Fiber optics are immune to electromagnetic emissions and have the potential to eliminate this concern especially in flight critical applications if they can be developed to the same level of technology as current systems using wire to carry the signals. As aircraft become more and more dependent of digital signals to control all systems, the Electromagnetic Environment (EME) will become more and more a concern for the safe long term operation. The International Severe HIRF electromagnetic environment (EME) is less than 2000 Volts per meter below 400 MHz and reaches a maximum of 6,850 Volts per meter in the 4-6 GHz range. The normal assumption is that a metal or composite aircraft skin with appropriate seals provides 20 dB attenuation of the external environment. This reduces peak levels at the avionics boxes to less than 200 Volts per meter below 400 MHz and a maximum of 685 Volts per meter in the 406 GHz range. MIL-STD-461D imposed an additional box level requirement to 200 Volts per meter from 10 KHz to 40 GHz. This requirement equals or surpasses the attenuated HIRF environment over significant portions of the spectrum and implies that the aircraft must be designed to achieve and maintain this value throughout its service life. Although wires can be shielded and designed to achieve these requirements, it is a more expensive process, adds the weight of shielding and requires maintenance of the shielding integrity at all times. The very light weight and high bandwidth of fiber optics also offer the potential of eliminating the number of connections and weight savings in aircraft. For example on a one to one replacement of wire by fiber, it is estimated that fiber would weight about 1/20 the weight of wire. Current wire buses used for duplex communications in aircraft applications have a bandwidth of about 1 MHz while equivalent buses using fiber optics have a bandwidth of 20 MHz. For other applications such as video and avionics interfaces, fiber buses in the hundreds of MHz are available. Applications of fiber optic buses would then result in the reduction of wires and connections because of reduction in the number of buses needed for information transfer due to the fact that a large number of different signals can be sent across one fiber by multiplexing each signal. The Advanced Research Projects Agency (ARPA) Technology Reinvestment Project (TRP) Fly-by-Light Advanced Systems Hardware (FLASH) program addresses the development of Fly-by-Light Technology in order to apply the benefits of fiber optics to military and commercial aircraft.

The worldwide proliferation of high intensity emitting sources and the more electric aircraft increase the intensity of the Electromagnetic Environment (EME) in which aircraft must operate. A FLASH program HIRF (High Intensity Radiated Field) EME requirement is derived to cover both commercial and military fixed and rotary wing aircraft. This requirement is derived from the radiated susceptibility requirement documents of both the FAA and U.S. military. Specific test data and analysis will show that we can meet this requirement.

In the face of shrinking defense budgets, survival of the United States rotorcraft industry is becoming increasingly dependent on increased sales in a highly competitive civil helicopter market. As a result, only the most competitive rotorcraft manufacturers are likely to survive. A key ingredient in improving our competitive position is the ability to produce more versatile, high performance, high quality, and low cost of ownership helicopters. Fiber optic technology offers a path of achieving these objectives. Also, adopting common components and architectures for different helicopter models (while maintaining each models' uniqueness) will further decrease design and production costs. Funds saved (or generated) by exploiting this commonality can be applied to R&D used to further improve the product. In this paper, we define a fiber optics based avionics architecture which provides the pilot a fly-by-light / digital flight control system which can be implemented in both civilian and military helicopters. We then discuss the advantages of such an architecture.

The Fly-By-Light Advanced Systems Hardware (FLASH) program is developing Fly-By-Light (FBL) and Power-By-Wire (PBW) technologies for military and commercial aircraft. FLASH consists of three tasks. Task 1 is developing the fiber optic cable, connectors, testers and installation and maintenance procedures. Task 3 is developing advanced actuators. Task 2, which is the subject of this paper, focuses on integration of fiber optic sensors and data buses with cable plant components from Task 1 and PBW, smart and thin wing actuators from Task 3 into complete centralized and distributed flight control systems. Laboratory demonstrations will show applicability to widebody transport and tactical aircraft. Products include high throughput processors, neutral network applications to maintenance diagnostics and reconfiguration, hardware upset recovery, active sidestick controllers and application of the SAE AS-1773A protocol to the flight control system bus. This paper overviews the requirements, objectives, key technologies and demonstrations for each of the FLASH flight control system developments. Companion papers provide additional description of the Tasks 1 and 3 as well as further details on the individual developments within Task 2.

