The Low Frequency Radio Range pretty much followed the original Transcontinental Airway System, defined by the hundreds of light beacons set up during the 1920’s. Not only did this serve as a reassuring back-up on clear nights, many airports and other aviation facilities were already well established along these paths. As the Civil Aeronautics Board (CAB) took over from the Department of Commerce in 1938, a network color-coded of east/west “Green” and “Red” airways and north/south “Amber” and “Blue” airways was being established. This convention has mostly fallen out of use in the US but is still used in many places internationally.
In many configurations, the station simply had four courses spread 90° apart. However, other stations managed to bend these legs in a “scissors” or “crow’s foot” fashion to match the alignment of airways. Each station's radio goniometer could adjust the overall alignment of its beam pattern, and/or power output to one or more antenna could be increased so their quadrants would expand and literally push back on the others, “bending and squeezing” the beams in-between to the desired angle. This effect also worked in 3D and could tilt the cone of silence, an important consideration for pilots.
The basic theory of flying from range to range was straightforward: as a pilot crossed the country, he (which was nearly always the case in that era) would fly the outbound leg of one station out to its maximum range of 50 to 100 miles then tune the inbound leg of the next station. The pilot would often need to fly a different heading from the actual airway to compensate for crosswind but the reference to the beam would assure the plane maintained a direct course. Fan markers would provide milestones to check progress against. The pilot would constantly have to adjust the headset volume as a station grew closer and louder, typically needing to lower the volume significantly just before the crossing. Suddenly, the volume would die off as the cone of silence was reached and, in many cases, a high-pitched Z marker confirmed passage. The signals would resume and fade again until the next range was tuned. Along the way, the pilot would receive regular updates of the weather ahead, and if equipped, could communicate with air traffic control. It worked well under optimal conditions but, as with any first-generation technology, there were many limitations:
Despite all these faults, the system was simple, cheap and it worked. For the first time, aircraft could reliably navigate without having any contact with the ground. Not too dissimilar from modern standards, instrument approaches and holds were developed that allowed pilots to land at airports with low cloud ceilings and to be delayed, if needed, by air traffic control. In short, this system ensured that weather simply wasn’t a factor and regularly scheduled airline service could be established regardless the conditions. This is one of the main technologies that allowed the arrival of that Golden Age of Travel. But as always, technology was ever evolving...
What follows below are actual sounds of what appears to match the Syracuse Range (Morse code identifier “SR” or “dot-dot-dot, dot-dash-dot”) in the 1950’s, in the waning days of LFR. These recordings were specifically made for pilot training by wiring a hi-fi recorder directly into the headset feed. They were generously provided by author Barry Schiff who owned the company that developed these educational materials, who is gratefully credited for these and other contributions under Resources.
Again, pilots would have to listen to these signals for hours on end on a noisy flight deck, often through static and other interference.
Of course, the ultimate point of LFR was to ensure that aircraft would find and safely land at their destination. Instrument “let down” or approach procedures were developed so arriving aircraft could safely emerge from the weather oriented toward the landing field, and equally importantly, knew how to successfully divert if they were still unable to see the runway. Starting in the 1930’s, pilots such as Elrey Borge Jeppesen began to document these procedures on “approach plates,” small diagrams that quickly briefed a pilot on all of the critical aspects in a portable, paperback-book sized format that could be easily carried in any cockpit. These quickly caught on, which lead him to found Jeppesen, which is still a major supplier of these charts and other aviation information today.
Today, LFR instrument landings would be considered “non precision” approaches analogous to VOR procedures where the arriving aircraft are directed toward the airport, but did not receive precise guidance up to the runway threshold as is possible with modern ILS or certain GPS procedures. The procedures for non-directional “homing” beacons were nearly identical, but as they lacked any beams it was completely up to the pilot's skill with a radio compass to keep on the designated course - not an easy task with a gusty crosswind (fortunately, these approaches are now near extinct). LFR approaches took on the same standardized format:
Interestingly enough, compared to modern approaches where altitude limits are prescribed for nearly every step starting with the initial fix, generally only altitudes for the procedure turn and final passage over the station were given. Instrument holds were entered and exited along a beam line and were maintained by repeated back and forth procedure turns 1 to 4 minutes apart along the same beam anchored by the repeated passage over a station, fan marker or an intersection with another LFR station beam.
Through World War II, the plate format became more standardized and after 1947 the CAB began to take over the role of designing and publishing these approaches. That said, many pilots to this day still prefer the format "Jepps” offers versus the FAA issued charts. Although VOR and ILS replaced LFR, the concept of the initial fix, timed procedure turns, and approach minima are still very much part of modern approaches - one major change being current systems allow greater latitude for locating the initial fix in a more convenient direction from an airport other than right over a station. For those instrument rated pilots out there who are curious, three LFR approach plates are shown below - the "real" world versions of the simplified approach diagrams shown above. Additionally, under Resources, one can find an early 1960's ADF/LFR briefing card and 1940's route planning charts.