VORs allow for more accurate navigation and provide an easier display to interpret. VORs are assigned the within the frequency range of 108.0-117.95 MHz (however, some of these frequencies are reserved for localizers). The signals transmitted by a VOR allow the receiver to determine its bearing from the station. Note that when navigating by reference to NDBs, the pilot references bearings to the station; VORs use bearings from the station. Bearings from a VOR station are referred to as “radials”. Deviation from a VOR radial is displayed on a course deviation indicator (CDI). More advanced aircraft use a horizontal situation indicator (HSI).
Course Deviation Indicator (CDI)
Most aircraft equipped for IFR flight have two VOR receivers and two CDIs, which look like this:
The needle oriented vertically on the indicators displays deviation from the course selected. Full scale deflection in either direction is reached when the aircraft is offset by 10 degrees from the selected course. The VOR receivers in the example above are tuned to two different VOR stations. Notice that the upper one has a course of 308° selected; because the needle is centered that means you’re on a course of 308° to or from the station. To determine if it’s to or from, you need to look at the ambiguity indicator which is marked with the “A” on both CDIs. The upper CDI’s ambiguity indicator is pointing up (which indicates a course toward the station); that means a 308° course will take you directly to the station (in this case, the needle would also center with a course of 128° selected; with 128° selected the ambiguity indicator would reverse because this would be the course directly from the station). The lower CDI is set to a course of 033°, and its ambiguity indicator is pointing down (or from the station). That means that a course of 033° will take you directly away from the station (in this case, the needle would also center with a course of 213° selected, with 213° selected the ambiguity indicator would reverse because that would be the course directly to the station). Like we discussed above, VOR courses are referred to as “radials”. The radial is always a course from the station; the upper CDI is displaying a course to the station, so the radial in that case is the value shown at the bottom of the instrument (128°); the lower CDI is displaying a course from the station, so the radial in that case is the value shown at the top of the instrument (033°). If the pilot were to fly a course of 308°, he would be tracking the 128° radial inbound to the station displayed on the upper CDI. If the pilot were to turn to a course of 033°, he would be tracking the 033° radial away from the station displayed on the lower CDI.
Here are some vor simulators you can practice on.