Impact of Magnetic Variation on PBN Systems

If you use an IFR-approved GNSS (GPS), you’ve probably noticed that the courses shown on the GNSS usually don’t match the numerical values printed on charts, if only by a few degrees. For example, as you fly an instrument procedure, a WAAS GNSS prompts you at each turn point to turn to a new course, but the number displayed on the screen is probably 2-3 degrees different from the number on the chart.

These differences are explained in AIM 1−1−17. Global Positioning System (GPS) , paragraph k. Impact of Magnetic Variation on PBN Systems (p. 1-1-27):

(1) Differences may exist between PBN systems and the charted magnetic courses on ground−based NAVAID instrument flight procedures (IFP), enroute charts, approach charts, and Standard Instrument Departure/Standard Terminal Arrival (SID/STAR) charts. These differences are due to the magnetic variance used to calculate the magnetic course. Every leg of an instrument procedure is first computed along a desired ground track with reference to true north. A magnetic variation correction is then applied to the true course in order to calculate a magnetic course for publication. The type of procedure will determine what magnetic variation value is added to the true course. A ground−based NAVAID IFP applies the facility magnetic variation of record to the true course to get the charted magnetic course. Magnetic courses on PBN procedures are calculated two different ways. SID/STAR procedures use the airport magnetic variation of record, while IFR enroute charts use magnetic reference bearing. PBN systems make a correction to true north by adding a magnetic variation calculated with an algorithm based on aircraft position, or by adding the magnetic variation coded in their navigational database. This may result in the PBN system and the procedure designer using a different magnetic variation, which causes the magnetic course displayed by the PBN system and the magnetic course charted on the IFP plate to be different. It is important to understand, however, that PBN systems, (with the exception of VOR/DME RNAV equipment) navigate by reference to true north and display magnetic course only for pilot reference. As such, a properly functioning PBN system, containing a current and accurate navigational database, should fly the correct ground track for any loaded instrument procedure, despite differences in displayed magnetic course that may be attributed to magnetic variation application. Should significant differences between the approach chart and the PBN system avionics’ application of the navigation database arise, the published approach chart, supplemented by NOTAMs, holds precedence.

The key text is:

a properly functioning PBN system, containing a current and accurate navigational database, should fly the correct ground track for any loaded instrument procedure, despite differences in displayed magnetic course that may be attributed to magnetic variation application.

Paragraph 2 in that section of the AIM also notes that:

(2) The course into a waypoint may not always be 180 degrees different from the course leaving the previous waypoint, due to the PBN system avionics’ computation of geodesic paths, distance between waypoints, and differences in magnetic variation application. Variations in distances may also occur since PBN system distance−to−waypoint values are ATDs [along-track distances] computed to the next waypoint and the DME values published on underlying procedures are slant−ranged istances measured to the station. This difference increases with aircraft altitude and proximity to the NAVAID.

A similar question also arises when comparing DME and GNSS-derived distances, such as when flying holding patterns.

AIM 5−3−8. Holding provides further details on this issue in:

  • 5. Distance Measuring Equipment (DME)/GPS Along−Track Distance (ATD)
  • 6. Use of RNAV Distance in lieu of DME Distance

Substitution of RNAV computed distance to or from a NAVAID in place of DME distance is  permitted when holding. However, the actual holding location and pattern flown will be further from the NAVAID than designed due to the lack of slant range in the position solution (see FIG 5−3−7). This may result in a slight difference between RNAV distance readout in reference to the NAVAID and the DME readout, especially at higher altitudes. When used solely for DME substitution, the difference between RNAV distance to/from a fix and DME slant range distance can be considered negligible and no pilot action is required.

For more information about using GNSS to substitute for or to complement conventional navaids, see Use of GPS on Conventional Approaches (Update) and Use of Approved GPS (RNAV) Systems on Conventional Procedures and Routes here at BruceAir.

For more information about how the FAA assigns magnetic variation to VORs and other navaids, see FAA Order 8260.19H.

Jeppesen also discussed this issue in one of its Chart Clinic series: The Chart Clinic – Database Series.

2 Responses to Impact of Magnetic Variation on PBN Systems

  1. James says:

    Good explaination. Pilots should also be aware that the radials from VOR can be quite different from their magnetic course. The variation value for a VOR station is defined when the station is commissioned. In some cases that could be 50 years ago.

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