page_title: Low-frequency radio range
section_title: Approaches and holds
hierarchical_section_title: Low-frequency radio range / Approaches and holds
caption_attribution_description: English: Joliet, IL LFR approach chart
context_page_description: The low-frequency radio range, also known as the four-course radio range, LF/MF four-course radio range, A-N radio range, Adcock radio range, or commonly "the range", was the main navigation system used by aircraft for instrument flying in the 1930s and 1940s, until the advent of the VHF omnidirectional range, beginning in the late 1940s. It was used for en route navigation as well as instrument approaches and holds.
  Based on a network of radio towers which transmitted directional radio signals, the radio range defined specific airways in the sky. Pilots navigated using low-frequency radio by listening to a stream of automated "A" and "N" Morse codes. For example, they would turn the aircraft to the right when hearing an "N" stream, to the left when hearing an "A" stream, and fly straight ahead while hearing a steady tone.
  As the VOR system was phased in around the world, low-frequency radio range was gradually phased out, mostly disappearing by the 1970s. There are no remaining operational facilities today. At its maximum deployment, there were nearly 400 stations exclusively using low-frequency radio range in the U.S. alone.
context_section_description: Final approach segments of low frequency radio instrument approaches were normally flown near the range station, which ensured increased accuracy. When the aircraft was over the station, the audio signal disappeared, since there was no modulation signal directly above the transmitting towers. This quiet zone, called the "cone of silence", signified to the pilots that the aircraft was directly overhead the station, serving as a positive ground reference point for the approach procedure.
  In a typical low frequency radio instrument approach procedure, final approach would begin over the range station, with a turn to a specific course. The pilot would descend to a specified minimum descent altitude (MDA), and if the airport was not in sight within a specified time (based on ground speed), a missed approach procedure would be initiated. In the depicted Joliet, IL low frequency radio approach procedure, minimum descent altitude could be as low as 300 feet AGL, and required minimum visibility one mile, depending on aircraft type.
  The low frequency radio range also allowed air traffic control to instruct pilots to enter a holding pattern "on the beam", i.e. on one of the low frequency legs, with the holding fix (key turning point) over the low frequency radio station, in the cone of silence or over one of the fan markers. The holds were used either during the en route portion of a flight or as part of the approach procedure near the terminal airport. Low frequency radio holds were more accurate than NDB holds, since NDB holding courses are predicated on the accuracy of the on-board magnetic compass, whereas the low frequency radio hold was as accurate as the low frequency radio leg, with an approximate course width of 3°.
caption: Joliet, IL low frequency radio instrument approach procedure
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