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Boeing 737 Aircraft 737 300 Version of the Boeing 737 Short Range Low Capacity Twinjets

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"Boeing 737 Aircraft 737 300 Version of the Boeing 737 Short Range Low Capacity Twinjets"
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Seeking to complete its family of quad-engined, long-range 707s and tri-engined, medium-range 727s with a twin-engined, short-range airliner, and capture some of the market already filled by the similarly-configured SE.210 Caravelle, BAC-111, and DC-9, Boeing had designed a low-wing aircraft which had deviated from these competing designs by using the same fuselage cross-section as its larger counterparts', retaining their six-abreast coach seating and attaching its engines to the wing underside, obviating the need for the t-tail. That aircraft had been designated "737."

Intended for the same short-sector, high-frequency routes as these other twinjets, it employed a wing short enough to reduce drag, structure weight, and direct operating costs, yet long enough to house the fuel capacity required for its intended sector lengths. Because these had been envisioned as relatively short, high cruise speeds had been less pivotal to the design than those of its long-range models. Although the wing-attached engines had resulted in some degree of lift loss compared to the now-standard, aft, fuselage-mounted powerplant configuration of the other twins, Boeing had been able to counteract this tendency with both leading and trailing edge high-lift devices on the wing.

Citing the design's commonality with its other Boeing types, and conceding that its existing 727s had offered excess capacity, particularly on internal German routes, Lufthansa had become the launch customer for the 737 in its initial version as the 737-100 when it had placed an order for 21 on February 19, 1965.

First flying in prototype form two years later, on April 9, and registered N73700, it had entered scheduled service on February 10, 1968, powered by two 14,000 thrust-pound Pratt and Whitney JT8D-7 engines and able to accommodate up to 103 single-class passengers. A slightly stretched version, the 737-200, had been launched with an order for 40 from United Airlines, and had first flown on August 8, 1967. The elongated version, with longer engine nacelles, had a maximum capacity of 130 and entered service the following year on April 28.

Despite a meager production run of only 30 for the former, the latter, particularly in its later "Advanced" guise, had achieved an impressive 1,114 sales, having earned it the title of world's "fasted selling" twinjet.

In order to incorporate evolving technology, improve performance, and reduce operating costs, Boeing offered yet a third basic version, the type's first significant upgrade and second passenger capacity increase.

Integral to both had been the new-technology, high bypass ratio turbofan which had offered greater thrust, lower fuel consumption, and reduced noise, eliminating the need for a costly redesign of the existing wing and hence restricting the development program's costs to the $250 million level. Unlike the narrower JT8D engine, however, the new turbofan featured a larger diameter and could no longer be directly attached to the wing underside, requiring, instead, attachment by means of a pylon, a configuration which left inadequate ground clearance without an equally costly main undercarriage strut lengthening and redesign.

Only close cooperation between Boeing and CFM International, intended engine manufacturer, could result in a co-solution to the engineering obstacles. The engine itself, the CFM56, had first run in June of 1974 after a considerable development period and had first become airborne on a McDonnell-Douglas YC-15, later also doing so in France mounted to a Caravelle. Retrofitted to the Super DC-8 as the CFM56-2, at which time the aircraft had been redesignated the DC-8-70 series, it had produced 24,000 pounds of thrust. At reduced rating, as the CFM56-3, it had been targeted at the new 737 version.

Relocating the ancillary equipment ordinarily installed in an engine's top and bottom portions, CFM International had redimensioned it to one of oval shape with an almost flat underside, reducing its diameter and therefore increasing its ground clearance, while Boeing had pylon-mounted it at a slight angle in order to avoid wing underside and trailing edge heat and exhaust interference. The configuration, which had placed most of the turbofan ahead of the leading edge and had resulted in downwardly canted thrust efflux, had liberated previously unusable wing volume for increased fuel tankage, indirectly increasing the aircraft's range. Coupled with a lengthened, repositioned nose gear strut, the arrangement had produced a 28-inch engine-to-ground clearance, which had been only two inches shorter than the 30 provided by the much narrower JT8D nacelle.

