A VOT (VOR Test Facility) is a special ground-based transmitter that lets pilots check whether their VOR navigation receiver is working accurately. Unlike a regular VOR station, which broadcasts directional signals used for in-flight navigation, a VOT exists solely for equipment testing. It transmits a single, known signal so pilots can compare what their instruments display against what they should display, revealing any error in the receiver.
How a VOT Differs From a Regular VOR
A standard VOR station sends out a rotating directional signal that changes depending on your position relative to the station. That’s what makes VOR navigation work: your receiver figures out which radial you’re on based on the signal it picks up.
A VOT does something much simpler. It replaces that variable directional signal with a second omnidirectional signal, effectively transmitting the equivalent of the 360-degree radial in every direction. No matter where you are relative to the VOT, your receiver should read the same thing. That predictability is exactly what makes it useful as a test. If your instruments show something other than the expected reading, you know your receiver has an error, and you can measure how much.
How to Use a VOT
The procedure is straightforward. Tune your VOR receiver to the VOT frequency listed in the Chart Supplement (formerly the Airport/Facility Directory) for that airport. Turn up the volume to identify the station, which broadcasts a series of dots or a continuous tone. Then rotate the omni-bearing selector (OBS) to center the course deviation indicator (CDI) needle.
When the needle is centered, you should see one of two correct readings:
- 0 degrees with a FROM indication
- 180 degrees with a TO indication
A common memory aid pilots use is “Cessna 182,” linking 180 and TO together. If your aircraft has a radio magnetic indicator (RMI) instead, it should point to 180 degrees regardless of the OBS setting. Any deviation from these expected readings tells you how many degrees of error your receiver has.
Allowable Error Limits
Federal regulations set a maximum permissible error of plus or minus 4 degrees when using a VOT. That’s the same tolerance as a designated ground checkpoint at an airport. For comparison, if you use an airborne checkpoint or fly over a known point along an established airway, the allowable error loosens to plus or minus 6 degrees. If your aircraft has two independent VOR systems, you can check them against each other by tuning both to the same VOR station: the maximum permissible difference between the two readings is 4 degrees.
These tolerances matter because even small bearing errors compound over distance. A receiver that’s off by a few degrees might put you miles from where you expect to be after flying a long leg, which is a serious problem during instrument approaches or when navigating in low visibility.
When VOR Checks Are Required
Under 14 CFR 91.171, you cannot fly under instrument flight rules (IFR) using VOR navigation unless your VOR receiver has been checked within the preceding 30 days and found to be within the allowable error limits. This applies every time you plan an IFR flight that relies on VOR. Visual flight rules (VFR) flights don’t carry this requirement, but checking your equipment is still good practice.
The regulation gives pilots several options for completing the check, listed in order of preference: a VOT or approved test signal, a designated ground checkpoint, a designated airborne checkpoint, or a self-check using a VOR airway and prominent ground point. The VOT is generally the most convenient and precise option when one is available at your departure airport.
Each check must be logged. Pilots are required to record the date, the location of the check, the bearing error found, and sign the entry. Without that logbook record, the check doesn’t count for regulatory purposes, even if you performed it.
Where to Find VOT Locations
Not every airport has a VOT. The facilities are listed in the Chart Supplement U.S., which notes the VOT frequency and any limitations on where at the airport the signal can be reliably received. Some locations may exclude certain ramp areas or taxiways where the signal is obstructed or unreliable.
VOTs are primarily designed for ground use, but airborne use is permitted in specific areas and altitudes when authorized in the Chart Supplement. If you’re planning to use one in the air, check the publication for those restrictions before relying on it. As VOR stations are gradually decommissioned in favor of GPS-based navigation, the number of available VOT facilities has decreased, making it worth confirming availability during preflight planning.