5.vor-navigation-detail. VOR Navigation
VHF Omnidirectional Range (VOR) is the backbone of the federal airway system and remains a primary short-range navigation aid for instrument flight. VOR ground stations transmit on frequencies between 108.0 and 117.95 MHz, broadcasting 360 separate signals (radials) that radiate outward from the station like spokes on a wheel. Each radial is referenced to magnetic north at the station, which is why VOR navigation is naturally aligned with magnetic course headings.
How the Signal Works
The VOR transmits two signals: a reference signal that is omnidirectional and constant in phase, and a variable signal whose phase rotates 360° per revolution. The aircraft receiver compares the phase difference between these two signals; that phase difference equals the magnetic bearing FROM the station. By convention, a radial is identified by the bearing FROM the VOR — the 090 radial extends due magnetic east of the station.
Cockpit Components
The pilot interfaces with VOR information through three instruments or display elements:
- Omnibearing Selector (OBS) — rotates the course card to select the desired radial or course.
- Course Deviation Indicator (CDI) — a vertical needle showing displacement left or right of the selected course. Full-scale deflection equals ±10° when navigating VOR (compared to ±2.5° for a localizer).
- TO/FROM indicator (ambiguity flag) — shows whether the selected course will take the aircraft TO or FROM the station. A red OFF or NAV flag indicates an unreliable signal.
Each dot on most CDIs represents 2°, so a full five-dot deflection equals 10°. At 60 NM from the station, one dot equals about 2 NM of cross-track error; at 30 NM it equals about 1 NM. This is the geometric basis for the 1-in-60 rule.
Types of VOR Stations
- Terminal (T) — usable to 12,000 ft AGL within 25 NM.
- Low (L) — usable to 18,000 ft AGL within 40 NM.
- High (H) — usable to 14,500 ft within 40 NM, to 60,000 ft within 100 NM, and to 45,000 ft within 130 NM.
Many stations are co-located with DME (VOR-DME) or with a military TACAN (VORTAC), providing slant-range distance information in addition to bearing.
Tuning, Identifying, and Monitoring
Proper VOR procedure follows a fixed sequence:
- Tune the published frequency.
- Identify the station by listening for the three-letter Morse code identifier or recorded voice ident. Never use a VOR that is not positively identified. Absence of the ident, or a station broadcasting "TEST" or coded with a maintenance signal, means the station is unreliable.
- Monitor the ident periodically during use, especially when relying on the VOR for primary course guidance.
Tracking a Course
To track inbound on a selected course, center the CDI with a TO indication, then fly the course heading. If the needle deflects right, the desired course is to the right — turn toward the needle. Apply a wind correction angle (WCA) to hold the needle centered. The standard intercept technique is to set the OBS to the desired course, then turn to an intercept heading typically 30° to 45° off the course, holding that heading until the CDI begins to center, then turning to the course heading plus WCA.
Station passage is identified by the first positive, complete reversal of the TO/FROM indicator — not by the needle behavior, which becomes erratic in the cone of confusion directly above the station (a roughly 60° cone where signal reception is unreliable).
Accuracy and Required Checks
VOR ground course accuracy is typically within ±1°. Under 14 CFR 91.171, no person may operate an aircraft under IFR using VOR for navigation unless the VOR equipment has been operationally checked within the preceding 30 days and found within tolerance. Permitted checks and tolerances are:
- VOT (VOR Test Facility): ±4° (set 360 should give a FROM, set 180 should give a TO — "cone of silence on the 180").
- Ground checkpoint: ±4°.
- Airborne checkpoint: ±6°.
- Dual VOR check: ±4° between the two receivers.
- Airborne over a known prominent landmark on a published radial above 20 NM: ±6°.
The check must be logged with date, place, bearing error, and signature of the person making the check.
Errors and Limitations
VOR is line-of-sight — terrain, the curvature of the earth, and altitude all limit reception range. Signal degradation can also be caused by scalloping (course bending from terrain reflection), station interference between two VORs on the same frequency, and reverse sensing when the aircraft is flown opposite the OBS course selection (heading and course do not agree). Maintaining heading agreement with the selected course prevents reverse sensing in conventional CDIs. Modern HSIs eliminate reverse sensing because the course arrow rotates with the aircraft heading.