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Boeing's new-generation 737is the same, only different

Guy Norris/SEATTLE

EXACTLY 30 YEARS ago, the first Boeing 737 was taking shape at the company's plant in Renton, Washington. At the time, not everyone was convinced that the "Baby Boeing" gamble would be a winner.

The concern was real. In 1966, Boeing was already behind in the marketing war with Douglas, which had its DC-9, and the British Aircraft Corporation, which offered its One-Eleven. Interest in the 737 was so slack during the initial years of production that Boeing even considered cancelling the programme.

In the long run, Boeing need not have worried. Successive improvements high reliability and its growing reputation as a money-maker made the 737 very popular. By 3 September, 1996, when the first -700 fuselage is due to arrive at Renton by train from Wichita, Kansas, sales of the type will be close to 3,500. Not only is this roughly comparable with total sales of all other Boeing standard-body aircraft put together (707/720/727 and 757), but it also equals the total commercial jet-airliner output so far of McDonnell Douglas (MDC) and exceeds the sells of Airbus Industrie by more than 1,000 aircraft.

Boeing expects sales of the new-generation 737 to reach or exceed 400 around the time of the Farnborough air show, breaking yet another record for a commercial aircraft which is yet to be built, let alone to be flown. Even Boeing has been caught off guard by the apparent rush of new-generation 737 sales. In early 1996, Boeing twice announced plans to increase 737 production and finally, at the end of June, it revealed plans to boost monthly production to 17 by 1998.

The build-up is due to begin in January 1997, about a month before the first flight of the -700. Monthly rates will move from ten to 12 by the second quarter. By the end of 1997, with both the -700 and -800 models in flight test, the rate will be 15 a month.

The jump to 17 begins in January 1998, coinciding with the first flight of the -600. For the first year, at least, the bulk of production will still be current-generation 737s, for which Boeing still holds an order backlog of more than 200, but this will quickly give way to the new-generation 737 by 1998. It is likely that the rate will be further pushed to around 23 a month by the turn of the century, exceeding even the halcyon days of the early 1990s, when 737 production peaked at 21 a month.

BIGGER WING

At first glance (especially on the ground), it will be hard to tell the difference between the current and new 737s. Even the -800, which is the only one of the new-generation 737s to be longer than its current equivalent, will differ visually only slightly from its -400 predecessor.

In flight, however, the larger (34.4m) span of the aircraft's extensively redesigned wing will be an instant giveaway. The slender wing, reminiscent of the 757 design, is the key to achieving three main improvements over the current family. With 25% more area and 30% more fuel capacity, it will enable each of the three versions to carry proportionately more payload, achieve a higher service ceiling and be flown further.

At a time when Boeing is on the verge of launching a re-winged 747, the experience gained on the 737 has become more significant. "This is because the new 737 is the first Boeing derivative with a new wing," says Jack Gucker, 737-600/700/800 programme director.

"The aircraft has a significantly changed wing, both in design and materials as well as manufacturing," says Doug Caton, former leader of the wing integrated-product team and now part of the 747-500/600X effort.

"We settled on one aerofoil shape, one planform and one chord, as well as sweep, dihedral and area, when we froze the high-speed line around January 1994," says Caton. That was the culmination of studies, which Boeing had been conducting since the early 1990s into its next-generation 100- to 200-seater.

Five major proposals were discussed, ranging from all-new designs to very simple derivatives. When the re-winged 737-X, as it became known, was selected (mainly for commonality reasons), the definition of the wing became one of the first priorities.

The new wing was designed for flight at speeds of up to Mach 0.82. An economical cruise speed of Mach 0.79 at a ceiling of 41,000ft (12,500m) is more likely in service, compared to around Mach 0.74 at 37,000ft for the current- generation 737. To help gain a few more knots, Boeing has designed a raked, low-drag wingtip similar to the much larger tip of the 777.

Wing area is increased to 125m2 (1,345ft2), primarily through a 5.4m stretch in span which is faired into the existing tip chord, and an entirely new wing-box. The longer-range mission required more fuel capacity, so the rear spar was moved aft to provide extra volume. The increased span also meant, newly designed extended spars and redesigned fuel and surge tanks. Each wing contains 4,900litres of fuel, taking total fuel volume (including a 16,230litre centre tank) to 26,036litres - giving roughly 1,660km (990nm) of additional range (see systems description).

Chord is increased by 430mm which, together with the new aft-camber cross-sectional profile, gives a lower thickness-to-chord ratio than in the 1960s-vintage design of today's 737.

Despite the bigger wing, the designers maintained maximum commonality with the current fuselage by keeping to the same side-of-body join location. A revised, low-drag aft wing-body composite fairing fits around the wing root.

