Is the bearing to the center of airport for a radio aid to navigation in the chart supplement, true course, magnetic course, is it based on the VOR's radials, or is it something else?


It's almost certainly magnetic. Page 12 of the Chart Supplement (sample airport information) has this information at the bottom (emphasis mine):

All bearings and radials are magnetic unless otherwise specified. All mileages are nautical unless otherwise noted.
All times are Coordinated Universal Time (UTC) except as noted. All elevations are in feet above/below Mean Sea Level (MSL) unless otherwise noted.
The horizontal reference datum of this publication is North American Datum of 1983 (NAD83), which for charting purposes is considered equivalent to World Geodetic System 1984 (WGS 84).

Because this information is in a kind of footnote at the bottom of the page, I suppose you could say that it's possible it only applies to that specific page. However, the mention of "this publication" strongly suggests that the information applies to everything in the Chart Supplement.

You can also see that it distinguishes bearings from radials, meaning that it's highly unlikely that the word "bearing" is being used to mean "radial" on p.27.

  • $\begingroup$ Wouldn't the 180 degree bearing at 4.1 NM be equivalent to the 180 degree radial & the same distance? Since both are referenced to magnetic north, I'm not following how there would be any difference. $\endgroup$ – Ralph J Sep 16 '20 at 19:04
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    $\begingroup$ @RalphJ Perhaps sometimes, but magnetic north moves around and VOR radials don't, or at least not as often. There's no guarantee that radial 180 is always aligned with 180° magnetic. $\endgroup$ – Pondlife Sep 16 '20 at 19:56
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    $\begingroup$ @RalphJ 2 VOR's next to each other can have significantly different settings of magnetic north based on variation differences at the time each VOR was commissioned. So 2 nearby VOR's can differ from each other and at the same time both differ significantly from the nearest isogonic line. $\endgroup$ – Ryan Mortensen Sep 16 '20 at 21:24

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