Background d with wider and more efficient
CFM56 turbofans Following the success of the
Boeing 737-200 Advanced, Boeing wanted to increase capacity and
range, incorporating improvements to upgrade the plane to modern specifications, while also retaining commonality with previous 737 variants. Development began in 1979, and in 1980, preliminary aircraft specifications were released at the
Farnborough Airshow.
Model developments In March 1981,
USAir and
Southwest Airlines each ordered 10 aircraft of the 737-300 series, with an option for 20 more. That aircraft, the initial model of the 737 Classic series, first flew in February 1984 and entered service in December of that year with Southwest Airlines. A further stretched model, the 737-400, was launched with an order for 25 aircraft with 30 options from
Piedmont Airlines in June 1986. That aircraft first flew in February 1988 and entered service later that year with Piedmont Airlines. The final model of the series, the 737-500, was launched with an order for 30 aircraft from Southwest Airlines in May 1987. That aircraft, which was designed as a replacement for the 737-200 and had similar passenger capacity and dimensions, as well as the longest range of any member of the 737 Classic family, first flew in June 1989 and entered service with Southwest Airlines in 1990.
Engines Boeing selected the
CFM56-3 exclusively to power the 737-300 variant. The 737 wings were closer to the ground than previous applications for the CFM56, necessitating several modifications to the engine. The fan diameter was reduced, which reduced the bypass ratio, and the engine accessory gearbox was moved from the bottom of the engine (the 6 o'clock position) to the 9 o'clock position, giving the engine nacelle its distinctive flat-bottomed shape, which is often nicknamed the "hamster
pouch". The overall thrust was also reduced, from , mostly due to the reduction in
bypass ratio.
Speed Trim System The 737 Classic saw introduction of Speed Trim System (STS), a flight augmentation system that adjusts the stabilizer automatically at low speed, low weight, aft center of gravity and high thrust with autopilot disengaged. Most frequently it can be observed during takeoffs and go-arounds. The system relies on most of the same hardware and software used in autopilot mode. STS is not fail-safe in that it uses only one of each sensor types required for its functionality and a single computer. Such a single-channel design is not common for augmentation systems that have full control of the stabilizer. This design was considered acceptable because of the ability for the aft and forward column cutout switches as well as center console cutout switches to constrain its malfunction. The limited flight envelope protections on the 737 Next Generation series, as well as MCAS on the 737 MAX, are later extensions of this system.
Further developments Throughout the 1980s, the 737 Classic series attracted large orders from airlines in the United States and Europe, with its order totals exceeding those of preceding 737 models. By far, the most successful model was the 737-300, with deliveries totaling 1,113 aircraft (the 737-400 and -500 reached 486 and 389 deliveries, respectively). Major operators included US carriers, small national airlines, and charter carriers. By the 1990s, when regular Boeing customer
United Airlines bought the
Airbus A320, this prompted Boeing to update the slower, shorter-range 737 Classic -400 into the rewinged, updated, more efficient, longer
737NG-800. Production of the 737 Classic continued alongside that of the Next Generation for a period of time; the first 737-700 was completed in December 1996; the last 737 Classic was completed in February 2000.
Modifications Six former
Southwest Airlines 737-300s are modified and operated for
aerial firefighting by
British Columbia-based Coulson Group, supported by a loan by the Canadian government for CA$3.4 million (2018) which is equivalent to CA$4.1 Million (or US$2.9 million) in 2024.