The U.S. aerospace industry currently provides high quality jobs for both large and small business and a significant positive balance of payments with its high value exports of military and commercial aircraft. This position is declining, however, as airlines purchase aircraft manufactured in Europe at a lower cost and with higher technology due to government subsidies and concessions. The FLASH program will help to reverse the loss of market share by the U.S. commercial transport aircraft (down to 60% in 1993 from 95%in 1980), and enhance the global competitiveness of the U.S. aerospace industry. It will help industry to capture significant portions of the $ 1 ,034 billion (1992 dollars) new aircraft market projected from 1 992 to 2011.

Hardware and software were developed for optical feedback links in the flight control system of an F/A-18 aircraft. Developments included passive optical sensors and optoelectronics to operate the sensors. Sensors with different methods of operation were obtained from different manufacturers and integrated with common optoelectronics. The sensors were the following: Air Data Temperature; Air Data Pressure; and Leading Edge Flap, Nose Wheel Steering, Trailing Edge Flap, Pitch Stick, Rudder, Rudder Pedal, Stabilator, and Engine Power Lever Control Position. The sensors were built for a variety of aircraft locations and harsh environments. The sensors and optoelectronics were as similar as practical to a production system. The integrated system was installed by NASA for flight testing. Wavelength Division Multiplexing proved successful as a system design philosophy. Some sensors appeared to be better choices for aircraft applications than others, with digital sensors generally being better than analog sensors, and rotary sensors generally being better than linear sensors. The most successful sensor approaches were selected for use in a follow-on program in which the sensors will not just be flown on the aircraft and their performance recorded; but, the optical sensors will be used in closing flight control loops.

An overview is presented of fiber optic smart structure research at Rockwell for Fly-by-Light technology. The methods emphasize the use of radio frequency (rf) amplitude modulation of the optical intensity and detection of phase and amplitude of the rf signal transmitted through various optical fiber systems. Strain is transferred from metallic or composite structures to the embedded optical fibers.

Distributed intelligence, fault tolerance, and fiber optic technology hold significant promise when applied to complex sensor/actuator systems such as those found in primary and secondary flight control systems. This paper outlines the theory of operation and configuration of a fault tolerant distributed control system jointly developed by Raytheon Company and Beech Aircraft Corporation. The system's benefits accrue from the union of fiber optic performance advantages with the low cost of fault-tolerant distributed sensing and control techniques. The initial configuration is comprised of low-cost fault-tolerant computers which control, monitor and display the functions of two JT15D-5 engines and their thrust reversers across redundant fiber networks. Pilot inputs are transmitted digitally over a redundant fiber optic network using a distributed fault-tolerant processing architecture. In the Distributed Control-By-Light (CBL) system, low-cost intelligent nodes are placed at the site of the sensors, actuators, control inputs, feedback devices, and displays across the entire aircraft. The nodes communicate across redundant fiber optic networks using an industry standard open architecture protocol. Firmware at each node monitors or controls the local function and responds to commands from or passes digitized processed sensor data to other functions on the aircraft. Proven hardware and software fault-tolerance techniques are applied to provide the ability to withstand multiple faults. The Distributed Control-By-Light system achieves all of the benefits traditionally ascribed to FlyBy-Light: reduced cost and weight, reduced EM!, HIRF, and lightning susceptibility, reduced system grounding problems, reduced certification costs, and increased aircraft range and payload. The distributed architecture also provides decreased production and assembly costs, improved reliability, enhanced operational utility and decreased pilot workload, and enhanced maintenance and diagnostics capabilities.

The ever increasing performance and economy of operation requirements placed on transport aircraft are resulting in very complex, highly integrated aircraft control and management systems, substantial improvements in reliability, maintainability, weight savings, manufacturability, and survivability are also required. Fly-By-Light (FBL) technologies and their integration offer the potential of providing light weight, highly capable, flexible, and robust aircraft control and power systems to meet the demanding requirements placed on future transport aircraft. This paper discusses some key FBL technologies and integration on transport aircraft.

This paper represents the results of a trade study performed under the Optical Backplane Interconnect System contract sponsored by the Navy's Advanced Avionics Subsystems and Technology program. The purpose of this program is to perform development, test, evaluation, and documentation of optical backplane technology suitable for application in next- generation military air platforms.

