Every so often pilots confess to having difficulty keeping track of where they are during IFR flight. In particular, this problem seems to occur most often when being radar vectored. I suspect there are several reasons for this difficulty, one of which deals with a pilot's inability to interpret his or her VOR indicator properly. Here are a few questions that any instrument student should be able to answer. If you're stumped, then it's time to talk to your instructor about VOR basics.
Answer to Question 1: Fly a heading of 330 degrees.
Explanation: First, the airplane's present heading is irrelevant to this question. This is an important point about VOR orientation. To determine the heading necessary to intercept a VOR course or radial, you only need to look at the VOR indicator, nothing else. Look at the present value placed under the index (the green triangle in Figure 1). Next, look at the position of the CDI (the needle) and the ambiguity indicator (the TO/FROM/OFF flag). Finally, there are two ways to communicate the idea of traveling to or from a VOR station. In this example, you can travel to the station on the 210 degree radial, which is the same thing as traveling to the VOR station on the 030 degree course. Either expression is correct. Therefore, if you fly heading 030 degrees, you'll fly parallel to the 030 course and the course will be on the left side of your airplane (we'll assume zero wind for these examples). To intercept this course at a 60 degree angle, you need to turn 60 degrees to the left of 030 degrees (simply count 60 degrees to the left of the value set under the index). This puts you on a heading of 330 degrees.
Question 2: Your airplane is on a heading of 310 degrees. Based on the VOR display in Figure 2, if you want to intercept the 080 degree radial (and track it inbound) to the station at a 30 degree angle, how many degrees to the left must you turn?
Answer to Question 2: Turn 20 degrees to the left of your present heading.
Explanation: In this case the airplane's heading is relevant to the question. Flying the 080 degree radial to the station is also referred to as flying the 260 degree course to the station. Flying a heading of 260 degrees allows you to parallel the course with the course located on the right side of the airplane. A heading of 290 degrees allows you to intercept the course at a 30 degree angle. Since you're presently flying 310 degrees, you need to turn 20 degrees to the left to arrive on the intercept heading of 290 degrees. Once again, you can determine your position (right or left of a course or radial) as well as the desired intercept heading by looking only at the compass rose on the face of the VOR indicator.
Question 3: Your airplane is on a heading of 270 degrees. Based on the VOR display in Figure 3, what heading must you fly if you want to intercept (and track outbound on) the 070 degree radial from the station at a 25 degree angle?
Answer to Question 3: Fly a heading of 095 degrees.
Explanation: The airplane's present heading is irrelevant to this question. Flying a heading of 070 degrees allows you to fly parallel to the 070 degree radial, with the radial to the right of the airplane. Turning further right an additional 25 degrees to intercept, places you on a heading of 095 degrees.
Question 4: What heading should you fly to intercept the 330 degree radial from the station in the shortest period of time?
Answer to Question 4: Fly a heading of 240 degrees.
Explanation: Flying a heading of 330 degrees allows you to fly parallel to the 330 degree radial. A 90 degree intercept angle provides the shortest route to the selected course or radial. If you look at the compass rose on the face of the VOR indicator, you'll see that 240 degrees is the heading that's 90 degrees to the selected radial. Therefore, fly a heading of 90 degrees to intercept this radial in the shortest possible time.
Summary of This Lesson
Here's the point of these questions. You should immediately be able to identify the headings necessary to intercept any radial or course by looking at the CDI's position (right or left of center), then glancing at the compass rose on the face of the VOR indicator. Flying the heading shown under the index always allows you to fly parallel to the selected course (zero wind assumed). Intercepting the course is simply a matter of turning the specified number of degrees in the direction of the CDI.
Here's the big payoff. Suppose you're being vectored for an approach shown below in Figure 5. This approach course consists of the 080 degree radial from BRD VOR to Yazoo airport. The missed approach point is identified by the 355 degree radial from RKT VOR. Assume ATC is vectoring you to the approach course (the 080 degree radial) on a heading of 030 degrees. Knowing that VOR No.1 shows a left needle with a FROM indication, you must be to the right of the 080 degree radial. In other words, you haven't intercepted the approach course yet. Knowing that VOR No.2 shows a right needle with a FROM indication, you must be to the left of the 355 degree radial. In other words, you haven't crossed the missed approach point yet (which is good since you're not even on the approach course yet). Therefore your airplane must be in the general vicinity of Airplane A as shown in Figure 5. This is about as precise as you need to be in identifying your position using VOR (you could be more precise by rotating the OBS and centering the needles for an exact bearing fix, but this is often unnecessary for most IFR flying).
Granted, all this may seem fairly simply, but there are a lot of instrument students who have great difficulty interpreting their VOR indicators in this manner. So, take some time and review the VOR basics if you're having trouble orienting yourself during an instrument approach.
For more information on this subject, see "Needle Mania: How To Use The VOR," "Checkride: Joining Up," and "Instrument Training: Understanding The VOR."
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