AIM Text
- The FAA VOR test facility (VOT) transmits a test signal which provides users a convenient means to determine the operational status and accuracy of a VOR receiver while on the ground where a VOT is located. The airborne use of VOT is permitted; however, its use is strictly limited to those areas/altitudes specifically authorized in the Chart Supplement or appropriate supplement.
- To use the VOT service, tune in the VOT frequency on your VOR receiver. With the Course Deviation Indicator (CDI) centered, the omni-bearing selector should read 0 degrees with the to/from indication showing “from” or the omni-bearing selector should read 180 degrees with the to/from indication showing “to.” Should the VOR receiver operate an RMI (Radio Magnetic Indicator), it will indicate 180 degrees on any omni-bearing selector (OBS) setting. Two means of identification are used. One is a series of dots and the other is a continuous tone. Information concerning an individual test signal can be obtained from the local FSS.
- Periodic VOR receiver calibration is most important. If a receiver's Automatic Gain Control or modulation circuit deteriorates, it is possible for it to display acceptable accuracy and sensitivity close into the VOR or VOT and display out-of-tolerance readings when located at greater distances where weaker signal areas exist. The likelihood of this deterioration varies between receivers, and is generally considered a function of time. The best assurance of having an accurate receiver is periodic calibration. Yearly intervals are recommended at which time an authorized repair facility should recalibrate the receiver to the manufacturer's specifications.
- Federal Aviation Regulations (14 CFR section 91.171) provides for certain VOR equipment accuracy checks prior to flight under instrument flight rules. To comply with this requirement and to ensure satisfactory operation of the airborne system, the FAA has provided pilots with the following means of checking VOR receiver accuracy:
- VOT or a radiated test signal from an appropriately rated radio repair station.
- Certified airborne checkpoints and airways.
- Certified checkpoints on the airport surface.
- If an airborne checkpoint is not available, select an established VOR airway. Select a prominent ground point, preferably more than 20 NM from the VOR ground facility and maneuver the aircraft directly over the point at a reasonably low altitude above terrain and obstructions.
- A radiated VOT from an appropriately rated radio repair station serves the same purpose as an FAA VOR signal and the check is made in much the same manner as a VOT with the following differences:
- The frequency normally approved by the Federal Communications Commission is 108.0 MHz.
- Repair stations are not permitted to radiate the VOR test signal continuously; consequently, the owner or operator must make arrangements with the repair station to have the test signal transmitted. This service is not provided by all radio repair stations. The aircraft owner or operator must determine which repair station in the local area provides this service. A representative of the repair station must make an entry into the aircraft logbook or other permanent record certifying to the radial accuracy and the date of transmission. The owner, operator or representative of the repair station may accomplish the necessary checks in the aircraft and make a logbook entry stating the results. It is necessary to verify which test radial is being transmitted and whether you should get a “to” or “from” indication.
- Airborne and ground check points consist of certified radials that should be received at specific points on the airport surface or over specific landmarks while airborne in the immediate vicinity of the airport.
- Should an error in excess of plus or minus 4 degrees be indicated through use of a ground check, or plus or minus 6 degrees using the airborne check, Instrument Flight Rules (IFR) flight must not be attempted without first correcting the source of the error.
- Locations of airborne check points, ground check points and VOTs are published in the Chart Supplement.
- If a dual system VOR (units independent of each other except for the antenna) is installed in the aircraft, one system may be checked against the other. Turn both systems to the same VOR ground facility and note the indicated bearing to that station. The maximum permissible variations between the two indicated bearings is 4 degrees.
Source: FAA Aeronautical Information Manual · current edition · paragraph 1-1-4.
The short answer
This AIM section is FAA advisory guidance explaining how pilots check VOR receiver accuracy using a VOR test facility (VOT), certified ground or airborne checkpoints, a radiated test signal, or a dual-VOR cross-check. It directly supports the IFR accuracy-check requirement of 14 CFR 91.171, describing acceptable tolerances and the procedures for verifying the airborne system before IFR flight.
Research Notes
Common Questions
What VOR error tolerance is allowed on a ground versus airborne check?
How do I use a VOT to check my VOR receiver?
Tune the VOT frequency. With the CDI centered, the OBS should read 0 degrees with a "from" indication, or 180 degrees with a "to" indication. On an RMI, it will indicate 180 degrees on any OBS setting. The AIM notes a VOT supports the 14 CFR 91.171 accuracy check.
What is the maximum allowable difference when checking a dual VOR system?
Per the AIM, with a dual-system VOR, one system may be checked against the other by tuning both to the same VOR ground facility and noting the indicated bearing. The maximum permissible variation between the two indicated bearings is 4 degrees.