In order to counter the additional weight exerted on approach speeds, a new leading edge slat, running between the engine pylon and the wing tip and featuring double the area of the precedent 737-200's, increased the chord by four percent, permitting an approach speed which had been only five knots higher than that of the earlier version and raising its altitude capability by 4,000 feet.

Program launch, on March 5, 1981, preceded actual first orders, by USAir and Southwest Airlines, by three weeks. The first roll-out, occurring three years later, on January 17, 1984, in Renton, Washington, of the prototype, which, for the second time, bore the N73700 registration originally assigned to the 737-100, initially took to the skies the following month, on February 24, attaining a 29,000-foot altitude and completing a successful two-hour, 56-minute maiden flight. FAA type certification, following a three-aircraft, 1,294-hour flight test program, had been attained on November 14 for an aircraft which had not only met new stringent noise level regulations, but had demonstrated greater performance and economy than calculated during its design phase.

The third basic version of the 737 family, designated 737-300, featured a fail-safe aluminum fuselage with a nominal, 8.8-foot extension over that of the 737-200, comprised of 3.8 feet ahead of the wing and five feet along its trailing edge, and resulting in a 109.7-foot overall length and a modest, 19-passenger capacity increase.

The cabin, incorporating larger, redesigned overhead storage compartments and revised lighting, standardly accommodates eight first class, two-two arranged seats at a 36-inch pitch and 120 economy class, three-three arranged seats at a 32-inch pitch. A maximum of 149, single-class, exit-limited passengers can alternatively be accommodated in a single-class configuration at a 32-inch seat pitch. Underfloor baggage, cargo, and mail holds are located forward and aft of the wing.

Cockpit commonality with the earlier 737 versions had been deliberately retained in order to facilitate a common type rating.

The aluminum alloy, two-spar wing, with a 94.9-foot span, features 11-inch tip extensions, a 4.4-percent leading edge extension and a lateral control spoiler panel outboard of the engines, and a new flap section and track fairing aft of them. Low-speed, high-lift devices include inboard, leading edge Krueger flaps; outboard, three-section, leading edge slats; and trailing edge, triple-slotted flaps. Three outboard, powered, overwing spoiler panels, of aluminum honeycomb, augment lateral control and act as inflight airbrakes and ground-based spoilers, while graphite composite ailerons provide roll control. Wing and fuselage center section tanks house 5,311 US gallons of fuel.

A variable incidence tailplane, whose span is greater than that of the earlier 737-200, is activated by two electric motors, with manual standby reversion capability, while the vertical tail introduced a dorsal fin atop the fuselage in order to counteract engine loss-created asymmetric thrust conditions.

All surfaces are hydraulically actuated and operate off of two independent systems.

The equally, hydraulically retractable and extendible, dual-wheeled, tricycle undercarriage is devoid of wheel well doors on its main units, their wheels forming aerodynamic seals in the stored position. The design decreases weight and fosters simplicity of operation and ease of maintenance access.

Power is provided by two thrust-reverser equipped CFM International CFM56-3C-1 turbofans, each rated at 20,000 pounds of thrust.

The basic gross weight variant, at a 124,500-pound take off weight, features a 1,625-mile range, while the high gross weight option, at 138,500 pounds, produces a 2,260-mile range.

A representative round-trip flight, from New York/La Guardia to Chicago/O'Hare with United Airlines, illustrates the typical two-hour sectors for which the 737-300 had been designed.

After some two hours of taxiing parallel to both the active (13-31) and the nonactive (4-22) runway at La Guardia Airport amid 50 other aircraft, most of which awaited westbound air traffic control clearances through the weather-obstacled Cleveland Center corridor, the United Airlines 737-300 followed a US Airways Express DHC-8-100 on to Runway 13, conducting its acceleration roll and rotating. Instructed to "contact departure," Flight 695 maneuvered into a tight left bank over Flushing Bay and the East River, as the Throgs Neck and Whitestone Bridges, draped in white light necklaces, passed below and beyond the left wing.

Given successive altitude clearances, from 5,000 through 9,000, 10,000, and 15,000 feet, the aircraft banked to a 320-degree heading, contacting New York Center on 132.6, and was cleared to its first VOR at a "climb and maintain" flight level of 230. After a 280-knot speed restriction, the 737 had been cleared to 36,000 feet and contacted Cleveland Center. A full orange moon, hovering above the left wing tip, faintly illuminated the thick cumulonimbus cloud deck below.

