The heading shown in the NavLog and the airway heading on a chart can differ due to:
1. Winds Aloft Inclusion
ForeFlight calculates a wind-corrected heading when the following conditions are met:
- Internet Connection: Weather data is available for download.
- Defined Altitude and TAS: A cruising altitude and true airspeed (TAS) are specified.
- Forecast Validity: The estimated time of departure (ETD) falls within the valid period for winds aloft forecasts.
This adjustment accounts for wind effects, resulting in a NavLog heading that may differ from the chart's airway heading.
2. Old VOR Station Magnetic Variation
NavLog headings incorporate updated magnetic variation values, while VOR radial headings may rely on older magnetic variation data. For example, the GEP VOR was calibrated in 1965 using a magnetic variation of 6E. The current magnetic variation in the area is 1E, creating a 5-degree difference. To track the 85° radial from GEP using the VOR signal:
- The OBS should be set to 085.
- To maintain the correct track along the radial, a magnetic heading of approximately 090 must be flown (in zero wind conditions) because magnetic variation is different today than in 1965.
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