SI Airways-from Lighted Beacons to Radio Navigation

By the end of 1927, the government had extended the lighted portion of the airway system from New York to Salt Lake City on the transcontinental route, and on portions of feeder and parallel segments, such as Los Angeles to Las Vegas, New York to Atlanta, Chicago to Dallas, and between Los Angeles and San Francisco. That year there were 4,121 miles of lighted airways operated by the Aeronautics Branch of the Department of Commerce. By 1933, there were 1,500 beacons in place, extending the lighted airway systems for a length of 18,000 miles. While the lighted airway was of significant aid in navigation, it had serious limitations in the context of an all-weather air carrier system. It was still a visual navigation system, dependent on reasonably good weather in order to operate effectively.

The Bureau of Standards in the Department of Commerce began, in 1926, to work with radio as a means of communication and navigation. As government involvement in aviation began to kick in as a result of the Air Commerce Act, money and effort were applied to solve problems and to attempt to eliminate limitations on the commercial development of air commerce. In 1926, for instance, there was no two-way voice communication possible with aircraft in flight. This amounted to a serious limitation in safety, including a lack of pilot awareness of developing weather. By 1927, the first radio transmitter was established at Bellefonte, Pennsylvania, allowing communication with aircraft in a 150-mile radius.

In 1928, the Bureau of Standards developed a new radio beacon system of navigation, the first non-visual navigation system in the world. The Aeronautics Branch, which had authority over the lighted airway system, took over the installation and control of the new radio navigation system in 1929. The system was known as the “four-course radio range,” and it would provide the first step in allowing a true all-weather air carrier system to begin to develop. It would remain the standard navigation system in use until World War П.

The four-course radio range utilized low frequency radio waves (190 to 535 kHz radio band) transmitted from powerful 1,500-watt beacons spaced 200 miles apart on the airway. The beacons transmitted two Morse code signals, the letter “A” and the letter “N.” In Morse code, these signals are opposite, “dot-dash” for A, and “dash-dot” for N. When the aircraft was centered “on the beam,” these signals merged into a steady, monotonous tone. If the aircraft ventured to one side of the airway, the signal heard was either the Morse A or N, depending on the aircraft’s position from the beacon. (See Figure 13-3.)

Each beacon defined four airways, thus the name “four-course radio range,” and the beacon’s identification was broadcast in Morse code twice each minute. The so-called beam width was 3 degrees, so that at the halfway point of 100 miles between beacons, the on-course deviation was about +/-2.6 miles. Station passage was

marked by a “cone of silence,” at which point the aural tone would disappear as the aircraft passed overhead. Distance from the station was later provided by marker beacons placed along the airway at intervals of 20 miles or so.

By today’s standards, the four-course radio range was primitive. Low frequency radio was subject to electrical static and other weather aberrations and distortions, but it constituted a quantum leap forward over the visual, lighted beacon system in use at the time. Pilots became very adept at flying the four-course system, and as the airlines began establishing schedules on their new routes. All-weather navigation allowed adherence to schedules that theretofore would have been impossible.