The Very High Frequency Omni-Directional Radio Range (VOR) is a ground based radio navigation aid that has an infinite number of radials that radiate outwards. In practice, we use 360 radials in relation to magnetic north.
The aircraft’s VOR receiver can determine which radial the aircraft is on and whether flying that track would take you towards (TO) or away from (FROM) the VOR.
The radials only radiate outwards, so the heading or track of the aircraft may be completely different from the radial it is on.
In the example below, the aircraft is flying towards the VOR on the 180° radial but it is flying a track of 360° (QDM=360°).
The pilot twists the Omni-Bearing Selector (OBS) knob on the Course Deviation Indicator (CDI) to select a radial from or bearing to the VOR. The needle will be centred when the aircraft is on that radial.
In this example, the pilot has selected the 090° (east) radial and flying a track of 090° will take the pilot away from the VOR (shown by the FROM indication). The needle (vertical bar) is centered when the aircraft is on the selected radial.
The centre circle can be thought of as the aircraft and the needle as the chosen radial. If the aircraft moves away from the chosen radial, the needle will indicate where the radial is relative to the aircraft.
The VOR receiver works independently of the aircraft heading. The indications on the cockpit instrument will only ‘make sense’ if your heading and the OBS selection are roughly the same.
In other words, the instrument is designed to give correct indications to a pilot who is flying along the selected radial from or bearing to the VOR.
The TO and FROM indications tell the pilot whether their OBS selection will take the pilot to the VOR or away from it.
When tracking along a radial inbound to a VOR, the TO indication changes to FROM as you fly overhead. During this change, a brief period of inaccuracy exists as you fly overhead and a warning flag may appear on the cockpit instrument.
The warning flag indicates inadequate signal or loss of power and the instrument cannot be used when the warning flag is showing.
Each dot on the CDI shows 2° of deviation from the chosen radial, up to the last dot which shows 10° or more.
So in the example here, the needle is telling the pilot they are 5° to the right of their chosen radial (i.e. the chosen radial is 270°, but they are currently on radial 275°).
The instrument is telling the pilot to ‘fly left’.
In order to obtain CDI indications in the correct sense when tracking towards a VOR on radial 105°, what should the pilot set on the OBS?
1 Always draw a diagram! Draw a VOR and radial 105°
2 Draw in an aircraft that is tracking to the VOR along this radial
Radial 105° radiates away FROM the VOR, so to fly along this radial TO the VOR, the pilot must track 285°.
3 The correct sense will appear on the CDI when the pilot sets their desired track (285°) with the OBS knob
Since 285° will take the pilot TO the VOR, the CDI instrument will have a TO indication.
So the answer is: 285° with TO indicated