TWA DC-3 Cockpit 1930’s (World Airline Hist. Soc.)
The Low Frequency Radio Range pretty much followed the original Transcontinental Airway System, as many airports and other aviation facilities were already well established along these paths. Although LFR supplanted the airway beacons, they were still a reassuring backup on clear nights – their numbers would actually increase to 2,112 beacons by 1946 before falling totally out of use by 1972. In 1938 the Civil Aeronautics Authority replaced the Bureau of Air Commerce, and in 1940 was itself split into the Civil Aeronautics Administration (CAA) responsible for managing the airways and the new ATC system, and the Civil Aeronautics Board (CAB) which provided independent economic regulation, investigation and safety rules. During this transition, the Federal Airways were reorganized into color-coded east/west “Green” and “Red” airways and north/south “Amber” and “Blue” airways. This convention has mostly fallen out of use in the US but is still used in many places internationally.
Some sources erroneously state that the first US airways were defined by homing beacons, now called non directional beacons. Although this technology preceded LFR, as described elsewhere, it still had severe limitations in the late 1920’s leading to LFR’s selection. By World War II, the automatic radio compass made this form of navigation more approachable for the larger military and civilian aircraft that could afford it and 34 homing beacons were then established, but they were still secondary to the 400+ LFR stations then extent on contemporary charts.
Under Federal law, LFR stations and their beams legally defined the nodes of the US network. Each airway was 10 miles wide and included all points 700’ or more above ground. Some stations simply had four courses spread 90° apart. However, many other stations managed to bend these legs in a “scissors” or “crow’s foot” fashion as needed to create the desired arrangement of the airways. As described in the previous section, 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 chosen angle. This effect also worked in 3D and could sometimes deform or tilt the cone of silence, an important consideration for pilots.
Pilot licensure became compulsory in 1927 but, at first, there were no other requirements for “blind flying” - any pilot that wanted to could attempt it, likely to the fatal detriment of many. By 1935, concerned with growing air traffic and close calls, the Bureau of Air Commerce temporarily banned non-commercial pilots from blind flying within 25 miles of an airway centerline until it implemented its new national Airway Traffic Control (ATC) system the following year. With this change, clear rules governed the conditions “Contact” (or “Visual”) Flight Rules were allowed, and when Instrument Flight Rules (IFR) were mandated. IFR flight now also required an “instrument rating” and all applicants that sought it would have to demonstrate to a pilot examiner their proficiency in flying a plane solely on instruments “under the hood” in a partially tarped cockpit with enough of the windscreen left open for the examiner to act as a “safety pilot.” Later on, orange plastic film would be custom fitted to the interior of the windscreen while applicants and aspiring students wore blue glasses which would effectively render their outside view black (today, eyeglass-like blinders called “foggles” are used). The applicant would also have to show their mastery of LFR by locating and establishing themselves on a beam via the complex “orientation” procedures described below, and then by executing an instrument approach. It was not an easy task: as stated in later CAA training materials “it separates the men from the boys.”
As is done today, pilots embarking on instrument flight would first develop their flight plans in consultation with published charts and procedures, consider their equipment and fuel requirements, and evaluate alternate airports. Next would be a visit or phone call to an Airways Communication Stations (ACS, now Flight Service Stations) where a briefer would pull the latest Notices to Airmen (NOTAM’S) and weather conditions from one of the constantly clacking teletypes. After careful review and analysis, the flight plan was filed, forwarded to ATC for approval and then quickly dispatched to all the airway stations en route. Once in the cockpit, checklists were ran through, engines started and the radios switched on. After an eternity by modern standards (about 10 to 30 seconds), the vacuum tubes warmed up and the first crackles were heard over the headsets. Once airborne and out of range of any control tower, the initial range station would be tuned. On well-equipped flight decks this would be done by a more precise “coffee grinder” type dial located over the pilot’s head; more modest aircraft had to make do by carefully adjusting the Bakelite knob on their sets. The station’s Morse code identifier was confirmed, and that familiar on-course tone would soon be heard as the plane established itself on the first leg of its journey.
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. In some cases, an “M” type or airway marker was placed at points between stations where a frequency change needed to be made. 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. Nearer to stations, 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 ATC by relaying messages through an ACS called up on a range station as “Radio.”
It all 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 using only a headset. 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.
For the audible tone engineers determined that 1,000 Hz was the ideal frequency as it fell right in the middle of the human auditory response spectrum and was readily distinguishable above cockpit noise. 1,020 Hz was selected for LFR as it was a 17x whole number multiple of the 60 Hz frequency of the US power grid which simplified oscillator design. Another source described how early testers deemed this note “pleasing”– this point may have been argued by veteran users of the system. What is also interesting to note is that the mechanical key switch technology wasn’t perfect: if one listens carefully to the on-course tone they can hear faint audible clicks as the tone switched between the A and N signals (3 clicks, pause, 1 click, pause, repeat). From the slight warbles that can heard in the tone, it’s also clear that audio oscillator stability has certainly come a long way since these recordings were made.
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. As stations were often located 3 to 5 miles away from the runway, LFR’s 3° accuracy could translate to 1,000’ to 1,500’ off centerline. 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. Multiple aircraft could be so “stacked” at 1,000’ intervals to manage traffic flow to airports.
During World War II, the plate format became more standardized and after 1947 the CAA 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. As time went on, Standard Instrument Departure (SID) and Standard Terminal Arrival (STAR) plates would be added to establish procedures for these phases of a flight. Although VOR, ILS and later GPS 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 is current systems allow greater latitude in locating initial fixes at more convenient points for arrivals other than just right over a station. For those instrument rated pilots out there who are curious, three LFR approach plates are above - the "real" world versions of the simplified approach diagrams shown earier. Additionally, under Resources, one can find an early 1960's ADF/LFR briefing card and 1940's route planning charts.
Next: Its Fate