By Stars, Beacons, and Satellites
The lost art-and intimidating science-of aerial navigation.
- By Peter Garrison
- Air & Space magazine, March 2006
(Page 2 of 6)
Pilot Jack Knight saved airmail. Meeting the surviving eastbound biplane at North Platte, Nebraska, Knight took off in darkness. Guided by bonfires and burning oil drums that had been lit by postal employees and helpful farmers, he flew all night in bitter cold, landing to refuel at Omaha and Iowa City, and reached Chicago in the morning. A relay pilot completed the trip to New York.
Having covered 830 miles in nine hours, Knight proved that the airmail could move even in darkness and bad weather. Lionized in the press as a hero (his name probably helped), he downplayed the difficulties he had faced, though he did concede that “if you ever want to worry your head, just try to find Iowa City on a dark night with a good snow and fog hanging around.” The trail of fire that Knight followed was the beginning of the nation’s first airway system: Funded after all, the Postal Service was soon erecting electric beacons to guide night fliers along its routes.
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.