Definition
The VHF Omnidirectional Range (VOR) is defined under ICAO Annex 10, Volume I, Section 3.3, and in the United States under FAA Advisory Circular AC 00-31C (United States Standard for Terminal Instrument Procedures). VOR stations transmit on frequencies between 108.0 MHz and 117.95 MHz, producing a directional signal that rotates 360° and encodes the aircraft's magnetic bearing from the station — called the radial. The pilot selects a desired course on the omni-bearing selector (OBS) of their CDI or HSI, and the instrument shows whether the aircraft is on, left of, or right of that radial. A full-scale CDI deflection on most instruments represents 10° off course (each dot equals 2°).
VOR stations are categorized by their certified service volume: Terminal (T-VOR) provides reliable signal within 25 NM and up to 12,000 ft AGL; Low (L-VOR) is valid to 40 NM and 18,000 ft AGL; High (H-VOR) provides service to 130 NM from 18,000 ft AGL up to FL450, and to 100 NM between 14,500 ft and 18,000 ft. The maximum permissible VOR bearing accuracy under ICAO standards is ±2° from the ground station, and inflight checks under 14 CFR §91.171 require a maximum allowable airborne bearing error of ±6° when using an airborne checkpoint or ±4° at an FAA-published VOR test facility (VOT).
Distance Measuring Equipment (DME) is standardized under ICAO Annex 10, Volume I, Section 3.5. The airborne interrogator transmits paired pulses on UHF frequencies between 1,025 MHz and 1,150 MHz, and the ground transponder (beacon) replies on a frequency offset by 63 MHz (placing it in the 962–1,213 MHz L-band range overall). Interrogation-to-reply round-trip time is converted to slant-range distance, displayed in nautical miles and tenths. DME frequencies are automatically paired with VOR or ILS localizer frequencies using a published frequency-pairing table, so tuning the VOR frequency on the pilot's NAV radio simultaneously tunes the associated DME without separate selection. The fundamental accuracy limit of DME is ±0.5 NM or 3% of range, whichever is greater. A critical operational consideration is slant-range geometry: at low altitudes close to the station, the aircraft altitude creates a significant error between slant range and ground distance. For example, an aircraft at 1,000 ft AGL directly over a DME station reads approximately 0.16 NM, not zero; at 6,000 ft AGL at 1 NM ground distance the DME reads approximately 1.16 NM.
The FAA's Minimum Operational Network (MON) program, established in 2016 and continuing through approximately 2030, is reducing the number of operational VOR stations from a historical peak of roughly 967 to a planned retained network of approximately 582 stations (later revised to approximately 606 stations following GA stakeholder input). The purpose of the MON is to provide a GPS-independent backup navigation capability sufficient to allow an aircraft to reach an airport within 100 NM in the event of a complete GPS outage, rather than to support every en route flight. ICAO's Aviation System Block Upgrades (ASBU) framework similarly designates legacy ground-based NAVAID infrastructure as progressively optional as GNSS/SBAS coverage matures globally, with VOR and NDB networks expected to reach maintenance-only status in most states by the late 2030s.
For pilots and schools in the current transitional period, VOR proficiency remains a Part 141 and Part 61 training requirement. The FAA Instrument Rating ACS (FAA-S-ACS-8B) requires applicants to demonstrate tracking, intercepting, and time/distance checks using VOR, and VOR-DME approaches remain published at many airports as primary non-precision procedures. EASA's FCL.615 IR theoretical knowledge syllabus likewise requires comprehensive understanding of VOR, DME, and their error sources.
Why It Matters for Flight Schools
VOR/DME proficiency is required knowledge for both private and instrument-rated pilots, and practical VOR navigation skill is tested during the IFR practical exam. At flight schools, teaching VOR intercepts and tracking is a standard early lesson in instrument ground training. Despite the shift to GPS-based navigation, VOR knowledge underpins students' understanding of CDI behavior — the same instrument used for GPS RNAV approaches — making VOR training a conceptual foundation rather than just a legacy skill. Schools must also keep VOR/DME equipped training aircraft serviceable, as a VOR receiver failure renders the aircraft non-IFR-current if the alternate navigation is GPS-only and the flight plan or approach requires conventional NAVAID backup.
DME is specifically required for certain approaches and some en route procedures. ILS approaches with no marker beacons require a DME for the outer fix, and DME arcs are published on many RNAV and ILS approaches as an alternative initial segment. Tracking DME serviceability in the school's maintenance program is therefore not just a logbook item but directly affects approach procedure availability for training flights — a DME squawk can make specific approaches impossible even if the GPS and VOR receivers are functional.
How Aviatize Handles This
Aviatize logs VOR and DME navigation training within the training management module, allowing instructors to record VOR intercepts, radial tracking, time-distance checks, and VOR/DME approaches as specific lesson elements against the ACS task codes. When a student's VOR-based instrument currency is approaching expiry, the system flags the gap so instructors can plan a lesson that includes VOR approach work before the six-month window closes under §61.57(c).
On the maintenance side, Aviatize's maintenance control module tracks the airworthiness status of each aircraft's VOR receivers and DME transponder. When a VOR or DME is written up as unserviceable in a squawk entry, the system automatically marks affected aircraft as unavailable for IFR dispatch and surfaces the squawk to the chief pilot and maintenance team in the unresolved items queue. This prevents inadvertent scheduling of an aircraft with a deferred VOR discrepancy onto an IFR training flight that legally requires conventional NAVAID equipment.