Boeing 737 Classic

The Boeing 737 Classic is a series of narrow-body airliners produced by Boeing Commercial Airplanes, the second generation of the Boeing 737 series of aircraft. Development began in 1979 and the first variant, the 737-300, first flew in February 1984 and entered service that December. The stretched 737-400 first flew in February 1988 and entered service later that year. The shortest variant, the 737-500, first flew in June 1989 and entered service in 1990.

Compared to the original series, the classic series was re-engined with the CFM56, a high-bypass turbofan, for better fuel economy and had upgraded avionics. With a 133,500–150,000 lb (60.6–68.0 t) MTOW, it has a range of 2,060 to 2,375 nautical miles [nmi] (3,815 to 4,398 km; 2,371 to 2,733 mi). At 102 feet (31 m) the -500 is similar in length to the original 737-200 and can fly 110 to 132 passengers. The 110-foot-long (34 m) -300 can seat 126 to 149 passengers while the 120-foot-long (37 m) -400 accommodates 147 to 168 seats.

It competed with the McDonnell Douglas MD-80 series, then with the Airbus A320 family which prompted Boeing to update its offer with the 737 Next Generation, thus designating the -300/400/500 variants as the 737 Classic. In total, 1,988 aircraft were delivered from 1984 until production ended in the year 2000: 1,113 -300s, 486 -400s and 389 -500s.

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. The new series featured CFM56 turbofan engines, yielding significant gains in fuel economy and a reduction in noise, but also posing an engineering challenge given the low ground clearance of the 737 – a trait of its 707-derived fuselage. Boeing and engine supplier CFM International solved the problem by placing the engine ahead of (rather than below) the wing, and by moving engine accessories to the sides (rather than the bottom) of the engine pod, giving the 737 a distinctive noncircular air intake.

The wing incorporated a number of changes for improved aerodynamics. The wing tip was extended 9 inches (23 cm). The leading-edge slats and trailing-edge flaps were adjusted. The flight deck was improved with the optional electronic flight instrumentation system, and the passenger cabin incorporated improvements similar to those on the Boeing 757. The family also featured a redesigned vertical stabilizer with a dorsal fin at the base.

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.

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 24,000 to 20,000 lbf (107 to 89 kN), mostly due to the reduction in bypass ratio.

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.