The Boeing 737-400 is an extended version of the 737-300, and several significant modifications were made to enhance its performance, capacity, and operational efficiency. Here are the key differences:

1. Length and Capacity:

   • The 737-400 is longer than the 737-300, measuring approximately 129 feet and 5 inches (39.5 meters) compared to the 737-300's 110 feet and 4 inches (33.6 meters). This increase in length allows the 737-400 to accommodate more passengers, typically seating around 150 to 188 passengers in a single-class configuration, compared to the 737-300's capacity of about 126 to 149 passengers.

2. Wing Design:

   • The 737-400 features a modified wing design with a larger wing area, which improves lift and fuel efficiency. The winglets, which were not standard on the 737-300, can be added to the 737-400 to further enhance aerodynamic performance and reduce drag.

3. Engines:

   • Both models use the CFM56 engines, but the 737-400 can be equipped with more powerful variants, providing better performance, especially on longer routes and in hot and high-altitude conditions.

4. Avionics and Systems:

   • The 737-400 includes upgraded avionics and systems compared to the 737-300. This includes improvements in navigation, communication, and flight management systems, enhancing operational efficiency and safety.

5. Cargo Capacity:

   • The 737-400 has a larger cargo capacity due to its extended fuselage, making it more versatile for airlines that also operate cargo services.

6. Operational Range:

   • The operational range of the 737-400 is slightly greater than that of the 737-300, allowing airlines to serve longer routes without needing to refuel.

These modifications made the 737-400 a popular choice for airlines looking for a medium-range aircraft with increased passenger capacity and improved operational capabilities.