Fiber Optics and Flight Control Systems

A prototype, quad redundant, fly-by-wire flight control system developed for the USAF Flight Dynamics Laboratory in 1967 was modified to incorporate a fiberoptic data transmission link in one of the four channels. This paper describes the system configuration and summarizes the test results which confirmed that the practical application of fiber optic data transmission to fly-by-wire or other avionics requirements was compatible with the 1973 device and system state of the art. The objective of this work was to demonstrate unique survivability and noise immunity characteristics of fiber optic data transmission in the flight control application. All data flowing between one of the elevator actuator channels and its control computer was transmitted on multiplexed optical data busses in serial-bit, serial-word format at a 500 KHz bit rate. The channel capacity was several hundred times greater than the maximum requirement anticipated for a fly-by-wire application. The system was mechanized with a 100-foot, fiber-optic cable and miniature optical transmitter and receiver modules, having a total bandwidth capability of about 4.0 MHz. System performance was verified, and the compatibility of the optical channel and the other three electrical channels was demonstrated. Various fiber-optic cable jacketing configurations were evaluated, and detailed environmental tests were performed on one type of jacketed cable. Immunity of optical data transmission to electrical noise was qualitatively demonstrated. The major problem area encounted was excessive fiber breakage associated with the extrusion of the fiber through the jacketing material, but improved extrusion techniques can solve this problem.