Speed brakes permitted rapid, initial descent 70 miles from O'Hare International Airport. Plying the moon-illuminated, silver skies, the aircraft briefly raced across the fluffy cumulous tops before plunging into them at 10,000 feet. United 695 contacted Chicago Approach Control.

The moon, periodically visible through the broken cloud deck, transformed the sky into a silver-and-black Halloween-scape on the other side of midnight. Instructed to "descend and maintain 6,000," the 737 momentarily turned left to 200 degrees before almost immediately banking right to a heading of 220. Maintaining 4,000 feet to the ADAM VOR, United 695 intercepted the ILS to Runway 27L, subsequently contacting the Chicago tower and completing its undercarriage and flap extension sequencing.

The orange geometric pattern of ground lights appeared through the mist as the aircraft commenced its flap and undercarriage sequencing. Applying partial power and maintaining a 135-knot approach speed, it passed over the blue-bordered taxiway patterns of the field and flared on to the runway during the wee hours of the morning.

Pushed back from Gate C31 amid a glowing copper dusk the following day on the return sector, the United Airlines 737-300, operating as Flight 690, commenced a lengthy taxi roll to the pad just short of runways 22L and 27L, where air traffic control restrictions necessitated its engine shut down for 30 minutes, before being granted take off clearance on the former of the two runways.

Contacting Chicago Departure Control and rolling into a tight left bank over the ground lights, the aircraft was instructed to "climb and maintain" 8,000 feet, cleared to its initial VOR. The orange, rectangular light geometries yielded to the black, referenceless surface of Lake Michigan. Angling from 10,800 to 13,000 feet, the 737 contacted Chicago Center on 126.47. Huge, moonlit cumulous mountains moved under the right wing as the aircraft was cleared to 35,000 feet.

Contacting Cleveland Center on 133.07, the aircraft rode "light chop" and was instructed to reduce speed to Mach .74 "for spacing." Contact with New York Center, on 128.57, subsequently followed. A published hold at the MILTON VOR, with a 0345 Zulu release time, had been canceled before reaching it, although the 737 reduced speed to minimize its then anticipated hold time upon reaching it.

Now cleared to La Guardia via Milton 3, the aircraft, maintaining 280 knots, was instructed to cross the Mark intersection at flight level 190, and then "descend and maintain" 10,000 feet and reduce speed to 250 knots. Contacting New York Approach on 127.43, Flight 690 was given the altimeter of "29.64" and requested to maintain a heading of 120 degrees and "reduce speed to 180 knots."

Instructed "direct to Green and intercept the localizer," the 737 was cleared for an ILS approach to Runway 4. The full moon, now visible through the thin, sheath-like clouds, assumed an apparitional profile.

Handed off to the tower, the aircraft was advised of "wind 090 at 5." Breaking free of the cloud cover, now with fully extended leading and trailing edge wing devices, it emerged above the ground lights of Jackson Heights, knifing through strong, slanting rain and rolling on its longitudinal axis as it counteracted winds.

Cleared to land, it passed over the black, reflective Grand Central Parkway, flaring on to the wet runway surface with a bounce before its raised spoilers firmly implanted it groundward.

When Boeing had rolled out its last, intermittently designated "Classic" 737, a 737-400, in Renton, Washington, it had achieved an impressive production milestone of having built 1,113 737-300s, 486 longer-fuselage 737-400s, and 389 shorter-fuselage 737-500s. In 1991, it had reached a peak of 21 aircraft completed each month. When these production figures include the 30 "First Generation" 737-100s and 1,114 737-200s, the 3,132 aircraft collectively produced had qualified the short-range, low-capacity twinjet as the world's best-selling commercial airliner. Usurped by the "Next Generation" 737-600, -700, -800, and -900, the basic design created to complete its family and compete with the then current twinjets, which had all since been removed from production, had achieved every goal its engineers had inceptionally set, as evidenced by the more than 6,000 airframes sold and the global coverage of their routes, which result in a take off or landing every few seconds, somewhere in the world, 24 hours per day.

More about this author: Robert Waldvogel

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