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by Friday Morning Flight Plan at [date]
Very High Frequency Omnidirectional Range (VOR) was developed in the late 1930s and entered widespread operational use in the United States beginning in the late 1940s/early 1950s. As it was the standard for US aeronautical navigation in the 1950s, it was likewise adopted globally during the following decade, after ICAO standardized its use.
Beginning in the late 1970s, with the launch of the first GPS satellites, the Global Positioning System—significantly more accurate than VOR—began laying the groundwork to eventually supplement and then reduce reliance on what had become an extensive VOR navigation structure. Because widespread civil adoption and certification of GPS did not occur until the 1990s and beyond, the FAA recognized early on that the operationally expensive VOR system could not be quickly or completely dismantled.
After several decades, during which time GPS became well-integrated into all tiers of aviation, including General Aviation, the FAA unveiled the VOR Minimum Operational Network (MON) in 2016. This initiative was designed to reduce the operating costs of the traditional VOR system while simultaneously providing a reliable structure that ensured backup navigation and instrument approach services for aircraft.
The advent of GPS has also spurred the development of Performance-Based Navigation (PBN), which offers various RNAV approach options to aircraft equipped with GPS/WAAS configurations. Despite these advancements, it remains prudent, and even essential in aviation, to have a backup system in place. This critical role is precisely what the VOR MON fulfills.
The number of dedicated VOR stations across the country is gradually being reduced. However, those stations included in the VOR MON serve a vital purpose.
Should a GPS outage occur, the stations within the VOR MON system provide essential backup by allowing pilots to continue through an outage area using VOR station-to-station navigation or proceed to a MON airport where they can fly an ILS, LOC, or VOR approach procedure without the necessity of GPS, DME, ADF, or surveillance. The signal is reliable above 5,000 feet AGL, and the VOR MON assures that at least one airport will be within 100 nautical miles.
As detailed in paragraph 1-1-8 of the Aeronautical Information Manual (AIM), "NAVAID Service Volumes," signal strength in the new VOR MON system has also been upgraded, categorized as VOR Low (VL) and VOR High (VH).
VOR Low (VL)
- From 1,000 feet ATH up to but not including 5,000 feet ATH at radial distances out to 40 NM.
- From 5,000 feet ATH up to but not including 18,000 feet ATH at radial distances out to 70 NM.
VOR High (VH)
- From 1,000 feet ATH up to but not including 5,000 feet ATH at radial distances out to 40 NM.
- From 5,000 feet ATH up to but not including 14,500 feet ATH at radial distances out to 70 NM.
- From 14,500 ATH up to and including 60,000 feet at radial distances out to 100 NM.
- From 18,000 feet ATH up to and including 45,000 feet ATH at radial distances out to 130 NM.
Additionally, new service volumes have been established for Distance Measuring Equipment (DMEs).
DME Low (DL)
- For altitudes up to 12,900 feet ATH at a radial distance corresponding to the LOS to the NAVAID.
- From 12,900 feet ATH up to but not including 18,000 feet ATH at radial distances out to 130 NM.
DME High (DH)
- For altitudes up to 12,900 feet ATH at a radial distance corresponding to the LOS to the NAVAID.
- From 12,900 ATH up to and including 60,000 feet at radial distances out to 100 NM.
- From 12,900 feet ATH up to and including 45,000 feet ATH at radial distances out to 130 NM.
Legacy VORs will continue to exist with their traditional service volumes and are designated as T, L, or H on IFR charts. VOR MON stations, in contrast, are listed as VL or VH. Both systems, however, remain limited by line-of-sight reception.
For instrument students and IFR pilots seeking currency renewal, understanding the new symbols on a low en-route chart is an expected competency during the oral portion of a checkride. Therefore, practicing their identification in local or nearby areas is highly recommended. Similarly, questions pertaining to these new designations may appear on the Instrument (or CFII/IGI Instrument) written examination.
Should a GPS outage occur, you will be required to revert to the traditional navigation system. Consequently, practicing ground scenarios where this could happen and rehearsing contingency plans are crucial. Utilizing a flight simulator is an excellent method to maintain proficiency in this regard.
The VOR navigation structure will likely never be completely removed, serving as an essential backup in the event of GPS failure. Even Non-Directional Beacons (NDBs) remain operational in certain locations. Traditional radio navigation has served aviation dependably for many years and will continue to do so, to some extent, into the foreseeable future.
Maintain sharp ILS/VOR skills, and remember to check en route and GPS NOTAMs before every IFR flight. Doing so will lead to safer flying and increased confidence, knowing you have a reliable navigational wingman as aviation accelerates into the 21st century.