History & Variants
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*** Updated 24 Nov 2013 ***
In late 1958 Boeing announced a design study for "A twin engined feeder airliner to complete the family of Boeing passenger jets". In Feb 1965 the first order was placed and the project went ahead. The 737 has since become the best selling commercial aircraft in aviation history with more than 5,900 orders from 225 customers so far. Here are some statistics to illustrate its success:
Boeing wanted a true short-haul jet to compete with the Caravelle, BAC One-Eleven & DC-9 but was way behind them. The DC-9 was about to fly, the One-Eleven was well into its flight test program and the Caravelle had been in service for 5 years. They had some catching up to do. Designers Joseph Sutter and Jack Steiner began work on the 737 in November 1964.
The original 1964 specification was for a capacity of about 60-85 passengers, an economical operating range of between 100 and 1000 miles and to be able to break even at a 35% load factor. As a result of final design talks with launch customer Lufthansa the capacity was increased to 100, but the range and load factor figures still stand.
Where to put the Engines
Joe Sutter (also considered to be the father of the 747) knew that Boeing was competing in broadly the same market as the Caravelle, BAC One-Eleven & DC-9 and needed something different. Sitting at his desk one day, Sutter took some scissors and cut up a drawing of the initial design for the 737 which also had a T-tail with aft mounted engines. He began moving the engines around to find a better layout. But putting the engines on struts under the wing like the 707 would block boarding access to the main cabin door on the shorter fuselage of the 737.
"I slid the cutout tight under the wing and felt a sudden flash of excitement. Instead of mounting the engines away from the wing on struts, why not mount them hard against the underside of the wing itself?"
The wing mounted engines gave the advantages of reduced interference drag, a better C of G position, quieter aft cabin, more usable cabin space at the rear, fore & aft side doors, easier access to engines for maintenance and required less pipework for fuel & bleeds. The weight of the engines also provides bending relief from the lift of the wings. Apparently this benefit was over-estimated and a set of wings failed in static tests at 95% of max load so the wing had to be redesigned.
The disadvantage of wing-mounted engines was that the size of the fin had to be increased for engine-out operation over centreline thrust aircraft. Also, due to the reduced ground clearance, the engines had to be almost an integral part of the wing, which in turn necessitated a short chord. The engines extended both forward and aft of the wing to reduce aerodynamic interference (further improved by the longer tailpipe of the target thrust reversers in 1969) and the straight top line of the nacelle formed a "stream tube" (aka streamline flow) over the wing to further reduce drag.
Initial worries about the low mounted engines ingesting debris proved unfounded, this was demonstrated by the Boeing 720B whose inboard engines are lower than the 737's and had been in service for four years without significant problems.
The final wing was a work of art as the specification required both good short field performance and economy at altitude.
Overall, the wing-mounted layout had a weight saving of 700Kgs over the equivalent “T-tail” design and had performance advantages. A further advantage of the wing-mounted engine design was its commonality with earlier Boeings such as the 707.
Jack Steiner had helped design the B29 & 707 and was Chief Designer of the 727. His major contribution to the 737 project was to use as much of the 727 in the 737, in particular the fuselage cross section. This gave not only cost savings in tooling commonality but also the payload advantage of 6 abreast seating, one more than the DC-9 or BAC 111 and allowed it carry standard sized cargo containers on the main deck. It gave the interior a spacious look and allowed Boeing to use standard cabin fittings from the 727 & 707 such as toilets and galleys. This together with its large hold capacity, gave it scope for using the aircraft as a freighter, a role to which many of the older 737's are now being converted, whilst the competitors are being scrapped.
In fact the 737 had a 60% parts commonality with the 727 which included the doors, leading edge devices, nacelles, cockpit layout, avionics, components and other fittings. The 727 in turn had a similar commonality with the 707, so parts of the 737 can be traced back to the early 1950's eg the fuselage cross-section above the floor.
Using off-the-peg components was quick and cheap for both design and production and also helped pilots and engineers convert to the new type, but they also dated the aircraft, a feature which was never really addressed until the NG's 30 years later.
On 19 Feb 1965, Boeing announced the 737-100 after an initial order of 10 from Lufthansa, who became the first foreign airline to launch a US airliner. Assembly started in the summer of 1966.
The maiden flight of the 737 was on 9 April 1967, just two years after the project launch. Boeing's assistant director of flight operations, Brien Wygle (Left) was in command and Lew Wallick (Right) the chief test pilot was co-pilot. The uneventful two and a half hour flight from Boeing Field to Paine Field. Wygle said after landing: "I hate to quit, the airplane is a delight to fly."
Flight testing continued at a blistering pace with the prototype clocking up 47hr 37min in the first month. Soon six aircraft, including the first -200, were on the flight test & certification program. Between them they flew 1300hrs of flight tests. Many changes were made to the aircraft in this time, e.g. trying inflatable main landing gear door seals, although these were soon changed to the present rubber strips.
