but not 700, 800 and 900, which are actually older models. Please see the following:
What's different about the 737 Max series compared with earlier 737s?
The 737 Max can fly farther and carry more people than the previous generation of 737s, like the 737-800 and 737-900. It also has improved aerodynamics and a redesigned cabin interior and flies on bigger, more powerful and more efficient CFM LEAP engines. CFM is a joint venture between General Electric and France's Safran.
Those engines, though, required Boeing to make critical design changes. Because they're bigger, and because the 737 sits so low to the ground (a deliberate design choice to let it serve small airports with limited ground equipment), Boeing moved the engines slightly forward and raised them higher under the wing. (If you place an engine too close to the ground, it can suck in debris while the plane is taxiing.) That change allowed Boeing to accommodate the engines without completely redesigning the 737 fuselage -- a fuselage that hasn't changed much in 50 years.
But the new position of the engines changed how the aircraft handled in the air, creating the potential for the nose to pitch up during flight. A pitched nose is a problem in flight -- raise it too high and an aircraft can stall. To keep the nose in trim, Boeing designed software called the Maneuvering Characteristics Augmentation System, or MCAS. When a sensor on the fuselage detects that the nose is too high, MCAS automatically pushes the nose down. (For background on MCAS, read these excellent in-depth stories from The Air Current and The Seattle Times.)
