Intercepting a course means to get on the course and start tracking it.
Tracking a course means to stay on the desired track while correcting any deviation, mainly caused by crosswind.
Intercept the course
Tune and identify
Before using a VOR for navigation, you must tune its frequency and make sure you are receiving the correct station.
Equipment varies, but this is the general procedure for identifying the VOR:
- Dial-in the VOR frequency
- Listen to and confirm you have the correct Morse code identifier.
- Check that no warning flags are present on the VOR display.
Newer equipment can automatically tune the VOR after you enter the identifier letters. Some units can automatically decode the station's id and display its letters on a screen.
1. Find your current position relative to the station
Rotate the Omni-Bearing Selector (OBS) until the needle centers. It is best to select the bearing to or from the staion that corresponds to the direction inbound or outbound of the desired track.
The bearing displayed on the top of the dial is your current bearing to or from the station.
2. Choose an intercept angle
To calculate an intercept angle, double the difference between the bearing to intercept and the current bearing to the station. If tracking inbound, use 'TO' bearings, otherwise, use 'FROM' bearings ("radials"). Cap the result at a minimum of 20º and a maximum of 90º.
Intercept Angle = |current bearing - desired bearing| x 2 20º ≥ Intercept Angle ≥ 90º
The FAA's Instrument Flying Handbook uses the above method. However, with experience, you may choose other methods, as long as the intercept angle is not larger than 90º. Several factors will affect this choice, including distance to the station, radial difference, and groundspeed.
So for example, you are on the 300º bearing TO the VOR and want to intercept the 350º inbound track:
Intercept angle = |300-350| x 2 = 50º x 2 = 100º
Since the result is larger than 90º, cap the intercept angle at 90º:
100º > 90º Intercept angle = 90º
3. Select the desired course to track
Rotate the OBS to the desired course. In our example, the desired course is 350º bearing to the station (the 170º radial inbound), so set the OBS to 350º.
4. Fly the intercept heading
Note to which side the needle deflected (left or right.) To intercept the course, fly towards the needle with the intercept angle you chose in step 2.
For this example, you are on the 300º degrees bearing to the station, the desired track is 350º inbound and you determined the intercept angle is 90º. You can now find the intercept heading:
350 - 90 = 260º
Note about reverse sensing
*This assumes you are flying in the general cardinal direction to match the desired track. Otherwise, you would get reverse sensing: CDI deflected in the opposite direction of the desired course. Avoid this by matching the OBS direction to the general direction you are trying to track. With an HSI, this problem is eliminated when tracking a VOR course.
5. Complete the intercept
As the CDI centers, begin turning towards the heading corresponding to the desired course. Choosing how fast or far to lead the turn may take some practice. Factors to consider for this turn include the distance from the VOR, the intercept angle, and the ground speed.
Track the course
The goal here is to keep the CDI needle centered.
In perfect, no-wind, conditions, the needle should remain centered while the heading matches the OBS.
However, perfect conditions are rare in the real world, and the CDI may start deviation to the left or right.
The direction to which the CDI deflects is the direction from which the crosswind is blowing. A way to remember this is to imagine the needle direction as the wind side: "The wind is the needle."
Reverse sensing, again
*Using a traditional, non-HSI VOR, you should be wary of reverse sensing while tracking the course. With reverse sensing, instead of "the wind is the needle," it becomes "you are the needle." Meaning, if the CDI goes right, you'll have to turn away from it to the left. To prevent this, use an OBS bearing that matches the direction you are going to, not its opposite.
If you don't know the wind direction, you'll have to find the best correction by trial and error. Otherwise, an E6B, FMS or a electronic flight computer can calculate it for you.
Fly a heading that is slightly into the wind to get back on track. Then decrease the correction angle to stay on it.
Tracking towards the VOR
As you get closer to the station, the CDI needle will become more sensitive. Once you pass over it, you may momentarily see a full deflection of the needle.
To keep on track, do not chase the needle. Keep the same heading that worked before. As you get further away, tracking outbound now, the needle should start moving back to the center.