A program was created with joint industry and government funding to apply fiber optic technologies to aircraft. The technology offers many potential benefits. Among them are increased electromagnetic interference immunity and the possibility of reduced weight, increased reliability, and enlarged capability by redesigning architectures to use the large bandwidth of fiber optics. Those benefits will only be realized if fiber optics meets the unique requirements of aircraft networks. Over the past two decades, considerable effort has been expended on applying photonic technologies to aircraft. Great successes have occurred in optoelectronic components development. In the development of these systems to link those components, known as the cable plant, progress has also been made, but only recently has it been organized in a coordinated, systems-oriented fashion. The FLASH program will expand on the nascent cable plant systems efforts by building upon recent work in individual components, and integrating that work into a cohesive aircraft cable plant. Therefore, the FLASH program will develop the low cost, reliable cables, connectors, splices, backplanes, manufacturing and installation methods, test methods, support equipment, and training systems needed to form a true optical cable plant for transport aircraft, tactical aircraft, and helicopters.

The high cost associated with the development and acquisition of new, highly complex and integrated digital control and avionics systems is leading the military and commercial aircraft industry toward increased standardization, modularization, and the use of flexible architectures and hardware which can be applied to multiple airframes. This paper describes the ARPA/Industry Fly-By-Light Advanced Systems Hardware (FLASH) program as it relates to multi-use transport aircraft fly-by- light development and discusses how this technology and hardware will be translated into commercial and military production applications.

A program was created with joint industry and government funding to apply fiber optic technologies to tactical aircraft. The technology offers many potential benefits, including increased electromagnetic interference immunity and the possibility of reduced weight, increased reliability, and enlarged capability from redesigning architectures to use the large bandwidth of fiber optics. Those benefits will only be realized if fiber optics meets the unique requirements of aircraft networks. The application of fiber optics to tactical aircraft presents challenges to physical components which can only be met by a methodical attention to what is required, what are the conditions of use, and how will the components be produced in the broad context of a fiber optics using economy. For this purpose, the FLASH program has outlined a plan, and developed a team to evaluate requirements, delineate environmental and use conditions, and design practical, low cost components for tactical aircraft fiber optic cable plants including cables, connectors, splices, backplanes, manufacturing and installation methods, and test and maintenance methods.

A multifiber mechanical splice concept has been developed for simultaneous mechanical joining of optical fibers in cable. This splice uses a novel 'multifiber positioner' to secure and precisely position the mating pairs of individual optical fibers to achieve excellent light throughput characteristics. The multifiber positioner is a micromachined structure that includes multiple V-grooves in silicon created via anisotropic etching. This concept has been used to create a 4-channel optical splice suitable for aircraft application. The multifiber positioner is mounted on a flexible polymeric 'elevator' and housed within a protective metal shell. The splice and shell design have been developed to facilitate user-friendly operation. The shell assembly includes a novel method for cable attachment and provides an environmental seal. The splice design concept should allow practical field use in adverse conditions.

Multifiber multimode connector concepts have been developed for use within MIL-C-38999 connector shells. These connectors use pairs of novel 'multifiber positioners' to secure and precisely position multiple mating pairs of individual optical fibers to achieve excellent light throughput characteristics. The multifiber positioner is a micromachined structure that includes multiple V-grooves in silicon created via anisotropic etching. In one embodiment, the multifiber positioner was designed to handle 4 fiber channels of 100/140/170 micron polyimide coated fiber. These concepts have been used to create preliminary designs for connectors suitable for aircraft application including a 4- channel connector housed in a 38999 shell size #11 and a 16-channel connector in a shell size #19. The multifiber positioner is mounted within the connector shell using a 'floating-contact' concept to provide vibration isolation and physical contact of fiber end surfaces. High temperature composite materials have been selected for certain key elements within the connector including resilient elements within an alignment sleeve to innovate a new approach to alignment. The fiber terminus and rear shell designs have been developed to facilitate user- friendly operation. The rear shell includes new methods for cable attachment and seals for both environmental and EMI/RFI protection.

