# Why is the treatment of magnetic variation different for NDB and VOR?

Why in solving numerals of NDB and VOR, do we apply variation at the station in case of VOR and at the aircraft in case of NDB?

• I removed your second question because we only allow one question per post. Feel free to ask that one in a separate post. Jan 5 at 7:54

## 1 Answer

The reason is, that for a VOR, the receiver in the aircraft determines the radial based on calculating the phase shift and as the VOR is oriented to the magnetic north of its location, we need the variation of the station. Think of it as a lighthouse which changes the color through a full turn and hence you can just tell by the color, which radial you are on.

For the NDB however, the ADF receiver uses a sense/loop antenna to determine a direction and the needle is overlaid onto a course card which is oriented to the magnetic north of the aircraft. Think of it as a pilot looking out of the window to see a lighthouse that has a white omnidirectional light.

So as you can see the angle is once determined at the VOR and once from the aircraft and hence the different variations apply. Nevertheless I think this was espacially emphasized for examination, as it is not that important in practical flying nowadays.

• For VOR it is not just variation of the station, but variation of the station at some past point in time when it was last aligned. Because magnetic variation changes over time, but the stations tend not to be realigned often, or ever at all, because then all the charts would have to be updated and that could be additional source of errors. Jan 5 at 13:44
• @Jan_Hudec what's your take on variation in aircraft for ndb Jan 5 at 16:00
• @SourabhJain you use whatever variation is depicted on the chart for the aircraft's position.They use a World Magnetic Model of the earth's field, that is computed and revised every 5 years, the "5 year epoch", by a joint US and UK defense geospatial project. The chart will include the date of the epoch model in use. The current one is 2020, and a new one will be issued for 2025. So if you have a chart published in 2019, the variation lines will be from 2015, but if you have a post 2020 issue chart, its variation data will be accurate. In any case, a degree or two of error is not a big deal. Jan 5 at 17:10