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Once it became possible to fly airplanes through clouds, the problem was how to find the way. It is worth recalling that tunable radios, in the 1920s and 1930s, were large clunky affairs that used vacuum tubes, required a fair amount of power and generated a lot of heat. (It was common for the radio sets to be mounted remotely in racks behind the cockpit, with remote control heads in the cockpit.)
The first commonly used system was the four-course range. This is what a range looked like:
The five antennas formed a broadcast pattern that, in idealized form, looked like this:
All you needed to fly it was a wet compass and a radio receiver. The frequencies were on a low-frequency band, just below the AM broadcast band. This is how it looked on a chart:
If you were off to one side, you heard the Morse Code for A: .- and if you were on the other side, you heard the Morse Code for N: -. and if you were "on the beam," the two signals merged into a steady tone.
You really had to have good situational awareness, for that is all the information you received. If the signal was getting stronger, you were flying towards the beacon. If you were over the beacon, you were in the "cone of silence" and you heard nothing. The system was augmented by narrow fan-beacons that broadcast a signal straight up, so if you heard the signal, you had to be over the beacon. If you lost the bubble and didn't know where you were, there were very complicated procedures to reorient yourself.
This system was augmented in larger planes, by radio-direction finders. At first, they were loop antennas that just hung out in the breeze. One is mounted on the top of this Lockheed 12:
Later, they were put in teardrop-style fairings that protected the antennas against ice, such as under the center of the wing of this DC-3:
The RDFs were worked by a pilot or radio operator who had to manually crank the antenna around until he heard the strongest signal. At that point, the antenna was perpendicular to the station and a needle in the gauge on the instrument panel indicated that the radio station was that way. Or it could be on the other side of the airplane; the ADF (automatic direction finder) used early signal processing to eliminate that ambiguity.
ADFs could home onto a four-course range station, a radio beacon (non-directional beacon) or a powerful AM radio station. Up until the 1980s or so, sectional charts showed the location of the broadcast antennas for radio stations such as WCBS and WGN.
The air traffic control system was very primitive and relied more on the "big sky, little airplane" theory. There was no radar control; pilots gave position reports and estimated times to the next reporting point. ATC's controllers had little plastic boats on map boards, they slid them along the maps to keep track of where the airplanes were.
In bad weather, the 4-course range was less than reliable. The range through the Banning Pass was notorious for wandering in snowstorms and Banning Pass collected a number of aircraft wrecks. In the summer, a line of thunderstorms would obliterate the ability to hear the ranges or the beacons (if you have never listened to an AM radio when thunderstorms are nearby, try it).
In bad weather, they really did "stack" airplanes. Airplanes would fly racetrack patters over beacons, with one-thousand foot spacing between them. The bottom airplane in the stack would shoot an approach, which was little more than fly over a beacon, start a descent, do a turn to the airport and level off at the floor for the approach. If the pilot saw the airport, he landed. Some would cheat because they knew the area and could (others cheated and died). Once that airplane was on the ground or had executed a missed approach, the next airplane was cleared to try and the other airplanes in the stack descended to the next level.
The "stack" system went away when jets were introduced. Jet engines at lower altitudes suck fuel at voracious rates. Piston-engined airplanes could reduce power and just motor around the stack pattern. When they tried to do that with jets, they had a lot of low-fuel emergencies and some crashes.
The 1940s saw the introduction of the Instrument Landing System and the VHF Omnidirectional Range, or VOR, but that'll be a later discussion.
The five antennas formed a broadcast pattern that, in idealized form, looked like this:
All you needed to fly it was a wet compass and a radio receiver. The frequencies were on a low-frequency band, just below the AM broadcast band. This is how it looked on a chart:
If you were off to one side, you heard the Morse Code for A: .- and if you were on the other side, you heard the Morse Code for N: -. and if you were "on the beam," the two signals merged into a steady tone.
Later, they were put in teardrop-style fairings that protected the antennas against ice, such as under the center of the wing of this DC-3:
The RDFs were worked by a pilot or radio operator who had to manually crank the antenna around until he heard the strongest signal. At that point, the antenna was perpendicular to the station and a needle in the gauge on the instrument panel indicated that the radio station was that way. Or it could be on the other side of the airplane; the ADF (automatic direction finder) used early signal processing to eliminate that ambiguity.