NO LEAKS

Boeing took advantage of the wing-box redesign to address some nagging manufacturing and maintenance headaches with the traditional wing, such as leaking fuel tanks. New in-spar skins, stiffeners and ribs were designed and produced from improved aluminum alloy used for the first time on the 777. The new design also saved weight. "All the in-spar ribs are fully machined, as opposed to being built-up. We saved literally thousands of fasteners [and] parts and weight, and we also saved on the tooling," says Caton. The new tooling was designed using an improved version of the Dassault CATIA computer-aided design and manufacturing system. The tooling allows improved manufacturing tolerances and, in doing, cuts the risk of fuel leaks.

Some weight was also unexpectedly saved in the wing-design process as late as 1996, when more sophisticated flutter-analysis techniques became available. The analysis showed that wing-skin thickness could be reduced aft of the engine pylon and part of the mass transferred outboard to an area near the tip. "It came out of nowhere and we saved up to 80lb [36kg] on the -800 design, around 120lb on the -700, and 200lb on the -600," says Caton. Alcoa's new 7055 alloy (which is also used on the 777 wing) is used for its high compression-resistance in the upper wing surface, stiffeners and in-spar ribs. A 2000-series alloy, 2324 is used for the lower surface skin which is subjected to high tension loads.

REDESIGNED SLAT

The leading edge of the new wing is modified with a new Kruger flap and an additional slat outboard. The slat is based on a simpler design made up of fewer parts and is expected to stand up better to corrosion, particularly around the trailing-edge wedge of the slat on the upper surface. There are two leading-edge flaps inboard of each engine and four leading-edge slats outboard.

"We've retained the same basic leading edge, because it was the simplest solution," says Caton. "However, because it is the aerodynamically most critical part of the wing, we are now building it differently, to improve the finished product and reduce variability. In the past, this has almost been hand-made, but now we are building it straight on to the front spar. It used to be a separate subassembly that was loaded on later. We have eliminated a whole part of the assembly process and therefore reduced cycle time," he adds.

Simplicity is also the key to the trailing-edge redesign, in which the complex triple-slotted flap mechanism of the current wing is replaced with a more straightforward double-slotted design. "Again, we're talking about reduced parts and lower maintenance costs with this design," says Caton.

A hydraulic motor drives a flap power-drive gearbox to operate all the trailing-edge flaps via a torque-tube drive to ball-screw actuators. If the normal hydraulic systems fail, the flaps can be operated using electrical power. A load-relief system is built in to protect the trailing-edge flaps from excessive air-loads. This will move the flaps up by one position if airspeed exceeds a set limit when the flaps are at 30¡-40¡.

Flap tracks, enclosed in new fairings, are made from stainless steel instead of the corrosion-prone high-strength carbon steel traditionally used. Composite ailerons are increased in span by 510mm, along with a proportionately increased trim tab.

The new-generation 737 has a similar engine nacelle/pylon design to that of the 777, for which Boeing has overall responsibility. As a result, the engine is supported at the wing by an attachment known as the R1 fitting, rather than being attached to the front and rear spars as on the current 737. "This helped us eliminate a fairing on the upper surface," Caton comments.

The dorsal fin and vertical stabiliser have been lengthened, and the span of the horizontal stabiliser increased to cope with the additional power of the CFM International CFM56-7B turbofans. Overall fin height off the ground has grown to 12.5m, compared with 11.1m for the current aircraft. The area of the vertical stabiliser increases to around 26.4m2, some 5.5m2 greater than that of the -300. The dorsal panels and fin trailing-edge fixed panels are of a honeycomb-sandwich construction, fabricated using a glassfibre-reinforced-plastic (GFRP) epoxy prepreg cured at 121¡C. The tailcone panels are also made out of a honeycomb sandwich using GFRP epoxy prepregs cured at 176¡C.

Horizontal-stabiliser span, meanwhile, has grown from 12.7m to 14.3m, while the area has increased by 1.3m2 to 32.8m2. The first all-composite -700 rudder, which is also around 1m longer than the current unit, was delivered from UK-based Shorts in early August. Rudder, elevator, aileron and thrust reverser, are of honeycomb-sandwich construction, using carbonfibre reinforced-plastic fabric epoxy prepregs cured at 176¡C.

The use of composites on the new-generation 737 is similar to that on the current series, although, in some cases, aluminum has been re-introduced. Some components with a history of frequent in-service damage problems, such as the outer engine cowl and main landing-gear doors, are now made of aluminum.

SYSTEMS IMPROVED

Much of Boeing's focus for cutting down maintenance costs and improving reliability was on improving the basic aircraft systems. One of these is the electrical system, which has been changed "with a lot of tweaks", says aircraft-systems chief engineer Mike Redmond.

