The Department of Commerce took over responsibility for the system in 1926 and eventually expanded it to 18,000 miles of airways with more than 1,500 beacons. Commerce also produced a series of aeronautical strip charts, the first of which, with topographic, airway, and beacon information for the route from Kansas City, Missouri, to Moline, Illinois, came out in 1927, the year of Lindbergh’s flight from New York to Paris. Only three years later, the first aeronautical sectional chart, depicting the Chicago area on a scale of 1:500,000, appeared. Rudimentary instrument approach charts were printed on the back. Aerial navigation, no longer an awkward improvisation upon sea and surface methods, had come into its own.
Home on the Range
It was evident that airplanes needed radio, both for communication between pilots and the ground and for defining airways that could be followed in bad weather. The 1930s saw the arrival of non-directional beacons and four-course ranges. NDBs mostly operated in the low-frequency band, between 170 and 600 kilohertz, broadcasting a three-letter Morse code identifier. A loop antenna on the airplane rotated (originally, the pilot or navigator turned a crank; later, rotation was made automatic), and the strength of the signal it received depended on the angle of the loop to the beacon. In the automatic version, the Automatic Direction Finder, a needle on the instrument panel showed the direction to the beacon.
Many airplanes still have these because NDBs, being the cheapest kind of ground navigational aids to install and maintain, are still in widespread use. The ADF operates not only in the low-frequency band but up through AM broadcast frequencies as well, so pilots can fly toward powerful broadcast stations in distant cities and, what they sometimes find equally important, entertain themselves by listening to the radio as well.
The four-course or Adcock ranges were low-frequency beacons with four directional antennas, each transmitting a Morse code signal in a lobe-shaped pattern over roughly a quarter of the compass rose. One antenna repeated the letter A—dot dash—while its neighbors had N—dash dot, and the fourth, opposite the first, had A again. Where neighboring quadrants overlapped, the A and N added up to a continuous tone called a course. If the range was near an airport, as most were, one of the courses led to a runway.
A pilot approaching the station likely heard one letter or the other. If he was unsure of his position, he flew until he crossed a course. He then executed a series of turns designed to determine which of the four courses it was, constantly adjusting the radio volume for greatest sensitivity to tell-tale changes in signal strength. He could now fly to the “cone of silence” over the range, track outbound along the approach course for 10 miles or so, then turn and begin his descent to the airport. A skilled pilot flew the “feather edge” of the course, where the faint clicking of a fragmentary A or N could be heard emerging from the continuous hum like a loose thread from a weave.
Over the years we have moved gradually away from aural indications and toward visual ones, and so the difficulty of steering an airplane by varying tones in a headset, and of judging direction by the swelling or fading of a scratchy signal, seems greater to us today than it apparently did to the DC-3 pilots of the 1930s. Author Ernest K. Gann, who omitted no tribulation of airline flying from his classic autobiography Fate Is the Hunter, passed over bad-weather range approaches almost without comment.
Just after World War II, a new type of four-course range, the VAR, or visual-aural range, appeared, broadcasting on static-resistant frequencies above the AM band. It was “visual” in that, in place of the sounds in the headset, a panel instrument presented the courses as a needle swinging between yellow and blue sectors. But it came too late; the four-course ranges were about to go the way of the open-cockpit biplane.
When I learned to fly in 1961, four-course ranges were still depicted on sectional charts, and I studied them before taking the written test for my instrument rating. Turned out I had wasted my time; the test bypassed the subject completely. In a dusty carton of outdated charts I find only one—a 1969 El Paso sectional—that shows a four-course range. It’s at Chihuahua, Mexico.
The war had given impetus to the development of new navigation systems, as it had driven all kinds of other aeronautical technologies. Bombers above clouds seeking targets below required some web upon which they could crawl to a given intersection. Methods had to be accurate to within a few hundred feet and resistant to jamming.
Several systems used a “master” and two or more “slave” transmitters, which created families of intersecting hyperbolic lines of position. Most of these systems were decommissioned at the end of the war, but one, Consolan (Consolidated Low or Medium Frequency Long-Range Aid to Navigation), was still broadcasting over the north Atlantic in the 1970s, and Loran still serves today (opposite). With these systems, latitude and longitude are determined from the different arrival times of sychronized signals from two or more fixed transmitters.