When you review TC's sample PPL exam you will see a few questions like the following:
With the VOR receiver tuned to the Coehill VOR (N44º40’ W77º50’), when you are over the town of Marlbank (N44°26’ W77°05’) the CDI should be
- centred with a “FROM” indication when the OBS is 123°.
- centred with a “FROM” indication when the OBS is 303°.
- deflected full left when the OBS is 123°.
- deflected full right when the OBS is 303°.
How do you go about solving the issue above? First you need to know where you are on the map in relation to the provided points of reference. Then, you will need to understand how VOR navigation works and where you are in relation to the given information.
You will however need to understand how the VOR works. Hopefully, below you will find an easy way to understand it.
VOR is short for VHF Omni-directional Radio Range, is a radio beacon located on the ground that transmits a signal which contains azimuth or the bearing with respect to magnetic north the station is from the aircraft. The most challenging for me is how to read the actual instrument...
The round dial with selectable azimuth information is the Omni Bearing Selector (OBS). The 'needle' is the Course Deviation Indicator (CDI). That needle will move/deviate from the centre to either left or to the right when the airplane gots off course.
When the aircraft is left of the course, the CDI needle will deviate to the right, showing the pilot which direction the desired course is located.
If the needle is to the LEFT, turn the plane to the LEFT to get back on course.
The TO and FROM indicator tells you if the VOR station is in front of you or behind you in the relation to the radial you set in the OBS. If you are tracking a 000 radial and the station is ahead of you the indicator will say TO. However, if you have the reciprocal radial of 180 set, then you will show a FROM heading.
You won't need to know the following for the exam but I think it will be certainly useful in practice.
A series of horizontal dots is shown on the instrument dial so that one can tell how much of a deviation you are tracking. Every dot represents a 2° deviation off your intended course. This will tell you how lost you are but also which new heading to choose to get back on course.
Here is a great simulation that helps you to visualize.
Here is a good Youtube video that explains the above.