Smart Skin Array Technology is a technology that has been developed and patented by Page Automated Telecommunications Systems, Inc. It is based on a new fabrication methodology which allows highly dense fiber optic parallel pathways to be manufactured in a repeatable manner with up to 100 fiber optics per inch, in widths of up to 12 feet and long lengths many meters > 500, the anticipation is with correct adjustment > 1000 meters. The technology allows for customization of fiber type, spacing between fibers and density from one upward. The ability to control spacing between fibers enables the base material to interface with various types of optoelectronic arrays and in diverse interconnect scenarios. Such a capability lends itself to be a repeatable and controllable, point to point, optoelectronic interface. In such a highly parallel system, performance need not be degraded. In this paper, we will describe some of those attributes which provide for the benefits achievable with this technology. It is these benefits which will provide for viable Fly by Light systems. The flash program will incorporate these in the development of the physical cable plant.

Fiber optic technology has been implemented within diverse areas of aircraft vehicle management systems, including propulsion and flight controls. At least four different fiber- based technologies have been demonstrated in the laboratory and some have accumulated flight hours while installed in technology demonstration aircraft. Some key technologies developed thus far include Time Division Multiplexing (TDM), Wavelength Division Multiplexing (WDM), dual wavelength analog, and Ladar. Some variations in these technologies have also been shown to have promise, such as transmissive vs. reflective encoders, where the number of interconnect fibers are reduced. TDM technology has been actively developed since 1982 as a result of the US Army sponsored Advanced Digital Optical Control System program. This paper provides an overview of the TDM technology and its status when viewed in light of the key flight control system requirements. Description of the TDM sensor concept, the associated electronics, delay line fiber technology and fiber connector requirements is provided. A comparison with WDM technology is also described.

An advantage to using a Fly-By-Light system with fiberoptic transducers and interconnects is immunity to disrupts and/or failure due to lightning strikes. The most vulnerable area of any flight control system on a helicopter is the swashplate region, where linear optical position transducers (LOPTs) would measure mean rotor actuator ram position. On the RAH-66 Comanche helicopter, LOPTs would be mounted inside the cylinders of these rams, providing protection from a direct lightning attachment to the ram. Lightning survivability testing was conducted at the Boeing Developmental Center's Lightning Facility the week of December 14 - 18, 1992. A time-division multiplexed LOPT was tested in a protected, aircraft-similar installation as well as unprotected. Transducer data during the strikes were compared to the results of similar tests performed on a linear variable differential transformer.

Frost condensing on the code plate of an optical transducer may cause degraded performance by diffusing reflected light. A test was performed to evaluate effects of temperature and humidity on a prototype time domain multiplexed optical position sensor. Temperature was varied from -55 to 125 degree(s)C and humidity from 0 to 100%. In addition, the test attempted to generate frost inside the transducer to interfere with operation of its code plate and read head. Overall, there were no significant losses due to humidity. There was no direct evidence that frost was present on the code plate at any time, though given the mechanical resistance of the transducer shaft at low temperature points, it appears likely that there was some frost buildup. Temperature effects were more pronounced as a loss of 2 dB or more was observed at low temperatures. There was no evidence of frost-induced losses.

Optical pulse reflections in the signal paths of a fly-by-light flight control system employing time division multiplexing can interfere with data returning date to the receiver. To determine how reflections of the interrogation pulse may interfere with optical data signals returning from an optical transducer to the flight control computer. Fresnel reflection theory and geometric optics are used to predict the intensity and return times of the pulse reflections at the receiver. A channel with multiple connectors and a 12-bit position transducer is considered as an example of a typical channel in a fly-by-light flight control system. The optical power and return times of the pulse reflections in the example channel are predicted and are compared with the optical power and return time window of the returning data signal. To check the analytical prediction, a dynamic simulation of the example optical channel is used to model the signal behavior. Although the analysis shows that the reflections from the connector interfaces can interfere with the transducer data by arriving at the same time as the data, these spurious signals can be rejected if the optical receiver is designed properly. It is recommended that the Fresnel reflection intensity, the number of disconnects in the interconnect cable and the relative insertion loss of the interconnect cable compared to that of the transducer must be minimized to warrant reliable operation and simpler receiver design.

