The acronym VOT in aviation stands for VOR Test Facility. This specialized ground station is a resource for pilots to verify the accuracy of their aircraft’s navigation equipment before flight. The reliability of navigation instruments is paramount for safe air travel, especially when flying under Instrument Flight Rules (IFR). The VOR Test Facility provides a highly precise, known signal that allows a pilot to confirm the operational status of their onboard VOR receiver.
What is a VOR Test Facility (VOT)?
A VOR Test Facility is a dedicated ground transmitter that broadcasts a single, omnidirectional test signal on a specific frequency. The sole purpose of this signal is to allow pilots to check the accuracy of their aircraft’s Very High Frequency Omnidirectional Range (VOR) receiver. The VOR system itself is a primary form of radio navigation that transmits 360 electronic courses, or radials, from a station on the ground.
The VOT signal is unique because it is calibrated to produce a specific, known reading on the aircraft’s VOR indicator, regardless of the aircraft’s physical position on the airport ramp. This eliminates the variable of location, making the test straightforward and highly reliable. The test is designed to confirm that the aircraft’s VOR receiver is interpreting the signal correctly, with a maximum permissible error of plus or minus four degrees. This strict tolerance ensures the navigation equipment is precise enough for the demands of instrument flight.
The Step-by-Step VOR Check Procedure
The VOR check procedure using a VOT is a simple, two-step process performed from the cockpit while the aircraft is stationary on the ground. The pilot first tunes the aircraft’s VOR receiver to the published VOT frequency and identifies the station’s signal to confirm it is active. The test then involves manipulating the Omni Bearing Selector (OBS) knob and observing the Course Deviation Indicator (CDI) needle and the TO/FROM flag.
For the first check, the pilot rotates the OBS until the CDI needle centers and the TO/FROM indicator displays a “FROM” flag. If the receiver is accurate, the OBS should indicate 0 degrees, or 360 degrees, with the centered needle and the “FROM” flag. The second part of the check requires the pilot to rotate the OBS to 180 degrees. At this setting, the CDI needle should again center, but the TO/FROM indicator must now display a “TO” flag.
The receiver is considered accurate if the CDI centers with the OBS set to 0 degrees (FROM) and 180 degrees (TO). The indicated bearing error must be no more than four degrees from the expected setting. This check is a regulatory requirement for pilots operating under Instrument Flight Rules, and it must be performed and logged within the 30 days preceding an IFR flight using the VOR system.
Finding and Understanding VOT Limitations
Information regarding the location and frequency of VOR Test Facilities is published in the Chart Supplement U.S., which was formerly known as the Airport/Facility Directory. Pilots must consult this resource to find the specific frequency for a VOT at a given airport, as not all airports are equipped with this facility. The Chart Supplement also provides any specific instructions or limitations for using the VOT at that location.
Most VOTs are designed for use only while the aircraft is on the ground, typically on the ramp or a designated run-up area. However, some facilities are authorized for airborne use, though this is strictly limited to specific areas and altitudes detailed in the Chart Supplement. In situations where a VOT is unavailable, pilots can use other approved methods to check their VOR accuracy, such as designated VOR ground checkpoints or by comparing the readings of two separate VOR receivers tuned to the same station.