15.vor-navigation. VOR Navigation
VHF Omnidirectional Range (VOR) is the backbone of the federal airway system and remains one of the most widely used ground-based navigation aids. VOR stations transmit in the very high frequency (VHF) band between 108.0 and 117.95 MHz, with frequencies ending in even tenths (e.g., 108.20, 108.40) reserved for VOR and odd tenths shared with localizers. Because VHF signals travel line-of-sight, range and reception altitude matter: a VOR is only usable when the aircraft is within radio line of sight of the station.
How a VOR Works
The ground station radiates two signals: a reference phase signal that is omnidirectional and a variable phase signal that rotates electronically. The aircraft receiver compares the phase difference between the two signals and converts that difference into a magnetic bearing FROM the station. Each VOR therefore defines 360 radials, each one a magnetic course outbound from the station. Radials are numbered the same as their magnetic direction from the station — the 090 radial extends due east of the VOR.
Classes and Service Volumes
VORs are categorized by their Standard Service Volume (SSV):
- Terminal (T): usable to 12,000 feet AGL within 25 NM.
- Low (L): usable to 18,000 feet AGL within 40 NM.
- High (H): usable from 1,000 to 14,500 feet AGL within 40 NM, to 18,000 feet within 100 NM, and to 45,000 feet within 130 NM.
The FAA has also published expanded VOR Service Volumes (VL and VH) to support the Minimum Operational Network (MON) as the VOR system is rationalized in the NextGen era.
Cockpit Components
A VOR receiver displays course information on a course deviation indicator (CDI) or horizontal situation indicator (HSI). Key elements:
- Omni-bearing selector (OBS): rotates the course card to select a desired course.
- CDI needle: shows lateral displacement from the selected course. Each dot represents approximately 2° of deviation, so full-scale deflection (5 dots) equals about 10°–12°.
- TO/FROM indicator: shows whether the selected course will take the aircraft TO or FROM the station.
- NAV/OFF flag: indicates an unreliable signal.
Identifying the Station
Before using a VOR for navigation, the pilot must positively identify it by listening to the Morse code identifier (or recorded voice ID). If the identifier is absent, garbled, or replaced by the word "TEST," the station is unreliable and must not be used. The frequency, identifier, and class are published in the Chart Supplement and on sectional/IFR charts.
Tracking To and From
The procedure for tracking a course consists of three steps:
- Tune and identify the station.
- Center the CDI with the OBS and note TO or FROM.
- Turn to the course shown under the index and correct for wind drift.
When tracking TO the station, fly the heading in the OBS window with a wind correction angle that keeps the needle centered. The CDI needle always shows the shortest direction to the selected course — if the needle deflects right, the course is to the right of the aircraft (when the heading roughly matches the selected course). This is sometimes called "flying toward the needle."
A bracketing technique works well: turn 20° toward the needle to intercept; once it begins to center, reduce the correction to 10°; once centered, establish a wind correction angle (typically 2°–10°) that holds the needle steady.
Station Passage and Reverse Sensing
Station passage is indicated by the first positive, complete reversal of the TO/FROM indicator. Directly over the cone of confusion (roughly 60° wide above the station), the CDI may oscillate; this is normal.
Reverse sensing occurs when the OBS course is set 180° opposite the aircraft's actual heading. The needle then indicates corrections opposite to reality (fly left to correct right). To avoid it, always set the OBS to approximately the course you intend to fly.
Accuracy Checks (14 CFR 91.171)
For IFR flight, the VOR must have been operationally checked within the preceding 30 days. Tolerances:
- VOT: ±4°
- Ground checkpoint: ±4°
- Airborne checkpoint: ±6°
- Dual VOR cross-check: ±4° between the two receivers
- Airway centerline check: ±6°
The pilot must log the date, place, bearing error, and signature.
Limitations
- Line-of-sight only — terrain, curvature of the earth, and low altitude block the signal.
- Cone of confusion directly overhead.
- Course sensitivity diverges with distance — 1° of course error equals about 1 NM of cross-track at 60 NM (the 1-in-60 rule).
- VORs are being decommissioned under the VOR MON program, leaving a network of approximately 590 stations to back up GPS outages.