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:

RNAV approach courses displayed on GPS navigators may differ by a few degrees from FAA charts because GPS calculates magnetic declination precisely and currently, while charts use less frequently updated, fixed values.
This course discrepancy is not a safety issue for flight, as the GPS guidance (CDI) is based on identical waypoint positions between chart and navigator, accurately guiding the aircraft along the intended path.
Some RNAV (GPS) "Z" approaches are LPV-only, requiring WAAS and offering no downgrade option if full accuracy isn't available, contrasting with "Y" approaches which might be LNAV-only.
Airports like Reid-Hillview (KRHV) present additional complexities such as parallel runways similar to a nearby airport, two tower frequencies, and notably short runways with large displaced thresholds that reduce available landing distance.

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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.