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Aviation History
1996
1996 - 2212.PDF
737 DESCRIPTION increased by 1.3 m2 to 3 2. 8m2. The first all-com posite -700 rudder, which is also around lm longer than the current unit, was delivered from UK-based Shorts in early August. Rudder, ele vator, aileron and thrust reverser are of honey comb-sandwich construction, using carbon- fibre 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, aluminium 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 aluminium. SYSTEMS IMPROVED Much of Boeing's focus for cutting down main tenance 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-sys tems 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 sup ply 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 115VAC power. A starter-generator is also available to startup 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-gen eration 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 gener ator control units which guard against differen tial current, over/under voltage or frequency, overcurrent and unbalanced phased current. As part of the drive for greater simplicity, much of the automated electrical-system con trol is installed in two power-distribution pan els 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 her itage, 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 ARTNC 429 databus to the flightdeck displays and flight-manage ment computer system. The wing design revision also resulted in the fitting of additional fuel boost-pumps. There The first 737-400 wing is moved to the next manufacturing position at Renton 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 vir tually identical to that of the current-generation 737, but operates at the same 207bar (3,0001b/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 die 757, is also being introduced. LANDING-GEAR LEG REDESIGNED The main landing-gear leg is "...totally redesigned" says Caton. Apart from being slighdy taller, the unit is manufactured with an integrated drag link and a one-piece outer cylin der. The drag brace and trunnion are also inte grated, saving weight, complexity and parts. Flight controls differ from those on earlier The nose for Southwest's first 737-700 is completed at Boeing's Wichita plant 737s mainly in the rudder and yaw-damper con trols. This area has been the focus for investiga tions following reports of rudder anomalies, I 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 turbu lence or dutch roll, is now fitted with an elec tronic 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 I main and standby rudder PCUs, but is operated independently of the rudder-control system, even though it uses the same actuator. It does j not give feedback to the rudder pedals. It takes I 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 j 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 con- I ditioning. 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 1 OftVmin [0.283mV min] per passenger", says Redmond. Pack air passes through an air-cycle machine (ACM), which is a refrigeration turbine running on an air bear ing. Boeing says that no scheduled maintenance for the ACM is needed. The output temperature of air moving from the pack to the 73 7-600/700 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 tem peratures downstream of the ACM, protecting the system from ice damage. A slighdy different method is used to control the temperature'in the larger -800 cabin. Atem- perature-control valve regulates the proportion 64 FLIGHT INTERNATIONAL 28 August - 3 September 1996
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