5.vor-receiver-check-procedure. VOR Receiver Accuracy Check
Before a VOR can be used for IFR navigation, the pilot must verify its accuracy. Under 14 CFR 91.171, no person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment has been operationally checked within the preceding 30 days and was found to be within acceptable tolerances. The pilot must also log the date, place, bearing error, and signature in the aircraft logbook or other reliable record.
The FAA authorizes several methods of accomplishing this check. Each has its own tolerance, and the choice usually depends on what is available at the departure airport.
1. VOT (VOR Test Facility)
The VOT transmits an omnidirectional 360° radial. Regardless of the aircraft's position on the airport, with the OBS set to 360°, the TO/FROM indicator must read FROM and the CDI must center. With the OBS set to 180°, the indicator must read TO. A handy memory aid is "Cessna 182" — Center, 180°, TO. The maximum permissible bearing error using a VOT is ±4°. VOT frequencies and locations are published in the Chart Supplement U.S.
2. Ground Checkpoint
Designated ground checkpoints are surveyed points on the airport surface (often marked by a painted circle or sign) from which the published radial and distance to a specific VOR are known. The aircraft is taxied onto the checkpoint, the published radial is set, and the CDI must center with the proper TO/FROM indication. Maximum bearing error: ±4°. Locations and frequencies are listed in the Chart Supplement.
3. Airborne Checkpoint
Airborne checkpoints are prominent landmarks (a road intersection, water tower, etc.) over which the aircraft flies at a specified altitude. The published radial from a specific VOR is then verified. Maximum bearing error: ±6°. These are also listed in the Chart Supplement.
4. Airway/Centerline Check (no published checkpoint available)
If no VOT, ground, or airborne checkpoint is available, the pilot may select a prominent ground point along a designated VOR airway centerline more than 20 NM from the VOR, fly over it at an altitude at least 1,000 feet AGL, and verify the published airway radial. Maximum bearing error: ±6°.
5. Dual VOR Check
If the aircraft is equipped with two independent VOR receivers (separate antennas and circuitry), the pilot may check one against the other. Both units are tuned to the same VOR ground station, and the OBS on each is centered with the same TO or FROM indication. The two indicated bearings must agree within 4° of each other. This check satisfies the 30-day requirement.
Logging the Check
Whichever method is used, the entry must include:
- Date of the check
- Place (e.g., "KAPA VOT," "ground checkpoint Rwy 17R run-up," "over Smith Bridge")
- Bearing error (e.g., "+2°" or "0°")
- Signature of the person performing the check
A typical entry reads: "7/14 — DEN VOT — 0° — J. Pilot." Without this entry, the airplane is not legal for IFR enroute or approach navigation on VOR.
Tolerance Summary
| Method | Tolerance |
|---|---|
| VOT | ±4° |
| Ground checkpoint | ±4° |
| Airborne checkpoint | ±6° |
| Airway centerline (≥20 NM, ≥1,000 ft AGL) | ±6° |
| Dual VOR (one against the other) | 4° between units |
Practical Procedure at the VOT
- Tune the VOT frequency listed in the Chart Supplement.
- Identify the station — VOT transmits a continuous series of dots or a steady tone (no Morse identifier like a normal VOR).
- Rotate the OBS to 360°; verify CDI centered, FROM flag.
- Rotate the OBS to 180°; verify CDI centered, TO flag.
- Note any bearing error and log it.
Common Errors
- Forgetting that the airborne checkpoint tolerance is ±6°, not 4°.
- Using a VOR on an aircraft whose last receiver check is older than 30 days — the aircraft cannot legally be flown IFR on VOR until a new check is performed and logged.
- Confusing the VOT identifier (steady tone/dots) with a station out of service.
- Forgetting that the 1,000 ft AGL requirement on the airway-centerline method is a minimum above terrain and obstructions, not above MSL station elevation.
Keeping a clean VOR check log is one of the simplest airworthiness items a pilot controls directly, and DPEs routinely ask to see it during the IFR practical test.