But unlike Airbus, Boeing’s old planes weren’t as aerodynamically suited for the newer, bigger engines. The engines would sometimes make the aircraft’s nose pitch up. So to avoid expensive redesigning, Boeing created new software (MCAS) to detect it instead. (4/X)