First flown in 1967, Boeing Co.’s 737 has become the world’s best-selling commercial aircraft. The fourth generation of jets, known as the Max, made its debut in 2017 when Indonesian low-cost carrier Lion Air became the first commercial operator. Yet two fatal crashes within five months -- Lion Air Flight 610 in October off the coast of Indonesia and Ethiopian Airlines Flight 302 in March outside Addis Ababa -- have led to a global grounding of the aircraft and put the 737’s previously sound saf