Fiber optic sensing techniques for measuring temperature, position, speed, and flame presence were passively demonstrated on the F404-400 augmented turbofan engine for the NASA FOCSI (Fiber Optic Control System Integration) program. From early definition through detail design, fabrication, and testing, these components began to meet requirements as candidates for future engine product applications. In this paper we describe a number of issues that were considered leading to engine ground and flight testing for FOCSI, and some issues that surfaced as a result of the program. Functionality of the FOCSI sensor set is described. Emphasis is placed on setting goals of fully meeting performance requirements over the entire range of service conditions. Some fundamental mechanical design ground rules are presented, and issues associated with using fiber optic cables and electro-optic circuitry are exposed. Finally, some methods of installing demonstrational sensor hardware and acquiring the sensor measurements while minimizing interference with normal engine operation are described.

Development of sensors for fly-by-light applications has focused more on development of airframe sensors than engine sensors. In this paper, we report on a limited suite of fiber optic sensors for on-engine sensing applications. The sensors are multimode and sense the parameters: engine speed, fuel presence and electrical current. Principles of sensor operation will be given and results of sensor testing on a Pratt & Whitney gas turbine engine will be provided. In addition, possible multiplexing options for these sensors will be discussed.

The inherent electromagnetic interference (EMI) immunity and other benefits of fiber optic technology provide numerous opportunities for it's use in aerospace applications. Rosemount Aerospace Inc. has focused significant resources on the advancement of fiber optic sensors for use in both aircraft and engine environments. These efforts include the development of a fiber optic temperature sensor based on the fluorescent time rate of decay (TRD) principle. General Electric Aircraft Engines recently completed an evaluation of a Rosemount Aerospace TRD system. The test consisted of 229 hours of testing on the inlet screen of a General Electric F404 Engine. The objective of the test was to monitor the performance of the optical sensor assembly on an aircraft engine and to compare the optical signal with the output of the corresponding electrical sensor. This paper will review the TRIJ system design and GE engine test results. Key Words: TRD, optical temperature sensor, engine inlet sensor, fluorescence, time rate of decay, F404

An optical sensor system extends gas temperature measurement capability in turbine engines beyond the present generation of thermocouple technology. The sensing element which consists of a thermally emissive insert embedded inside a sapphire lightguide is capable of operating above the melting point of nickel-based super alloys. The emissive insert generates an optical signal as a function of temperature. Continued development has led to an optically averaged system by combining the optical signals from four individual sensing elements at a single detector assembly. The size of the signal processor module has been reduced to overall dimensions of 2 X 4 X 0.7 inches. The durability of the optical probe design has been evaluated in an electric-utility operated gas turbine under the sponsorship of the Electric Power Research Institute. The temperature probe was installed between the first stage rotor and second stage nozzle on a General Electric MS7001B turbine. The combined length of the ceramic support tube and sensing element reached 1.5 inches into the hot gas stream. A total of over 2000 hours has been accumulated at probe operation temperatures near 1600 degree(s)F. An optically averaged sensor system was designed to replace the existing four thermocouple probes on the upper half of a GE F404 aircraft turbine engine. The system was ground tested for 250 hours as part of GE Aircraft Engines IR&D Optical Engine Program. Subsequently, two flight sensor systems were shipped for use on the FOCSI (Fiber Optic Control System Integration) Program. The optical harnesses, each with four optical probes, measure the exhaust gas temperature in a GE F404 engine.

The FOPMN is a fiber-optic signal collection system for primary flight control applications. An avionics bay protected electro-optic interface unit transmits light down fiber optic cable to an optical sensor housed in the harsh environment of a hydraulic actuator. The interface unit also receives the sensor's reflected pattern and calculates independent positions from the multiplexed signals. This paper discusses the FOPMN method for fiber-optically sensing and multiplexing two channels of position of a TEF actuator's main ram cylinder. Currently installed in NASA Dryden's SRA F/A-18, the FOPMN has accumulated approximately 15 hours of flight time. A performance comparison is made between the FOPMN positions and the flight control computer's feedback mechanism (the actuator LVDTs). Included is a discussion of some of the lessons learned as a result of testing the FOPMN in the lab and in flight. The FOPMN is well on its way to proving itself as a robust fiber optic system with the ability to multiplex numerous optical sensors for primary flight control. The success of the FOPMN leads to the second phase of the project--optical loop closure. Our goal for this phase is to have four FOPMN sensor channels on the main ram and/or the main control valve of the actuator to serve as the quad redundant feedback mechanism for flight control.

