The Boeing 737 MAX has gone from one of the most scrutinized aircraft programs in modern aviation to one of the most important workhorses of the global commercial narrowbody market. For airlines, it is now a core piece of pretty much every major North American fleet. The jet is an extremely fuel-efficient single-aisle jet used on everything from short domestic hops to longer regional and leisure routes. For Boeing, the MAX is even more significant. It is the company’s most important commercial program by volume, and its main challenger to the bestselling Airbus A320neo family.