Engines on the 737 Classic series ( 300 , 400 , 500 ) and Next-Generation series ( 600 , 700 , 800 , 900 ) do not have circular inlets like most aircraft .
Engines on the 737 Classic series ( 300 , 400 , 500 ) and Next Generation series ( 600 , 700 , 800 , 900 ) are not shaped like a circle like most planes .