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Aviation History
1994
1994 - 0931.PDF
BOEING 777 TECHNICAL UPDATE Fly-by-wire first T he 777 is Boeing's first fly-by- wire (FBW) airliner, and was designed to incorporate this sys tem after the benefits of FBW technology were thoroughly evaluated for potential use on the 7J7 programme. One of the first design advantages of FBW comes from using flight-control computers to provide stabilising functions which allow the use of lighter wing and tail structures. The FBW system also replaces complex, mechanical, cables, pul leys, brackets and linkages, making the control system inherently easier to build. The company also predicted that an FBW system would be more reliable, easier to maintain and improve handling characteristics, and also protect against inadvertent manoeuvres. The Boeing sys tem, while ensuring that ultimate control authority remains with the pilot, has functions which "assist" the pilot in "avoiding or recovering from exceedances of operational boundaries". At the system's core are three identical GEC-Marconi Avionics primary flight computers (PFCs), each used to calculate control commands for primary surface actuators, the trim system and the con trol-column-feel system by Rockwell- Collins Air Transport. In manual mode, the PFCs receive flightcrew demands, via Control laws • Pitch The flight-control laws of the 777 differ mainly from other Boeing types in pitch. The aircraft is trimmed to a speed and any alteration from this speed causes a pitch change. If the speed drops, for example, the law will cause the nose to drop to regain speed. In turn, this means that changes in speed, rather than configuration and power, will cause trim alterations. This law is called the C* U law. C* represents a manoeuvre demand (which in turn demands g load), while U is positive stability. • Roll Beyond 35° of bank, the 777 system introduces a positive spiral sta bility, which makes the aircraft want to return to 35° of bank through aileron deflection. This is made obvious to the crew through stick movement, but can be overcome with force if needed. • Yaw Although it will be very like those of the 757 and 767, the new system has a thrust-asymmetry compensation sys tem, which automatically applies rud der in the event of an engine failure. • Operating limits Artificial changes in stability or control forces to warn the pilot as operating limits approach. Elevator J f\ (single span) \_ y? Spoilers L\ Flaperon ^J^^JJ^ (7 per side) "^T ^W^ ^^~~~~^~~^ <^L_I /-^^^ „<• 0^><Z~Cr ^^-^ Outboard £ ,.<''"''^— —"^^^> ^^5^ aileron © Reed Business Publishing Surfaces controlled by PFCS // Single i rudder Stabiliser the ARINC 629 databus, through the conventional control-column yoke and rudder pedals. In automatic mode, the commands are received from the triplex autopilot and flight director (AFDS). This system, again built by Collins, consists of three re dundant AFDS computers, one mode-control panel and six backdrive control actuators. The physical feed back to the control column, yoke and pedals is via an in tegrated dual-pro cessor backdrive. Surface, trim and feel actuators are driven by four actuator-control electronics (ACE) units produced by California-based Lear Astronics and Teijin Seiki America, the Washington-based subsidiary of the Japa nese parent company. Teijin Seiki pro duces the powered control units (PCUs). Each ACE receives command data via a specific ARINC 629 bus but, if the bus or PFC has failed and no data comes through, it switches automatically to one of the other two buses. Flightcrew de mands are routed through the ACE on to the ARINC 629 bus so, in the unlikely event of a total PFCS failure, a reversion ary analogue command path is available to control the aircraft through the ACE. Despite safety requirements imposed to ensure against an overall FBW system failure, a back-up mode is available, consisting of an independent me chanical link con trolling the stabil iser trim system and two flight spoilers. The aircraft can therefore be con trolled by mechan ical reversion or three FBW system operational modes — normal, secon dary and direct: • normal mode is when all PFCS functions are avail able. It provides envelope protec tion from stall, overspeed, exces sive bank angle and full-time pitch-stability augmentation; • secondary mode is reversionary, for when data from the primary and back-up inertial sensors are unavailable. It gives a direct digital control path between flightcrew inceptors and control surfaces; • direct mode by-passes the PFCS and the ACE-analogue link is engaged. GEC Avionics 777 programme manager Malcolm Earl says: "Black Label hardware will be delivered in October. We've got software in the [test] rigs and we're continuing final integration and testing. We also have software on the aeroplane." Although Earl says that the programme has endured "all the normal software problems", GEC has "...so far achieved the major milestones". One such was the validation of the Boeing-developed flight- control laws, completed in late 1992 with a modified 757 flying testbed. "The biggest challenge is to get the autopilot engaged with the PFC. The Collins autoland algorithms are well de fined and, when we produced boxes for the 757, we locked into the autopilot back in those," says Earl. B Flap/slat electronic units Engine ndication crew- alerting system * t Aeroplane information * management system Control column & pedal-position transducers Trim switches 1 * —» Autopilot flight- director computers _, ,_ -f-*- Actuator- control slectronics Trim actuators Power- supply assemblies Air-data inertial- reference unit ^ , Secondary attitude air-data reference unit ^± TriDlex ARINC 829 i Primary flight computers data busses Pitch-feel actuators Power- control units (31) Electronic flight control system 1/GW INTERNATIONAL 6 - 12 April, 1994 29
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