Magnetic variation

Dimme wrote:

GPS overlays are true

Huh? They are not, they are magnetic… If they were true they couldn’t be flown.

OTOH a VOR is not necessarily recalibrated every time the variation has changed by one degree, while I suppose that GPS overlays are always recomputed with the current variation every time a new database is generated (i.e. every 28 days). That could account for a few degrees discrepancy.

AIM 1−1−17. Global Positioning System (GPS), paragraph k.
Impact of Magnetic Variation on PBN Systems

(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.

Is it worth moving the PBN magnetic variation points to a separate thread?

[ done – not sure if the title is right ]

If I understand correctly the published tracks on an RNP approach are magnetic but may be different to the ground station track (VOR or LOC) due to the magnetic deviation drift (or because the terrestrial may be offset, eg VOR at Southampton).

The AIM cited fails to mention another feature which is that the IAF, Intermediate and Final fixes are fly byes (some STAR in mountainous terrain may have fly overs?) , while the Missed Approach fix is a fly over. As a result the final track and distance only correspond perfectly to the published only on the FAF – MAPpt leg. This always agrees between the GPS and approach plate in my experience. Also if there is an ILS, the final approach track (magnetic) also agree.

In the US, there is often 3 to 6 degrees difference between a VOR radial and the magnetic track shown on a GPS. Many of the VOR’s were not aligned since the 1960’s and as magnetic north has drifted, the current magnetic variation has changed. The GPS will use the current magnetic variation with an exception for navigation to a VOR or when in OBS mode with the VOR as the navigation fix. In those cases the GPS uses the original variation of the VOR. Airports and runways have a variation that is not necessarily updated when the magnetic variation changes. So an ILS approach chart will use the runway variation and it there is a difference between the runway variation and the current variation, a GPS will show the track based on the current variation. So the displayed DTK on an approach can vary from the ILS chart by 2 to 3 degrees.

The GPS will use the current magnetic variation with an exception for navigation to a VOR or when in OBS mode with the VOR as the navigation fix

That’s an amazing little fact

AFAIK, in the UK, each VOR has a dedicated phone line going to it, and gets aligned periodically.

The FAA is looking at this issue here local copy

The FAA is not going to realign VOR’s just because the magnetic variation changes. It is a massive undertaking. Each airway would need to be updated as well as every fix defined by a VOR-VOR radial intersection or a VOR radial and VOR DME distance. In the US, one must go thru the complete regulatory process to change an airway which itself can take 6 months or more. The new airway needs to be flight tested and a charting cycle needs to be involved to update the charts. Every instrument procedure that uses the VOR would have to be evaluated and changed. A VOR fix may be used for a missed approach hold fix or as part of a feeder route for an airport quite some distance from the VOR. Instrument approach procedures go thru the regulatory process to be changed. With many VOR being phased out or not repaired when they fail to pass flight tests. The vast majority of pilots have no clue that the VOR radials are not typically aligned to the current magnetic variation and because the difference is usually less than 6 degrees, they rarely notice this, must be a crosswind today?

NCYankee wrote:

The new airway needs to be flight tested and a charting cycle needs to be involved to update the charts. Every instrument procedure that uses the VOR would have to be evaluated and changed. A VOR fix may be used for a missed approach hold fix or as part of a feeder route for an airport quite some distance from the VOR. Instrument approach procedures go thru the regulatory process to be changed.

Realigning a VOR as the magnetic variation changes doesn’t change the airway or instrument procedure at all – it only changes what figures are written on the charts.

So of course you would need a quality control process for updating the charts, but apart from that nothing changes in reality.

BTW when I updated Jepp Nav database in G1000 this morning, I got dialog box asking me to update magnetic variation.

Airborne_Again wrote:

Realigning a VOR as the magnetic variation changes doesn’t change the airway or instrument procedure at all – it only changes what figures are written on the charts.

So of course you would need a quality control process for updating the charts, but apart from that nothing changes in reality.

That is only technically true. But in the US any such change requires a new regulation to be issued, the procedure needs to be flight tested with the new values. In the interim, an FDC NOTAM would have to be issued and the procedure change would have to go into an already full production update schedule that is already out a few years. So a single realignment of a VOR could generate tons of NOTAMs and workload on flight test and charting,