The Avionics Multifiber Array Connector, AVMAC, is an optical connecting device that utilizes circular connector technology with MACII technology to align 18 channel fiber optic ribbon cable. There presently is no inexpensive avionic or military grade high-density fiber optic connector. The AVMAC connector will use a ruggedized MACII v-groove alignment array enclosed in a circular protective housing. This contact assembly will be rear insertable into and removable for a MIL-C-83723 connector. A MIL-C-83723 connector incorporating a special insert will be the initial testbed. However, with small modifications to the basic design, a multitude of different circular and rack-in-panel connectors might be used. The AVMAC connector offers a gang terminated 18 channel fiber optic interconnection all within a very small footprint of approximately 0.5 inch in diameter, and is designed to be installable through 0.5 inch conduit. The design hurdles are: (1) developing a new polishing technique to prevent intimate contact of the fiber faces to prevent cracking during extreme vibration and shock, (2) selection of an adhesive and cure schedule development to handle the extreme temperature requirements, and (3) in general, material and process development in order to ruggedize the design to survive the harsh environment and performance requirements of an avionic and possibly military application. A preliminary design of the AVMAC connector has been completed and a prototype model built. This model is currently undergoing vibration and optical performance testing. The AVMAC connector is scheduled to be finalized and fully qualifiable by end of 1995 as part of the FLASH (Fly-by-Light Advanced System Hardware) contract.

Organic polymeric materials offer great promise for the creation of optical guided-wave structures. We have developed a number of new polymeric compositions which can be used to fabricate optical waveguide circuitry characterized by low loss and high thermal stability (up to 75 years at 120 degree(s)C for 840 nm wavelength). This technology makes possible the fabrication of complex point-to-point optical interconnections with controlled numerical aperture and geometry, allowing creation of right angle bends, splitters, combiners, etc. on a wide variety of substrates including circuit board laminate, silicon, silica, polymeric films and glass. To complement this technology, we have developed a unique capability for interconnecting glass optical fibers to multimode planar waveguide structures with low loss (0.5 dB per connection or less). As part of the FLASH program, we are utilizing this technology to construct a backplane which will incorporate multimode guides with both in-plane and out-of-plane bends, and which will be integrated with a connector to allow interfacing with glass fiber. We are also researching the implementation of single-mode guides employing our materials for possible future application.

We will describe the nature of the activity which surrounds the development of the 'Fly-by-Light' Cable Plant in the FLASH Program in conjunction with McDonnell Douglas Aerospace. We will describe the characteristics of the Technology and the activities which we are engaged in to provide a robust, environmentally stable system. The environmental constraints which have been modelled and analyzed to ensure the applicability of this technology will be described. We have dealt with worst case conditions, and fed back this information to the design for aircraft systems. The Cable Plant involves the optic cables, PATSI-developed connectors, FORSS integrated connectors and avionics applicable optical backplanes. This program is oriented to satisfy the needs of fighter aircraft, with an extendibility to helicopters. The products developed should see use in large volume markets, such as, devices used as shared storage for clusters of computers, in local area networks, High Performance, highly parallel computers and multimedia systems. It represents a new technique of wiring buildings for all kinds of optical transmission whether it be parallel, or multichannel serial based. It represents a new technique to deal with embedded sensors.

Requirements for future advanced tactical aircraft identify the need for flight control system architectures that provide a higher degree of performance with regard to electromagnetic interference immunity, communication bus data rate, propulsion/utility subsystem integration, and affordability. Evolution of highly centralized, digital, fly-by-wire flight/propulsion/utility control system is achieved as modular functions are implemented and integrated by serial, digital, fiber optics communication links. These adaptable architectures allow the user to configure the fly-by-light system to meet unique safety requirements, system performance, and design to cost targets.

