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:

Magnetic declination differences cause displayed RNAV approach courses on charts to vary from those calculated by GPS navigators, with GPS using more current data.
This course discrepancy is not a safety issue for navigation; pilots should fly the CDI, as the underlying waypoint positions are identical on charts and in the GPS database.
Some RNAV approaches are LPV-only, requiring WAAS accuracy with no LNAV downgrade option, and pilots should thoroughly brief all airport-specific details, including runway length, displaced thresholds, and glideslope variations.

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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.