The Boeing 737-400 is a narrow-body airliner and a variant of the 737 Classic series. Like other 737s in the Classic series, this variant features upgraded turbofan engines and an extended fuselage. The 737-400 is the longest & largest variant in the Classic series and at the time, of the entire 737 family. It is no longer in production but it is still in service with many airlines. Some aircraft have been modified into a 'Combi' variation where they carry cargo as well as passengers. Like other 737s, the Boeing 737-400 typically flies short to medium haul flights with medium to high demand. It is well tasked to fly trunk routes between major airports or less busy routes to secondary airports. This flexibility, shared across all 737 aircraft, has made this plane very popular with airplanes.