I recently got an email from a user of Nav Trainer asking what the red lines represent on the sides of the VOR station symbols.
Those red lines represent areas where the VOR signal is ambiguous and the VOR display is unreliable.
Flying through this airspace would cause erratic behavior of your CDI needle (Course Deviation Indicator) and of the to/from flag on the VOR displays. Therefore it’s important to know about this pitfall when using VORs for navigation.
From a top-view it looks like two cones extending from the VOR station perpendicular to the course you selected on the OBS (Omni Bearing Selector).
From a side view, it looks like a cone extending upwards from the VOR. This is the “Cone of Confusion”.
As you get closer to the VOR station, the needle will become unreliable and may even reach a full-scale; the to/from indicator will first turn into an “Off,” or “Nav” flag before completely reversing.
The farther you are from the station (in ground distance or in altitude), the cones would become wider and take longer to fly through.
So, when approaching a VOR station, you should ignore the needle oscillations. If you’re trying to continue on the same course to track outbound on the VOR, simply keep flying the same heading that you used to track it so far, until the needle centers or stabilizes again.
If you’re going to track a new course outbound from the station, simply turn to a heading that equals the course or a heading estimated for wind correction, and keep it until the needle is reliable again.
When passing abeam a VOR , the farther you are from the station, the longer the to/from indicator will take to reverse. So take this into consideration when flying holding procedures with the VOR and trying to time your outbound legs. If you have an extra VOR receiver, you can use it to precisely find when you are abeam the station on the outbound leg.
Simply tune the second receiver to the same station but set its OBS to a perpendicular bearing to your holding course. (For example, if you’re holding on the 360 inbound course, tune Nav 2 to 090 or 270.)
You’ll know that you past abeam the station once the second VOR’s needle centers. If you don’t have a second VOR to spare, you can wait until the full reversal of the to/from flag.