McDonnell Douglas has developed, installed, and tested a number of fiber optic systems on various commercial and military aircraft. From our experience installing and maintaining many different types of fiber optic flight test systems, it has become evident to us that the problems associated with installation and maintenance of fiber optic systems is perhaps the greatest impediment remaining to their incorporation on aircraft. This paper discusses the problems and issues associated with installing and maintaining fly-by-light systems on transport aircraft as well as some approaches and concepts being developed at McDonnell Douglas Aerospace--Transport Aircraft to alleviate these problems.

Aircraft manufacturers are developing fiber optic technology to exploit the benefits in system performance and manufacturing cost reduction. The fiber optic systems have high bandwidths and exceptional Electromagnetic Interference immunity that exceeds all new aircraft design requirements. Additionally, aircraft manufacturers have shown production readiness of fiber optic systems and design feasibility.

We have previously reported on the development of a temperature compensated, self- referenced, microbend fiber optic pressure transducer for total pressure measurement. This transducer outputs the sum of altitude-dependent background pressure and dynamic pressure proportional to air speed. During flight tests at NASA Dryden, this fiber optic pressure transducer was installed in the same pitot pressure line as the existing electronic pressure transducer in the air data computer. Pressure data were simultaneously recorded from the transducers during flight tests. Presented in this paper is the comparison and analysis of the data from both transducers and a summary of the results.

A linear displacement transducer has been developed for use in fly-by-light control systems. Based on a digitally coded plate, it utilizes wavelength division to multiplex the digital channels on the plate. The sensor is connected to a remote processing unit via a single optical fiber. The transducer described here has a resolution of 12 bits over a 4 inch stroke, or 0.001 inch resolution, and the same configuration can be used for other strokes as well as for rotary transducers. In this paper we first describe the sensor system design. We then present the system's performance in terms of optical power budget and environmental performance, and conclude by discussing the strengths and limitations of our current design and by describing our future efforts in this field.

The Reliable Optical Card-Edge (ROC) Connector is a blind-mate backplane unit designed to meet military stress requirements for avionics applications. Its modular design represents the first significant advance in connector optics since the biconic butt-coupled connector was introduced twenty years ago. This multimode connector utilizes beam optics, micro-machined silicon, and a floating, low mass subassembly design to maintain low coupling loss under high levels of shock and vibration. The ROC connector also incorporates retracting doors to protect the unmated termini from environmental contamination and abusive handling. Design features and test results for the ROC connector are presented in this paper.

As efforts to include fiber optic technology in aircraft flight control electronics have progressed, the need has arisen for a compact optoelectronic interface with an integral multipin optical connector. The United Technologies Research Center optoelectronic Connectorized Module (CM) was designed and built to satisfy this need. This paper will discuss the background, design, fabrication and testing of a completed Connectorized Module. The prototype CM is a four channel speed sensor interface that incorporates established ceramic multichip module (MCM-C) technology with optical emitters and detectors and a multipin fiber optic connector. This combination of technologies yields a compact and rugged interface module. In addition, the CM removes optical fibers, and their associated difficult to repair pigtails, from within the electronic control box. The CM achieves this because: it contains all necessary optoelectronic circuitry, has integral electrical and optical connectors, and is mounted directly on the electronic control box wall, not on an internal circuit board. Although this CM is a speed sensor interface, the flexible nature of MCM-C technology will enable a wide variety of sensor and data communication interfaces to be implemented.

Fiber optic smart structures and fly by light have often evolved in the aerospace industry as separate and distinct programs. As these areas mature and start to move into demonstrations the overlap and merger of these two fields can be expected to become increasingly apparent. This paper explores how this merger is likely to occur.

Current fly-by-light (FBL) sensors represent a proliferation of different unique electro-optic interfaces and transducers. Development of standard electro-optic interfaces for diverse measurements (position, pressure, temperature, speed, etc.) offers potential to improve affordability of FBL sensor systems. Ladar fiber-optic sensor (LFOS) is a promising sensor technology that has demonstrated such a capability. A position transducer, temperature transducer, rotary speed transducer, liquid level transducer, and switch have all been demonstrated as plug compatible. In addition to providing a standard common interface, LFOS technology also offers the benefits of small and robust transducers, inherent multiplexing capability, and inherent fault detection and isolation capability. Current versions of the LFOS electro-optic interface consist of two VME circuit cards that are capable of interrogating and processing four multiplexed sensors. LFOS has been demonstrated in several flight and propulsion control laboratory testbeds.