Civil air transport: a fresh look at power-by-wire and fly-by-light

Power-by-wire (PBW) is a key element under subsonic transport flight systems technology, with potential savings of over 10% in gross take off weight and in fuel consumption compared to today's transport aircraft. The PBW technology substitutes electrical actuation in place of centralized hydraulic, uses internal starter-motor/generators, and eliminates the need for variable engine bleed air to supply cabin comfort. The application of advanced fiber optics to the electrical power system controls, to built-in-test (BITE) equipment, and to fly-by-light (FBL) flight controls provides additional benefits in lightning and high-energy radio frequency (HERF) immunity over existing mechanical or even fly-by-wire controls. The program plan is reviewed. The key technologies and their benefits to future aircraft, civil and military, are discussed.<<ETX>>

The application of advanced fiber optics to the electrical power system controls, to bulIt-in-test (BITE) equipment, and to fly-by-llght (FBL) flight controls provides addltlonal benefits In llghtnlng and hlgh energy radio frequency (HERr) immunity over exist-Ing mechanical or even fly-by-wire controls.
Thls paper wlll review the program plan and glve a snapshot of the key technologies and their benefits to a11 future alrcraft -civil and military.

Introductlon
NASA In response to a directive from the U.S. Senate has developed a multlyear technology development and validation plan that wlll help the United States retaln Its leadershlp In aeronautics research and technology and compete In the international marketplace for future clvil aircraft.
One of the key elements under subsonic transport f11ght systems technology Is powerby-wlre (PBW) wlth potential savings of over I0 percent In gross take-off-weight (GTOW) and fuel consumption compared to today's alrcraft (Ref. I). Fly-by-llght (FBL), whlch Is the replacement of electronic data transmission, mechanical control linkages, and electronic sensors wlth optical components and subsystems, Is another key element.
In the NASA Lewls Research Center study reported in Ref. I the stated benefits are shown to be possible through the integration of an advanced secondary electrical power system Into a civil transport aircraft uslng a Boeing 767 as a baseline.
While the particular benefits may depend on aircraft slze and type, engines, and specific electrical and flight control systems, the range of improvements Is consistent with available advanced technologies.
The primary weight reduction occurred In the secondary power system when the baseline hydraulic, pneumatic and electrlcal subsystems were replaced wlth a slngle, advanced high frequency, slnusoldal power management and distribution (PMAD) system wlth controlled energy flow and load management.
An advanced fly-bywlre f11ght control system using electrlcal actuators and advanced low fixed bleed hlgh bypass ratio engines contribute the remaining major advantages according to the NASA Lewis study.
The single electrlcal power system provides hlgher component ut111zatlon wlth slgnlflcant reductions in parts count, weight, failure modes, and cost of ownership.
The PBW technology el_mlnates the need for supply hydrau 11c actuatlon and for englne bleed air to cabin comfort and anti-Iclng.
It enables Integral starter/ generators for engine startlng and power generatlon to be used wlth advanced energy-efflclent engines without gearboxes.
These changes significantly Improve rue! efflclency and reduce alrcraft weight.
Improved safety and dispatch rellab111ty comblned wlth lower malntenance and direct operating costs are additional tanglble benefits.
According to the study, e11mlnatlng the englne bleed for powerlng the envlronmental control system (ECS) and for anti-lclng provided the largest single fuel savlngs of any proposed change.
An electrlc motor driven ECS wlth electric Impulse driven de-icers are the proposed replacements.
Subsequent to the NASA Lewis study, addltlonal technologies have emerged.
The app11catlon of advanced flber optics to the electrlcal power system controls, to bullt-ln-test (BITE) equipment, and to fly-by-llght (FBL) flight controls provides additional benefits In 11ghtnlng and hlgh energy radio frequency (HERr) Immunlty over existing mechanical or even fly-by-wlre controls. This paper will review the program plan and take a fresh look at some of the key technologles and their beneflts to all future aircraft -civilian and military. Figure I shows the payoffs and the major tasks planned for the PBW element. Figure 2 summarizes the prlmary elements of the FBL/PBW 5-yr program plan under the ClvlI Transport Inltlatlve.