"The electrical system is generally a lot more powerful than the -300 system and is now based on a 757-style architecture," says Redmond. Unlike the current system, which takes its supply from two 50kVA variable-speed, constant-frequency, generators, the new series will be supplied by 90kVA integrated-drive generators (IDGs). Each IDG is driven by an engine and supplies 115V AC power.

A starter-generator is also available to start up the auxiliary power-unit (APU) and act as a 90 kVA generator up to 32,000ft. The 757-style system is being adopted to protect the new-generation 737 from "bus trips" which "-is the cause of a number of issues with the 737", says Redmond. If electrical power is lost to either of the two main transfer busses, a bus-protection control unit closes the bus tie-breakers to supply power from the opposite bus, and sheds non-essential loads. Each bus is protected by generator control units which guard against differential current, over/under voltage or frequency, over-current and unbalanced phased current.

As part of the drive for greater simplicity, much of the automated electrical-system control is installed in two power-distribution panels in the revamped electronic equipment (EE) bay. "On the -300 there are a lot of switching relays and circuit breakers. We've now put a lot of it down in the EE bay and out of areas where they are often more of a bother," says Redmond.

The fuel system also shows some 757 heritage, particularly the fuel-quantity indicating system (FQIS) which uses a microprocessor to analyse a capacitance signal from units in each tank. The signal contains data on fuel quality and temperature, which is used by the FQIS to calculate density. The processor then sends a fuel-weight signal via the ARINC 429 databus to the flight-deck displays and flight-management computer system.

The wing design revision also resulted in the fitting of additional fuel boost-pumps. There are two boost pumps each for main tank 1 and 2, and for the centre tank. The centre-tank pumps have higher output pressures than those in the main tanks, and the engines therefore receive centre-tank fuel first. The APU can take fuel from any tank.

The triple-redundant hydraulic system is virtually identical to that of the current-generation 737, but operates at the same 207bar (3,000lb/in2) normal pressure with larger pumps. "We went for bigger pumps to provide room for growth," says Redmond. Engine- and electric-motor-driven main systems are backed up by a third, standby, system. This electric-motor-driven, pump supplies power for the rudder control as well as secondary power for the thrust reversers and leading-edge devices.

The higher operating weights and capacity of the new family are also reflected in changes to the wheels and brakes. Boeing tests of worn brakes revealed that higher-capacity multi-disc steel brakes were needed for the higher-weight -800 aircraft. Standard-size main tyres are used on the -600/700 with an optional larger tyre, while the -800 is only available with the larger units. A digital anti-skid brake system, based on that of the 757, is also being introduced.

LANDING-GEAR LEG REDESIGNED

The main landing-gear leg is "-totally redesigned" says Caton. Apart from being slightly taller, the unit is manufactured with an integrated drag link and a one-piece outer cylinder. The drag brace and trunnion are also integrated, saving weight, complexity and parts.

Flight controls differ from those on earlier 737s mainly in the rudder and yaw-damper controls. This area has been the focus for investigations following reports of rudder anomalies, although Boeing stresses that yaw-damper changes are related to ride quality, rather than issues of flight safety. Changes have been made to the power-control unit (PCU) which moves the rudder. "Because we had to have a bigger rudder, we need a bigger PCU," says assistant chief project engineer Peter Rumsey. During the scaling-up, Boeing took the opportunity to "-clean up some of the bearings and update the pressure-relief valves so that, under multiple failure conditions, it didn't trap any pressure".

The yaw-damper system, which moves the rudder to decrease yaw rates caused by turbulence or dutch roll, is now fitted with an electronic gyro in place of a mechanical unit. "We knew the yaw damper could be improved, and most of the failures have been related to the mechanical gyro which wears out and fails. We also changed the electronics, which go around it, which give us better life and allows us to add built-in test to them. So if they fail, they tell you they're failing and why," says Rumsey.

The yaw-damper system connects to the main and standby rudder PCUs, but is operated independently of the rudder-control system, even though it uses the same actuator. It does not give feedback to the rudder pedals. It takes inertial-reference inputs from its own gyro to get yaw rate and lateral acceleration. Using much more accurate and immediate data than was available to the previous system, the yaw dampers send commands to the rudder PCUs to move the rudder and stop the dutch roll.

A wheel-to-rudder interconnect system (WRTIS) is available to assist manual turns when the standby hydraulic system is on. The WRTIS senses control-wheel movement and sends commands to the standby rudder PCU to move the rudder. A similar WRTIS is employed full-time on the 777 and the system may be developed as a primary mode for the 737.

