Seeing Double

Instrument flight is a game of single degrees, so it should get your attention when numbers dont match. Were used to this when flying VOR radials with GPS. The magnetic declination of the VOR is infrequently updated, while GPS calculates it exactly from the latest data and the current location. This can be disconcerting on a VOR approach where the GPS youre using for situational awareness (or, the actual approach, now that you can) shows a course several degrees off from the chart.

jwanwest

Thursday, October 20, 2016 at 12:00 AM ET

Verified

Key Takeaways:

GPS navigators calculate magnetic courses for RNAV approaches based on dynamic declination, often differing by a few degrees from static FAA chart values and VORs.
Despite these magnetic course differences, flying the GPS Course Deviation Indicator (CDI) is safe as the underlying waypoint positions remain identical between charts and GPS, ensuring the correct flight path.
Pilots must understand specific RNAV approach types (e.g., "Z" vs. "Y") which may mandate WAAS-enabled precision (LPV) with no downgrade option, requiring full accuracy or an abort.

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Instrument flight is a game of single degrees, so it should get your attention when numbers don’t match. We’re used to this when flying VOR radials with GPS. The magnetic declination of the VOR is infrequently updated, while GPS calculates it exactly from the latest data and the current location. This can be disconcerting on a VOR approach where the GPS you’re using for situational awareness (or, the actual approach, now that you can) shows a course several degrees off from the chart.

What we don’t think about too often is that airports have magnetic declination assigned to them as well, and it’s not just for the runway numbers. The FAA also uses it for RNAV approaches—and the GPS navigator doesn’t.