McDonnell Douglas Aerospace (MDA) is developing a Neural Network based Intelligent Flight Control System that utilizes fly-by-light data transmission combined with high capacity flight processors to implement control advancements and fault monitoring processes. In this control system design, high speed data transfer and electromagnetic interference protection obtained through fiber optic technology is linked with Neural Network based flight control hardware processors that are programmed with damage adaptive control capability, thus providing maximum survivability for fighter aircraft. This system also provides enhanced component fault diagnostics that can identify subsystem failures during flight, thus providing reduced life cycle cost through efficient maintenance action and less downtime of the aircraft. The Intelligent Flight Control products apply to fighter and transport aircraft. This program is Task 2C of the ARPA Fly-by-Light Advanced Systems Hardware (FLASH) Technology Reinvestment Program. The principal partner with MDA for the Task 2C Intelligent Flight Control development is Martin Marietta Control Systems. The program will mature the system hardware and software for laboratory demonstrations of component fault diagnostics and highly adaptive flight control performance.

Driving the need for optics in flight control are the issues of wire weight, HIRF, and system bandwidth. With the advent of advanced Vehicle Management System (VMS) systems and the higher level of functional and physical integration of subsystems such as the More Electric Aircraft (MEA) and the Subsystem Utility Integration Technology (SUIT) program, optics in flight control offer attractive system benefits. Advanced VMS control functions generate large quantities of data to be transmitted over the VMS buses for processing. This paper will address the implementation of a module that will support the high throughput requirements of an advanced VMS system. The module is defined as the Optical Bus Interface Module (OBIM) and utilizes the AS1773A 20 mbps fiber optic bus for the VMS system bus. In addition, the module supports 5 mbps fiber optic Cross Channel Data Links (CCDLs) for the exchange of flight critical data in redundant architectures.

The Fly-by-Light Advanced Systems Hardware (FLASH) program is developing Fly-by-Light technologies for flight control and vehicle management of military and commercial aircraft. Fiber optics offers greatly improved data transfer rate, very low susceptibility to electromagnetic interference (EMI), lower on-board weight, and lower operating and maintenance costs. FLASH will result in demonstrations needed to gain confidence in using Fly-by-Light for flight control and vehicle management. This paper reviews the requirements and the trade study leading to selection of the dual-rate 1773A fiber optic data bus to interconnect fly-by-light advanced system hardware.

The increasing performance and reduction of life cycle cost requirements placed on commercial and military transport aircraft are resulting in more complex, highly integrated aircraft control and management systems. The use of fiber optic data transmission media can make significant contributions in achieving these performance and cost goals. The Honeywell portion of Task 2A on the Fly-by-Light Advanced System Hardware (FLASH) program is evaluating a Primary Flight Control System (PFCS) using pilot and copilot inputs from Active Hand Controllers (AHC) which are optically linked to the primary flight Control Computers (PFCC). Customer involvement is an important element of the Task 2A activity. Establishing customer requirements and perspectives on productization of systems developed under FLASH are key to future product success. The Honeywell elements of the PFCS demonstrator provide a command path that is optically interfaced from crew inputs to commands of distributed, smart actuation subsystems commands. Optical communication architectures are implemented using several protocols including the new AS-1773A 20 Mbps data bus standard. The interconnecting fiber optic cable plant is provided by our Task 1A teammate McDonnell Douglas Aerospace (West). Fiber optic cable plant fabrication uses processed, tools and materials reflecting necessary advances in manufacturing required to make fly-by-light avionics systems marketable.