The Electrical Powerp]ant
The heart of the electrical power system is a multlredundant, fau]t tolerant, microprocessor controlled, power management and dlstrlbutlon (PMAD) system.
It incorporates bidirectional Inverters driven through a hlgh frequency, resonant utlIlty bus connected to internal starter-n_)tor/generators, thereby ellmlnat-Ing gearboxes and the need for separate APU's and engine starters.
The hlgh frequency utlllty bus permits all the advantages of ac for stability, fault clearing, dlfferentlal monltorlng and control, as well as slgnlficant crew/technlclan safety because of Its low energy per pulse nature and ease of ground fault Interruption. Figure 3 shows a typical utlIlty bus architecture for an ac PMAD system using advanced electronic switching for power conditioning, control, dlstrlbutlon, protection and a fault tolerant architecture (Ref. I).
Thecharacteristics oFtheadvanced PMAD system provlde the conduit to the benefits For PBW In civil transport and perhaps certain military aircraft.
The resonant, high Frequency (>]0 kHz) ]Ink drive enables either muItiphase low frequency ac sources or dc sources to operate at their optimum voltage and Frequency at the Input.
Since the main Inverters switch at the zero crossing of either current or voltage, they mlnlmlze power losses, component stress, EMI/EMC and the need For heavy, bulky filters.
The hlgh Frequency bldirectlona] conversion and synthesis slgnIFIcantly reduces the slze and mass of the electronics components, controls and systematlcs.
The key beneflt, however, comes in De ablllty to drive all kinds of motors (Including rugged Induction motors wlth high temperature capability) In either direction with Independent control of torque, speed and maximized, efficient operation over the entire speed range.
In effect, all load control including variable speed motor drive is accomplished by sorting and steer-Ing the hlgh frequency slnusoldal pulses to the appropriate power switch, motor winding or energy storage element. Voltage regulation, power quallty and energy flow are determined and managed at all tlmes and within specified limits.
Multiple levels of redundancy are easily accommodated In the system, prov_dlng fault containment, fault recovery, and maximized end-to-end efficlency.

Electrical Actuatlon
Existing electrical actuation technology in the 5-to 50-hp range will be adapted to the flight control and other actuation requirements on an aircraft.
Prototype electrical actuators operating from the distributed power bus will be built and demonstrated In a full avlonIcs control environment.
The electric actuators would replace hydraulic actuators, servovalves, and mechanical control linkages. An advanced electrical actuation system Includes electromechanlcal and/or electrohydraullc actuators, load receivers, redundant digital data buses, and remote terminals.
Electrical actuators perform the same functions as hydraulic actuators with lower welght, hlgher efflciency and without slzlng restrictions.
The dlgltal data bus and electro-optlcal sensors and controls promise to be a much lighter welght, more fellable and EMI Immune approach than fly-by-wlre or mechanical controls.
Also, many functions prevlously done wlth hardware can now be done with software. Thls may Include such functlons as control surface damping and trlm.

Status and Health
MIcrochlp level BITE w11l be built Into the hardware to provide a "F1ngerprlnt", which may include component characterlstIcs, test InFormatlon and valldatlon parameters.
Such smart BITE chips could provide health self-testlng for pre-FIIght checkout, For In-flight status and For malntenance assistance and records.
The keys to autonomous, growable power and control systems are simple, smart, repllcatlve logic structures.
Pushing the Inte111gence down to the power switch and circuit level enables easy verIflcatlon, validatlon, status, and malntalnablllty. It provides step-by-step transltlons from manual to autonomous controls.
Integrated health monltorlng, Inciplent Fault prediction and a controlled evolution of power and avlonlc systems are readily accommodated wlth each node communicating wlth other nodes vla simple, common words.
Thls enables dlstributed Intelllgence For Fault containment, Fault tolerance, and autonomous control without massive software Investments.

Electro-optlcal Controls
The Fly-by-llght (FBL) controls are proposed as a replacement of electrical data transmission, mechanical control linkages and electronic sensors wlth optical components and subsystems_ They circumvent electromagnetic Interference (EMI) concerns In applying dlgltal controls by providing lifetime immunity to signal EMI without need for shielding.
The FBL technology will demonstrate optical sensors and Interfaces wlth Improved lightning and HERF Immunlty.
The FBL program wlll identify, develop, and evaluate an optical sensor suite.
The sensors will be Integrated Into Innovatlve electro-optlcal based Faulttolerant archltectures using optical networks and multlplexer/demultlplexer techniques. Performance and reliability assessments of the fault tolerant processors and architectures will provide a basis For developlng hardware and software For fllght test and Inservlce evaluation.
Several ongoing actlvlties wi11 feed technology Into the FBL program: the Fiber optical control system integration (FOCSI) program, the optical propulsion management Interface system being designed Into the advanced transport operatlng system aircraft, and a f|ber optical transmltter/recelver with adc 4-GHz bandwidth.
An extensive data base and experimental Investlgation of 11ghtnlng effects on digital electronics will serve as a baseline For assessing FBL enhancements to the fllght control system.

Summar Z
The goal of the FBL/PBN program is to accomplish credible flight tests and demonstration of Full author-Ity, all dlgltal FBL/PBN transport alrcraft systems. Performance will be evaluated In the stress of flight environments.
The program will use the NASA advanced transport operating system aircraft.
Using parallel operation of experimental equipment on the basic alrcraft will maximize Flight test safety.
The f11ght tests will be designed to verify and evaluate the Integrated system. FAA participation and coordination wlll be Integral to developing a prototype certification model.