VOR Navigation Reference
Radial structure, CDI & HSI operation, TO/FROM flag logic, errors, and DME integration — complete VHF omnidirectional range reference for PPL through ATPL.
What is VOR?
A VOR (VHF Omnidirectional Range) is a ground-based radio navaid that transmits on frequencies between 108.0–117.95 MHz. Aircraft receivers lock onto the station and determine their radial — the magnetic bearing from the station to the aircraft.
- 360 radials radiate outward from each beacon; Radial 090 lies to the east of the station
- Each radial represents a magnetic bearing FROM the station — not toward it
- To fly outbound on R090, track 090°M; to fly inbound, track 270°M (the reciprocal)
- Stations are identified by a 3-letter Morse code ident broadcast continuously (e.g.
LONfor London VOR) - Always identify a VOR before using it for navigation — never rely on frequency alone
VOR Types
| Type | Designator | Service Volume | Notes |
|---|---|---|---|
| Terminal VOR | T | 25 nm | Approach & terminal area use |
| Low Altitude VOR | L | 40 nm | Below FL180 |
| High Altitude VOR | H | 130 nm (above FL450: 100 nm) | En-route IFR airways |
DVOR: Most modern VORs are Doppler VORs (DVOR). They use the Doppler principle rather than a rotating pattern, achieving better accuracy (±1°). The airborne receiver and associated procedures are identical to conventional VORs. Many are collocated with DME equipment, forming a VOR/DME or VORTAC station.
VOR radials radiate FROM the station. R090 = east of station. Aircraft on R090 tracks 090° outbound or 270° inbound.
CDI Operation
The Course Deviation Indicator (CDI) is the primary cockpit instrument for VOR navigation. It consists of a vertical needle that deflects left or right to indicate deviation from your selected course.
- OBS (Omnibearing Selector): rotate to set the desired radial or inbound course
- TO flag: if you fly the selected course, you will fly toward the station
- FROM flag: if you fly the selected course, you will fly away from the station
- Full-scale deflection = 10° off course; each dot represents 2° deviation
- Fly toward the needle: if the needle is right, turn right to intercept
Reverse sensing: If you fly a heading that is the reciprocal of the OBS setting and a FROM flag is showing, the CDI needle indicates in reverse — turning toward the needle will take you further off course. Always confirm the correct TO/FROM flag for your phase of flight.
HSI Operation
The Horizontal Situation Indicator (HSI) combines a directional gyro with a CDI. It gives a more intuitive picture of the aircraft's position relative to the selected course.
- A course arrow is set to the desired track using the course knob
- The aircraft symbol (or lubber line) shows the actual heading relative to the course arrow
- The CDI needle deflects left/right as on a standard CDI — same 10° full-scale deflection
- A TO/FROM arrowhead on the course arrow confirms bearing sense
- The "picture" directly shows whether you are left or right of course and by how much — eliminating the reverse sensing confusion of a basic CDI
VOR Errors
| Error Type | Cause | Effect |
|---|---|---|
| Scalloping | Multipath interference, terrain reflections, poor station | Rapid CDI needle oscillation; use position for information only |
| Cone of silence | No usable signal directly overhead the station | Brief unreliable indication; normal once past station |
| Twilight effect | Ionospheric interference at dawn/dusk | Increased random errors; most pronounced on lower frequencies |
| Ground station error | Station calibration tolerance | ±1° (Class A DVOR); ±2° conventional VOR |
| Airborne error | Receiver tolerance, installation | ±3° typical; total VOR system error ±4° |
VOR Accuracy Check Requirements
Before conducting an IFR flight using VOR, pilots must verify VOR receiver accuracy. Maximum permissible errors:
- VOR ground checkpoint: ±4°
- VOR airborne checkpoint: ±6°
- Dual VOR check (two receivers on same VOR): within 4° of each other
- Check must be logged with date, location, bearing error, and signature
DME — Distance Measuring Equipment
DME provides slant-range distance from the aircraft to the ground transponder, typically collocated with a VOR. When tuning a VOR/DME or VORTAC, DME is automatically paired — no separate frequency entry is required.
- Operates on UHF (962–1213 MHz); interrogation/reply principle
- Displays slant range (not ground distance) — includes altitude in the calculation
- Slant-range error is negligible beyond approximately 10 nm from directly overhead
- Also computes groundspeed and time-to-station when tracking inbound at constant speed
Slant range example: At 10,000 ft directly above a DME station, the receiver reads approximately 1.6 nm (10,000 ft ÷ 6,076 ft/nm) even though ground distance is 0. At 30 nm from the station at 10,000 ft, the slant-range error is less than 0.1 nm — operationally negligible.