Low-frequency radio range was the chief system of navigation for instrument flying in the 1930s and 1940s. Using this system, when blind flying, or, ‘flying the beam,’ pilots heard a steady buzz over AM radio headsets when on direct course to an airport. When a plane’s course deviated to one side of the beam, they would hear the Morse code letter ‘A’ (.-). If they drifted to the other side, they heard the Morse code letter ‘N’ (-.). As pilots neared the ‘On Course’ signal, the letters ‘A’ and ‘N’ would meld to become a uniform tone termed the ‘Twilight’ zone. As the pilot approached the radio sending station at an airport, the signal would fade into a ‘Cone of Silence’.

William J. Chapman was an Army captain and pilot. In 1939 he worked as a flight instructor at the Army Air Corps' Lincoln Flying School in Lincoln, Nebraska.