A Tuned-Reed Course Indicator for the Four- and Twelve-Course Aircraft Radio Range

For the 12-course radio range system, in which three modulation frequencies are used, a type of reed indicator has been developed to indicate when the aircraft is on any one of the twelve courses, and if off, approximately how many degrees and whether to the right or left. In addition, it indicates to the pilot in case he becomes lost which is his nearest course, how to turn to get on it, and which way he is flying on it. This is accomplished by the use of three reeds in the visual indicator, each reed being tuned to one of the modulation frequencies sent out by the radio range, namely 65 cycles, 86.7 cycles, and 108.3 cycles. Unequal amplitudes of vibration of the reeds indicate the plane is off the course to the side of the reed having the greatest amplitude. A simple shutter with windows, in front of the vibrating reeds, exposes any two at a time. The correct two for a given course is determined by a color system which is exposed by the window to correspond to the color of the particular radio range route marked on the map. A second shutter and color system is provided so that the rule, "longest reed indicates side off course," may be made to hold regardless of the course being flown or the direction of flight.