Another system which is updated with more sophisticated technology is the aircraft's air conditioning. The system uses two independent AlliedSignal air-cycle cooling packs, a cabin-temperature control system, an air-distribution system and recirculation system to produce a fresh air rate "to near 10ft3/min [0.283m3/ min] per passenger", says Redmond. Pack air passes through an air-cycle machine (ACM), which is a refrigeration turbine running on an air bearing. Boeing says that no scheduled maintenance for the ACM is needed.

The output temperature of air moving from the pack, to the 737-600/700 cabin is controlled by a mixer valve. This mixes cooled and uncooled pack air to the right temperature. A "2¡C control system" prevents freezing temperatures downstream of the ACM, protecting the system from ice damage.

A slightly different method is used to control the temperature in the larger -800 cabin. A temperature-control valve regulates the proportion of air, which does not flow through the cooling components of the pack, and this produces the correct discharge temperature. The cabin zone needing the coolest air sets the pack output temperature. Hot trim air is then added to the other two zones.

FLIGHTDECK FLEXIBILITY

Boeing faced a dilemma when it came to the new flight deck design: how could it combine airline demands for maximum commonality, simplicity and the same type rating with the tempting maintenance and performance benefits of new flat-panel-display technology?

The answer was surprisingly simple: have both. A common display system (CDS) was developed, using six Honeywell multi-function liquid-crystal displays (LCDs) identical to those developed for the 777. The CDS can show the primary flight and navigation data in two optional formats, electronic flight-instrument system (EFIS), or primary flight display/navigation display (PFD/ND).

The EFIS format shows information, which replicates that on the 737-300, -400 and -500, thereby satisfying existing 737 users such as launch customer Southwest Airlines. The PFD/ND format looks like the 747-400 and 777-type displays and has therefore attracted non-737 users such as SAS.

Boeing also felt that the time was right to standardise future flight-decks around the new LCD technology, and the competition for a Saudi Arabian Airlines contract, which specified a PFD format, gave it the chance to explore the glass-cockpit option for the 737.

"We were told the flat panels wouldn't fit because of the wishbone frame, but CATIA said they would. So we just had to try it," says Capt Mike Hewett, a main driver behind the 737 CDS initiative and chief pilot for the -600/ 700/800 project. "It fits, but only if the panel is pushed back 15¡ from the vertical," he adds.

Advanced avionics introduced into the aircraft for the first time include an air-data inertial-reference system (ADIRS). The air-data function is active when electrical power is on, while the inertial-reference function, which uses laser gyros and accelerometers to measure aircraft movement, is active when the pilots select it "on". The global-positioning system (GPS) is available as an optional part of the aircraft's multi-sensor navigation suite. The ADIRS provides inertial-reference and air-data values to the GPS sensor unit, which then uses this information to locate the best satellites during system initialisation.

ENGINE ECONOMY

The CFM56-7B power plant is fundamental to the operating economics of the new-generation 737. The engine is designed to operate with 15% lower maintenance costs than those of the present series, and have up to an 8% lower fuel consumption. It is also designed to be changed, in a shorter time and offers improved maintenance access to systems.

The engine matches the core and low-pressure turbine of the -5B (used on the Airbus standard-body series) with a new single-crystal-blade high-pressure-turbine design, a smoother flow path and a FADEC II (full-authority digital engine-control). One of the biggest innovations is a wide-chord fan.

Development has had its problems. FADEC software governing stall-detection-and-recovery logic needed to be rewritten. Some redesign and stiffening of the fan containment collar was required, to cope with the higher energy of the fan. Rotor weight is up by about 35% compared with that of the CFM56-3 which powers current-generation 737s. The design of the fan-blade retainer also needed strengthening after several more blades than expected detached during a blade-off test. The engine also gained weight when the exhaust duct needed to be stiffened after cracking was found.

Results have been better than expected in most cases, however. Cruise specific fuel-consumption has been found to be 0.6% better than expected, and exhaust-gas temperature margin has been a healthy 20¡ wider than predicted. Near its highest rating of 115kN (26,000lb) thrust, the engine indicated a 4% thrust margin, most of it attributed to the fan.

A total of seven AlliedSignal 131-9(B) APUs in the test programme have completed more than 4,150h and 12,200 starts to date. The first production APU will be completed in late August and tested for 50 starts before being shipped to Boeing on 23 September. The APU incorporates a novel starter-generator, which effectively plays a dual role as the standard starter-motor, and therefore saves weight. AlliedSignal director of Boeing programmes, Scott Brandenburg, says that the DC-based system is now working effectively.

With the -700 due to be rolled out in December, and orders rolling in, Boeing's biggest challenge appears to be how to meet the demand. Its new 737 series will not only be replacing older 737s, but 727s too, as well as rival products. The model seems destined to capture a large chunk of the 6,700 units Boeing has estimated will be sold in this category between now and 2015.

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