History
The Idea
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. Here are some statistics which illustrate its success:
- The Boeing 737 is the best-selling commercial jetliner in history, with, as of January 2017, orders for almost 14,000 aircraft from 290 customers. Over 9,300 737s have been delivered.
- With approximately 6,500 airplanes in service, the Boeing 737 represents a quarter of the total worldwide fleet of large commercial jets flying today.
- On Feb. 13, 2006, Boeing delivered the 5,000th 737 to Southwest Airlines. Guinness World Records acknowledged the 737 as "the most-produced large commercial jet" in aviation history.
- 31% (24,000) of all commercial flights are operated by the 737s.
- More than 338 airlines in 112 countries fly the 737.
- On average, over 2,900 737’s are in the air with nearly 500,000 passengers at any given time.
- On average, one 737 takes off or lands every 1.5 seconds.
- Boeing 737’s have carried more than 16.8 billion passengers; that is equivalent to every single man, woman and child flying at least twice. (2012 world population was 7 billion).
- Boeing 737’s have flown more than 115 billion miles; equivalent to approximately 624 round trips from the earth to the sun.
- Boeing 737’s have flown more than 180 million flights.
- Boeing 737’s have flown more than 257.6 million flight hours; the equivalent to one airplane flying more than 29,416 years nonstop.
Design
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.
Heritage
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. Both Jack Steiner and Joe Sutter’s names are on the patent for the 737.
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.
First Flight
 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." (follow this link to more background on the first 737 flight)
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.
Teething Troubles
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. |
History, Development & Variants of the Boeing 737
