Most everyone has heard of a VOR accuracy check. But have you ever heard of a VOR sensitivity check? It is possible to have a VOR that's accurate within the required tolerance for IFR flight, yet have wildly varying degrees of sensitivity.
Figure 1 shows the VOR's "dot" scale. Each dot represents two degrees of deviation from the course selected by the OBS (Omni Bearing Selector). The small circle in the center is sometimes called the donut (police officers really like this term). The donut represents the first two degrees of course deviation followed by another four dots, for a total of 10 degrees of measurable course deviation.
Figure 2 shows the four step process that I use in performing a VOR sensitivity check. Begin by tuning the VOR receiver to any strong signal. If a VOT (VOR Test Signal) is available, this also works well for sensitivity checks.
Step one: Rotate the OBS and center the needle with either a TO or a FROM indication.
Step two: Read the course value shown under the course index (the white triangle). Our VOR shows a value of 360 degrees (0 degrees).
Step three: Rotate the OBS an additional 10 degrees (right or left) from its previously selected value. Let's rotate the OBS until 010 degrees is shown under the course index.
Step four: Observe the number of dots the CDI (the needle) has moved from the center position. Our example shows that the CDI moved five dots. Since each dot represents two degrees of course deviation, our VOR in this example demonstrates perfect sensitivity.
Figure 3A and 3B show examples of an under sensitive and over sensitive VOR indication. So what does this mean to you as a pilot?
With an under sensitive indication, you may be farther from the airway centerline than you think you are. The CDI doesn't show your true angular deviation from the course centerline you selected.
In Figure 3A, the VOR has a six degree sensitivity error. In other words, if you?re flying with a two dot (4 degree) needle deflection, you may actually be 10 degrees off course.
Figure 3B shows an over sensitive VOR indication, which is equally troublesome. In this instance, you may be closer to the airway centerline than you suspect. Is that such a bad thing? It is if you're planning to use your VOR to navigate around Class B airspace.
Many sections of Class B airspace are identified by VOR radials. Suppose you're planning to descend below the progressively lowering floor of this airspace. You select a VOR radial which identifies the lower wall of this airspace and plan on commencing your descent when the CDI indicates a two dot deflection. You're thinking that this gives you enough time to avoid entry into Class B airspace. With an over sensitive VOR indication, a two dot deflection could actually represent less than two dots worth of angular deviation from the wall of this airspace. In other words, you're closer to this airspace than you think you are and may not descend quickly enough to avoid penetrating it.
When VORs are bench tested at the avionics shop, they're usually sensitive within (+) or (-) one degree. But this doesn't mean that they remain accurate over time. So, make it a point to do a sensitivity check before your next flight.
For more information on this subject, see "Needle Mania! How To Use The VOR."
