Dr. Andrew Sarangan
The vast majority of pilots use the VOR as a command instrument – turns left when the needle deflects left, and turn right when the needle deflects right. Then they learn about reverse sensing. If the OBS is set to the reciprocal course, the commands become reversed - turn right when the needle deflects left, and turn left when the needle deflects right. Naturally, most prefer to stay away from reverse sensing. As a result, they do a lot of knob twisting to keep the desired number at the top of the OBS. Those who use the five-T’s mantra would recall that one of the T’s stands for twisting the OBS knob. This technique works fine for the most part, but there is a simpler and more elegant way. It is a less known fact that the original VOR receiver was designed as a course instrument. It was not designed as a “fly left” “fly right” indicator. This is why the needle was called a Course Deviation Indicator. This is also why there is a full circle of numbers on the face of the VOR. The needle points to the hemisphere where the selected course lies. The triangle pointer (also called the TO/FROM flag) points to the hemisphere where the station lies. Together, they point to a quadrant of headings that will intercept the desired course. Perhaps this is best illustrated through a few examples.
Let’s say we are flying a VOR approach, and the final
approach course is 210. And let’s also assume that the
VOR station is on the field. You want to intercept the 210
radial and fly inbound towards the station. When we select
210 on the OBS, the needle deflects as shown. Which
heading should we fly? Pause for a moment and think
about how we normally do this. Most pilots would turn the
OBS until the needle centers, get their current radial
position, form a mental image of their relative position to the station, and then determine which heading to fly. This works fine, but it is a lengthy process and takes too much mental effort. The alternative method is a lot simpler, and requires less handwork and brainwork. Look at all the numbers along the needleside (left-side) of the VOR face. The needle is pointing to a hemisphere of headings between 030 and 210. Turn to any one of those headings, and the needle will eventually center. Yes, it is really that simple! The fastest way to get there of course is to fly 120, which is directly against the needle. This will make a 90-degree intercept to the desired course. All other headings will intercept the course at a shallower angle. Now look at the triangle pointer. It points to a hemisphere of headings between 300 and 120. This is where the station lies. In order to fly towards the station and intercept the selected course, we need to pick a heading from the bottom left quadrant of the VOR. For example, 080 would be a good heading to fly. Once the needle centers, its hemisphere collapses to just two numbers – 030 and 210. Of these two numbers, only 030 lies in the direction of the station. This is the heading we need to fly to track the course towards the station.
Let’s look at a second example: We want to intercept the
selected course and track outbound from the station. This
time we have to look at the needle and the tail of the arrow. They point to a quadrant of headings between 280 and
010. Pick one and fly it. Whether we turn left or right is
immaterial. What matters is that we turn to the desired
heading. When the needle centers, fly 280 to track
Notice that it doesn’t matter which number we put at the top of the OBS. We could put the desired course, or its reciprocal, and the indications will not change. There is no reverse sensing.
Here is another scenario. We are cruising along an airway. VOR1 is set to the airway radial, and VOR2 is set to an
intersecting radial from another VOR. We want
to know whether we have passed that
intersection or still headed towards it. Here is
how to do this in less...