cdhutch

Potential Discrepancy on Plotted VOR Course Lines

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I'm running PMDG 737 NGX version 1.10.6461 on P3D v3.3.5.17625.

When recently flying the DIRTY3 STAR into Atlanta, I noticed that the VOR course lines, with the chart specified courses,  do not line up with the track lines between two VORs on the STAR. 

The course between the Spartanburg (SPA) VOR and Foothills (ODF) VOR is about 254 degrees Magnetic. When I entered 254 into the MCP course windows for each VOR, the two course lines are parallel but non-colinear.

Capture2.thumb.PNG.86e3577607d44d44a4b663b4b7942687.PNG

I had to change the MCP course windows to 260 to make them overly the track between the VORs.

Capture3.thumb.PNG.93682dc09eaa2061f0c57abfbeb45604.PNG

 

It may be coincidental, but the magnitude of the difference between the charted course (254) and the course window entries that make the two course lines colinear is 6 degrees, which is close to the charted magnetic variation. I am certain that the ND is not plotting the courses at true bearings, since the true bearing equivalent of 254 degrees magnetic at 6 degrees West variation is 248 (using the mnemonic Can Dead Men Vote Twice at Elections: Magnetic + east Variation = True).

I would appreciate the developers or other users testing to see whether this behavior exists in other contexts.

 

Thank you,

Craig Hutchinson

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According to the FAA navaid database at http://www.airnav.com the magnetic north radial of the ODF VOR is referenced to the variation that existed in 1965, which (at that time) was zero degrees.

I double checked by looking at the ODF VOR on a VFR sectional chart, and indeed, the magnetic north radial appears to be exactly parallel with the chart's true north-based longitude meridians, indicating a variation of zero degrees.

Toccoa, GA airport (KTOC) just a few miles south of ODF shows a variation of 5 degrees west, based on 2015 data per Airnav.

It appears that this is the case with many US VORs, which are often using magnetic variation data that is 30 to 50 years old. 

This is probably due to the fact that recalibrating the VOR north reference would be a disruptive and time-consuming process. The VOR would have to be taken out of service for a protracted amount of time, and after the realignment is completed, it would have to be extensively flight tested by FAA aircraft before being recertified for use. Then, all published aeronautical charts with airways based on the VOR would have to be republished with new radial numbers etc.

Another probable factor is the (now) almost universal use of GPS for enroute navigation by almost all aircraft, and the FAA's plan to decommission many existing VORs in the next few years.

 

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Thanks for your recommendations. I'll reload the MAGVAR in P3D and retest and also relook at the VOR bearings that Jim described.

 

 

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