However the earliest 737's had some problems, including clamshell door thrust reversers (from the 727) that didn't work properly, and a shimmy in the landing gear, but it was a good airplane from the start, recalls Brien. FAA type certification A16WE was gained on 15 Dec 1967.
The -100 was 94ft (28.65m) long, carried 115 passengers and had an MTOW of just 42,411Kgs, less than half that of the current -900 series. The original choice of powerplant was the Pratt & Whitney JT8D-1 at 14,000Lbs thrust, but by the time negotiations with Lufthansa had been completed the JT8D-7 was used. The -7 was flat rated to develop the same thrust at higher ambient temperatures than the -1 and became the standard powerplant for the -100.
The prototype subsequently went to NASA and was in regular experimental use until 1997. It is now stored at Boeing Field where it is on permanent display. NASA 515 was involved in numerous pioneering flight investigations including control systems, 3D and 4D navigation, in-flight energy management, computerized flight management systems, electronic displays, Microwave Landing System (MLS) development (overrun in late development by the advent of GPS), slippery runway studies, and clear air turbulence and wind shear detection and warning. The airplane has a second flight deck, fully functional, installed in the main cabin, that was used for much of the flying. Control systems used included the original Boeing control column/wheel, Brolly handles (like bicycle handlebars), and the current side-stick controllers. Numerous glass cockpit CRT display configurations were tried, that attacked many questions regarding display arrangements, colours and symbology. Much of this work wound up on Boeing, Douglas, and Airbus airplanes as well as the Space Shuttle. The airplane has a unique fourth hydraulic system with a reservoir, pump and filter system. Other studies were performed of drag-reducing external coatings, cockpit displayed traffic information, takeoff performance monitoring, and precision flare guidance during landing touchdown.
Photos: Boeing & NASA
The 2 / 3 Crew Issue
The 737 was the first 2 crew aircraft that Boeing had produced, all others had a flight engineer station which was necessary as early airliners had been more complex and less reliable. The 3 crew issue had been around since the late 1950's with the Lockheed Electra and some fighting between airlines and the pilots and engineers unions which lead to a high profile four month strike that in the end only deferred a new policy decision. When the 737 was announced ALPA and the FAA were on the case of the 737 from the outset as the 3 crew issue had still not been resolved.
To finally determine if the aircraft was capable of safe 2 crew operation, a 737 was flown with an FAA pilot and a Boeing pilot over the busiest week of the year (Thanksgiving) in the busy Boston - Washington corridor. They flew 40 sectors in 6 days, including approaches to minimums, go-arounds, diversions, simulated instrument failures and crew incapacitation. In December 1967 the FAA issued a statement declaring that "the aircraft can be safely flown with a minimum of two pilots."
Even after the FAA statement, American, United and Western continued to operate with 3 pilots until 1982. Fortunately the rest of the world was not so limited and this helped sales recover.
The first 737 went into service with Lufthansa on 10 February 1968. Generally operators were very impressed with the reliability of the aircraft from the start, although inevitably there were some technical issues found during line work. The APU had a tendency to shut down under load, this was solved by developing a new acceleration control thermostat. Engine starter valves were found to clog with sand from treated runways, this was fixed by using a finer mesh on the filter screen. Perhaps most troublesome were the integral airstairs which have a complicated way of folding and simultaneously collapsing the handrail as they retract or extend. The airstairs can still be very frustrating and in 2005 at least two UK based 737 operators decided to remove all airstairs from their fleets to avoid despatch delays, further maintenance costs and a 177Kg weight saving every sector. The first 737's had twice the problems though as many aircraft were fitted with rear airstairs which were if anything, more complicated. Other minor issues were nosewheel corrosion, ram air inlet problems and hydraulic line failure, all of which were sorted out within a couple of years.
737 Originals Key Dates:
11 May 1964: Formal design begins.
9 Nov 1964: 737 Program go-ahead
19 Feb 1965: First order from Lufthansa.
5 Apr 1965: First order for 737-200 from United.
9 Apr 1967: First flight of 737-100.
8 August 1967: First flight of 737-200, the 5th 737 to fly.
15 Dec 1967: FAA Type certification of 737-100 and -200.
Production (See also Production page)
At present it takes 11 days to build a 737 with about 5800 airplane unit hours of work. However, the order backlog is over 800 aircraft, with further orders coming in at around 200 a year so production methods are being speeded up. By 2006 Boeing are aiming for a 6 day assembly time and just 4000 airplane unit hours. This will be achieved by connecting the "join and installation" process to the moving assembly line and by changes to the wing-body join process.
Not all of the 737 is built at Renton, for instance the fuselage is built at Wichita. Also much of the components are outsourced beyond Boeing; eg the doors are built by Vought at Stuart in Florida, the spoilers by Goodrich at Charlotte, NC, the vertical fins by Xi'an Aircraft Industry in China and the horizontal stabilisers are built by Korea Aerospace Industries.
Any special modifications or conversions (eg for the C40A, AEW&C or MMA) are done at Witchita after final assembly of the green aircraft.