Development began in 1979 for the 737’s first major facelift. Boeing wanted to increase capacity and range, incorporating improvements to upgrade the aircraft to modern specifications, while also retaining commonality with previous 737 variants. In 1980 preliminary aircraft specifications of the variant, dubbed 737-300, were released at the Farnborough air show.
The CFM56-3B-1 turbofan engine was chosen to power the aircraft, which yielded significant gains in fuel economy and a reduction in noise, but also posed an engineering challenge given the low ground clearance of the 737 and the larger diameter of the engine over the original Pratt and Whitney engines. Boeing and engine supplier CMFI solved the problem by placing the engine ahead of the wing, and by moving engine accessories to the sides of the engine pod, giving the engine a distinctive non-circular air intake.
The passenger capacity of the aircraft was increased to 149 by extending the fuselage around the wing by 2.87 meters. The wing incorporated a number of changes for improved aerodynamics. The wing tip was extended 23 cm and the wing span by 53 cm. The leading-edge slats and trailing-edge flaps were adjusted. The flight deck was improved with the optional EFIS (Electronic Flight Instrumentation System), and the passenger cabin incorporated improvements similar to those developed on the Boeing 757. The prototype -300, the 1,001st 737 built, first flew on 24 February 1984 with pilot Jim McRoberts. It and two production aircraft flew a nine month long certification program.
In June 1986 Boeing announced the development of the 737-400, which stretched the fuselage a further 3.0 m, increasing the passenger load to 170. The -400s first flight was on February 19, 1988 and, after a seven-month/500-hour flight testing run, entered service with Piedmont Airlines that October.
The -500 series was offered, due to customer demand, as a modern and direct replacement of the 737-200. It incorporated the improvements of the 737 Classic series; allowing longer routes with fewer passengers to be more economical than with the 737-300. The fuselage length of the -500 is 1 ft 7 in (48 cm) longer than the 737-200, accommodating up to 132 passengers. Both glass and older style mechanical cockpits arrangements were available. Using the CFM56-3 engine also gave a 25% increase in fuel efficiency over the older -200s P&W engines.
The 737-500 was launched in 1987 by Southwest Airlines, with an order for 20 aircraft, and flew for the first time on June 30, 1989. A single prototype flew 375 hours for the certification process, and on February 28, 1990, Southwest Airlines received the first delivery. The 737-500 has become a favorite of some Russian airlines, with Aeroflot-Nord (now Nordavia), S7 Airlines, and Rossiya Airlines all buying second-hand models of the aircraft to replace aging Soviet-built aircraft.
After the introduction of the -600/700/800/900 series, the -300/400/500 series was called the 737 Classic series.
The price of jet fuel has skyrocketed in the past five years; airlines devote 40% of the retail price of an air ticket to pay for fuel in 2008, versus 15% in 2000. Consequently, carriers have begun to retire the Classic 737 series to reduce their fleet sizes; replacements consist of more efficient Next Generation 737s or Airbus A320/A319/A318 series aircraft. On June 4, 2008, United Airlines announced it would retire all 94 of its Classic 737 aircraft (64 737-300 and 30 737-500 aircraft), replacing them with Airbus A320 jets taken from its Ted subsidiary, which has been shut down.
Prompted by the modern Airbus A320, Boeing initiated development of an updated series of aircraft in 1991. After working with potential customers, the 737 Next Generation (NG) program was announced on November 17, 1993. The 737NG encompasses the -600, -700, -800 and -900, and is to date the most significant upgrade of the airframe. The performance of the 737NG is essentially that of a new aircraft, but important commonality is retained from previous 737 models. The wing was modified, increasing its area by 25% and span by 4.9 m which increased the total fuel capacity by 30%. New, quieter, more fuel-efficient CFM56-7B engines were used. All three improvements combined increase the 737’s range by 900 NM, now permitting transcontinental service. A flight test program was operated by 10 aircraft; 3 -600s, 4 -700s, and 3 -800s.
The first NG to roll out was a -700, on December 8, 1996. This aircraft, the 2,843rd 737 built, first flew on February 9, 1997. The prototype -800 rolled out on June 30, 1997 and first flew on July 31, 1997. The smallest of the new variants, the -600s, is the same size as the -500. It was the last in this series to launch, in December 1997. First flying January 22, 1998, it was given certification on August 18, 1998.
In 2004, Boeing offered a Short Field Performance package in response to the needs of Gol Transportes Aéreos, which frequently operates from restricted airports. The enhancements improve takeoff and landing performance. The optional package is available for the 737NG models and standard equipment for the 737-900ER.
Boeing has already hinted that a “clean sheet” replacement for the 737 (internally dubbed “Boeing Y1”) could follow the Boeing 787.
Engines on the 737 Classic series (300, 400, 500) and Next-Generation series (600, 700, 800, 900) appear not to have circular inlets, as most aircraft do. The accessory gearbox was moved from the 6 o’clock position under the engine to the 4 o’clock position (forward looking aft). This was done because the 737 sits lower to the ground than most airliners and the original 737s were designed for small P&W engines, but additional ground clearance was needed for the larger CFM56 engines. This side-mounted gearbox gives the engine a somewhat triangular rounded shape. Because the engine is close to the ground, 737-300s and later are more prone to engine foreign-object damage (FOD).
737s are not equipped with fuel dump systems. Depending upon the nature of the emergency, 737s either circle to burn off fuel or land overweight. To save weight and reduce cost and complexity the 737 lacks full doors to cover the main landing gear. The main landing gear (under the wings at mid-cabin) rotate into wells in the aircraft’s belly, the legs being covered by partial doors, and “brush-like” seals aerodynamically smooth (or “fair”) the wheels in the wells. The sides of the tires are exposed to the air in flight. “Hub caps” complete the aerodynamic profile of the wheels. It is forbidden to operate without the caps, because they are linked to the ground speed sensor that interfaces with the anti-skid brake system. When observing a 737 takeoff, or at low altitude, the dark circles of the tires can be plainly seen.
The primary flight controls are intrinsically safe. In the event of total hydraulic system failure or double engine failure, they will automatically and seamlessly revert to control via servo tab. The 737 is the only modern passenger aircraft this size or larger that can operate completely without hydraulics
Most 737 cockpits are equipped with “eyebrow windows” positioned above the main glareshield. Eyebrow windows were a feature of the original 707. They allowed for greater visibility in turns, and offered better sky views if navigating by stars. With modern avionics, they became redundant, and many pilots actually placed newspapers or other objects in them to block out sun glare. They were eliminated from the 737 cockpit design in 2004, although they are still installed in military variants and at customer request. These windows are sometimes removed and plugged, usually during maintenance overhauls and can be distinguished by a metal plug which differs from smooth metal which appears in later aircraft that were not originally fitted with the windows.
Blended winglets are available as retrofits and in production on newer 737 aircraft. These winglets stand approximately 2.4 m tall and are installed at the wing tips. They help with reduced fuel burn (by reducing vortex drag), reduced engine wear, and less noise on takeoff.
A short-field design package is available for the 737-600, -700 and -800, allowing operators to fly increased payload to and from airports with runways under 1,500 m. The package consists of sealed leading edge slats (improved lift), a two-position tail skid (enabling reduced approach speeds) and increased flight spoiler deflection on the ground. These improvements are standard on the 737-900ER. The 737 models can be divided into three generations, including nine major variants. The “Original” models consist of the 737-100, 737-200/-200 Advanced. The “Classic” models consist of the 737-300, 737-400, and 737-500. The “Next Generation” variants