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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. In March 1981, USAir and Southwest Airlines each ordered 10 aircraft, with an option for 20 more.
The new series featured CFM56turbofan 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 CFMI 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 non-circular 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 EFIS (Electronic Flight Instrumentation System), and the passenger cabin incorporated improvements similar to those on the Boeing 757.
Engine inlet of a CFM56-3 engine on a Boeing 737-400 series showing the non-circular design
Boeing selected the CFM56-3 to exclusively 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 prototype of the -300 rolled out of the Renton plant on January 17, 1984, and first flew on February 24, 1984. After it received its flight certification on November 14, 1984, USAir received the first aircraft on November 28. A very popular aircraft, Boeing received 252 orders for it in 1985, and over 1,000 throughout its production. The 300 series remained in production until 1999 when the last aircraft was delivered to Air New Zealand on December 17, 1999, registration ZK-NGJ.
In December 2008, Southwest Airlines selected Boeing to retrofit the 737-300 with a new set of instruments, hardware and software, to improve commonality with the 737-700, as well as to support the Required Navigation Performance initiative, but that order was later cancelled and the retrofits never took place.
The 737-300 can be retrofitted with Aviation Partners Boeing winglets. The 737-300 retrofitted with winglets is designated the -300SP (Special Performance). Used passenger -300 aircraft have also been converted to freighter versions. The 737-300 has been replaced by the 737-700 in the Boeing 737 Next Generation family.
The 737-400 design was launched in 1985 to fill the gap between the 737-300 and the 757-200, and competed with the Airbus A320 and McDonnell Douglas MD-80. It stretched the 737-300 another 10 ft (3.45 m) to carry up to 188 passengers. It included a tail bumper to prevent tailscrapes during take-off (an early issue with the 757), and a strengthened wing spar. The prototype rolled out on January 26, 1988, and flew for the first time on 19 February 1988.
The aircraft entered service on September 15, 1988, with launch customer Piedmont Airlines (25 aircraft ordered).
The 737-400F was not a model delivered by Boeing, but a 737-400 converted to freighter. The Boeing 737-400 never included winglets as an option, just like the Boeing 737-600. Alaska Airlines was the first to convert one of their 400s from regular service to an aircraft with the ability to handle ten pallets. The airline has also converted five more into fixed combi aircraft for half passenger and freight. These 737-400 Combi aircraft are now in service.
The 737-400 was replaced by the 737-800 in the Boeing 737 Next Generation family.
The -500 series was offered, due to customer demand, as a modern and direct replacement of the 737-200, incorporating the improvements of the 737 Classic series in a model that allowed 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 (47 cm) longer than the 737-200, accommodating up to 140 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 30 June 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 Nordavia, Rossiya Airlines, S7 Airlines, Sky Express, Transaero, UTair and Yamal Airlines all buying second-hand models of the aircraft to replace aging Soviet-built aircraft and/or expand their fleets. Aerolíneas Argentinas replaced its 737-200s with second-hand 737-500s. The 737-500 is now replaced by the 737-600 in the Boeing 737 Next Generation family. However, unlike the 737-500, the 737-600 has been a slow seller for Boeing since its introduction, with only 69 aircraft delivered.
As the retirement of all 737 Classic models has accelerated, with retirement of 300s and -500s climbing 40% in 2012, the 737-500 has fared worse because of its smaller size. Aircraft older than 21 years old are being retired (vs. at least 24 years old for the 737-300). While a few 737-300s are slated for freighter conversion, there is no demand at all for a -500 freighter conversion. On September 5, 2016, Southwest Airlines flew their last 737-500 revenue flight, flight 377 from El Paso to Dallas.
As of August 2016[update], 55 hull losses of Boeing 737 Classic series aircraft have occurred, with a total of 1,174 fatalities. Notable accidents and incidents involving the 737 Classics (-300/-400/-500) include:
May 24, 1988: TACA Flight 110, en route to New Orleans, suffered double engine failure due to a severe hail storm. The pilot conducted a successful forced landing on a grass levee with no injuries. The aircraft was repaired and returned to service. As a result of this incident, further engine development was carried out to prevent flame-out in severe weather conditions.