Because of the advantages of high bandwidth and EMI immunity, optical communication systems and sensors continue to be the focus of development in the aerospace industry. Existing electrical sensor systems are being modified to support the optical communications and optical fiber sensors are being developed to take advantage of the proposed optical fiber data buses. This paper discusses the development of a time-rate-of-decay optical fiber total air temperature sensor and its integration with an existing air-data sensor system. The combined temperature/air data system interfaces with the flight control computer over an optical data bus. The advantages of the time-rate-of-decay temperature sensor and the issues to the integration of the optical communication system are described.

Fiber optic sensing technology has now reached the point at which it is mature enough to be applied to practical aerospace applications. In particular, closed loop control of flight surfaces in which the sensor continuously monitors actuator position in order to provide feedback to the control system is of interest. In this paper, we describe the principle of operation, design and preliminary testing of a long stroke length (20 cm, 7.8 inches) wavelength encoding multimode fiber optic linear position sensor. The sensor provides a significant reduction in size over conventional LVDT sensors for the same application.

In support of the Fly-by-Light Advanced Systems Hardware (FLASH) Program, Moog has developed and continues to refine a high performance optical input servovalve. This servovalve features no external electrical connections, with all control inputs commanding the valve via an optical fiber. This valve has already demonstrated dynamic and static performance that exceeds most aerospace servovalve requirements, requiring less than 100 milliwatts of optical input power.

Smiths Industries (SI) is developing a Fiber Optic Gyroscope (FOG) Inertial Measurement Unit (IMU) for incorporation into the Advanced Research Projects Agency (ARPA) Technology Reinvestment Project (TRP) Fly-by-Light Advanced Systems Hardware (FLASH) laboratory demonstration. An optically interfaced FOG IMU satisfies the FLASH top-level flight control requirements for improved aircraft upgrade capability, reduced weight, enhanced EMI immunity, and improved system performance capabilities. The FLASH IMU design is leveraged from SI's Standard Compass/Attitude Heading Reference System (C/AHRS) Inertial Sensor Assembly (ISA) and attitude processing hardware and software. The ISA incorporates a triad of single-axis FOGs which use low cost, commercially available single mode fiber in the rate sensing coils and make maximum use of integrated optics to reduce cost and complexity. This sensor has demonstrated excellent tactical grade performance in a wide range of strenuous test environments. The dual rate 1773 fiber optic bus was selected by the FLASH team following a trade study which evaluated many factors including component availability and maturity, cost, speed, and applicability to flight control. The implementation of the bus components, as well as an overview of the overall IMU design, is presented.

A Litton LN-200 fiber optic gyro IMU is being adapted to provide flight control and AHRS data massages via a 1773A fiber optic interface to the FLASH demonstration system. Software modifications to generate the requisite output data from the LN-200 IMU incremental angle and velocity data are being developed for the VFCS program. To provide a rugged, reliable demonstration unit, the production LN-200 IMU is mounted in an adaptive housing along with the circuitry to convert the LN-200 RS-485 SDLC electrical interface to the FLASH 1773A fiber optic interface. A dc/dc converter is also incorporated to provide the additional voltages required for LN-200 and conversion circuitry operation.

Future More Electric Aircraft, which use electric actuators instead of hydraulics, will need high speed optical data buses to enable diagnostics, health monitoring, power management, and failure protection in a high EMI environment. This paper describes these future needs and the resulting optical data bus requirements.

The Fly-by-Light Aircraft Closed-Loop Test (FACT) program is a flight test program sponsored by NASA-Lewis Research Center. The objectives of the FACT program are to demonstrate optical closed-loop control of flight critical and non-flight critical control surfaces and to demonstrate installation and maintenance aspects of fiber optics for application to commercial aircraft. This paper summarizes the FACT program optical maintenance, test architecture, and hardware developments to be flight tested on the NASA-Dryden F/A-18 Systems Research Aircraft (SRA). The modifications include replacing Fly-By-Wire (FBW) main ram feedback LVDT's with optic position sensors and an electro-optic decoder, and using electrical to optic converters and reverse for commands. The performance and handling qualities will be validated by laboratory, ground, and flight tests. The goal is to demonstrate system performance equivalent to the production system.