January 8, 1989: Kegworth air disaster: British Midland Flight 92, using a 737-400, crashed outside of East Midlands Airport. Of the eight crew and 118 passengers, 47 passengers died. The left engine had suffered a fan blade fracture and the crew, unfamiliar with the 737-400, shut down the still-functional right engine, causing the aircraft to lose power.
February 1, 1991: USAir Flight 1493, operated by a 737-300, collided with a SkyWest AirlinesFairchild Metro III while landing at Los Angeles. All 12 people on the Fairchild Metro died, while 20 passengers and two crew members out of six crew members and 83 passengers died on the 737.
September 16, 1998: Continental Airlines Flight 475, using a 737-500, received wind-shear while landing at Guadalajara, Mexico. None of the passengers and crew received injuries. The aircraft was written off.
April 7, 1999: Turkish Airlines Flight 5904, using a 737-400 with six crew members, crashed in Turkey. All of the crew on board died; no passengers flew on that flight.
March 3, 2001: Thai Airways International Flight 114, a 737-400 bound for Chiang Mai from Bangkok, was destroyed by an explosion of the center wing tank resulting from ignition of the flammable fuel/air mixture in the tank. The source of the ignition energy for the explosion could not be determined with certainty, but the most likely source was an explosion originating at the center wing tank pump as a result of running the pump in the presence of metal shavings and a fuel/air mixture. One flight attendant died.
January 3, 2004: Flash Airlines Flight 604, using a 737-300 with 135 passengers and 13 crew members, crashed into the Red Sea, killing everyone on board, making it the deadliest involving the Boeing 737 Classic.
August 14, 2005: Helios Airways Flight 522, using a 737-300, suffered a gradual decompression which incapacitated five of the six crew members and all 115 passengers. The plane circled in the vicinity of Athens International Airport on its pre-programmed flight plan before running out of fuel and crashing near Grammatiko, killing everyone on board.
June 15, 2006: TNT Airways Flight 352, using a 737-300 Freighter and operating from Liège Airport in Belgium to London Stansted Airport in the UK had to divert to East Midlands Airport due to bad weather. On final approach, the autopilot was disengaged for a short period. The aircraft touched down off the runway to the left, resulting in the right main landing gear being detached and the right wing tip and engine scraping the ground. The pilots managed to lift off again and subsequently made an emergency diversion to Birmingham International Airport, where a landing was performed on the remaining two landing gear, during which the aircraft scraped on its nose and right engine. There were no injuries. The cause of the crash was determined to be a poorly timed message from local air traffic control which the pilot misinterpreted, causing him to descend too quickly. The team of pilots were said by the airline to have managed the situation with skill once the error had been detected, but were dismissed from service with the company as a result of the incident.
October 3, 2006: Turkish Airlines Flight 1476, using a 737-400, was hijacked by Hakan Ekinci in Greek airspace. All 107 passengers and six crew members on board survived. The aircraft landed safely at Brindisi Airport in Italy.
January 1, 2007: Adam Air Flight 574, using a 737-400 with 96 passengers and six crew members, crashed off the coast of Sulawesi. The occupants were never found, and were presumed dead.
Adam Air Flight 172, showing the collapsed rear fuselage
June 5, 2015: Aero contractors Boeing 737-500 En Route from the Domestic wing of Murtala Mohammed Airport, Ikeja, Lagos to Kaduna. 20 minutes after takeoff the aircraft lost altitude, depressurization in the aircraft cabin leading to the dropping of the oxygen masks. After an hour of wobbling in the air the Boeing 737 was diverted to Abuja, despite the landing gear failing initially the aircraft landed safely at Abuja.
August 5, 2016: ASL Airlines Hungary flight QY-7332, a 737-400 cargo registration HA-FAX, landed on Milan Bergamo's Orio al Serio Airport but overran the runway. The aircraft broke through the airport perimeter fence, a parking lot, a guardrail and came to a stop on a road 520 meters past the end of the runway. The aircraft sustained substantial damage, losing both engines and all landing gear legs. Of the two flight crew members, only the captain was injured.
^Epstein, N (1981). "CFM56-3 High By-Pass Technology for Single Aisle Twins". 1981 AIAA/SAE/ASCE/ATRIF/TRB International Air Transportation Conference, 26–28 May 1981, Atlantic City, New Jersey. AIAA-1981-0808.