magnetic variation question

[yankeegolf3 51]

Hello,

 

I have noticed some discrepancies between the magnetic variation indicated on the sectional charts and the FAA A/ FD pages:

Let's take KBPI as an example:

sectional: mag VAR 11.25 E (current)

FAA  FA/D: mag VAR 16 E  (valid august october 2017)

it makes a 4.75 ° difference which is huge

which one is  correct ? Why is there a difference between two valid documents?

THX

[BrianW 124]

Strange, the data on Skyvector says the magnetic variation data is from 1975.  If that's true it makes sense, but I wonder why no update since? 

KPNA is on about the same isogonic line and is shows 13 E from 2005, so maybe it is just old data.

[JRBarrett 3,034]

In this case the variation references the co-located BPI VOR, which has not had its magnetic north zero degree radial bearing changed since 1975, when the variation was 16E.

This is not unusual. There are dozens of US VORs which are calibrated to very old variation data. Many, like ABQ (Albuquerque) were last done in 1965.

There are many good reasons why this is rarely changed once set. With a VOR, the important thing is not whether a specific radial magnetic bearing corresponds to 2017 variation. The important thing is the path a given radial takes over the underlying terrain - and that never changes, no matter how long ago the zero degree reference was set.

Recalibrating a VOR north reference is an extremely expensive and time-consuming process. The FAA flight testing required to re-certify the VOR could take many weeks, which is why it is rarely done.

[John_Cillis 2,406]

Will aviation ever entirely switch to GPS navigation?  I am surprised VOR's are still used this far into the twenty first century.  The GPS in my cell phone can track me to a few feet of my location, I found that when I went to Europe in June.  Why hasn't aviation switched over to GPS navigation, what are the shortfalls?

John

[yankeegolf3 51]

30 minutes ago, JRBarrett said:

In this case the variation references the co-located BPI VOR, which has not had its magnetic north zero degree radial bearing changed since 1975, when the variation was 16E.

This is not unusual. There are dozens of US VORs which are calibrated to very old variation data. Many, like ABQ (Albuquerque) were last done in 1965.

There are many good reasons why this is rarely changed once set. With a VOR, the important thing is not whether a specific radial magnetic bearing corresponds to 2017 variation. The important thing is the path a given radial takes over the underlying terrain - and that never changes, no matter how long ago the zero degree reference was set.

Recalibrating a VOR north reference is an extremely expensive and time-consuming process. The FAA flight testing required to re-certify the VOR could take many weeks, which is why it is rarely done.

Your explanation makes sense but if I use VFR navigation techniques (not supposed to rely on VORs), I could end up making a compass heading error, even with current documents....

[JRBarrett 3,034]

Variation shown on a sectional is normally as current as possible - as are the variations for airports. That's why airport runway numbers occasionally change - say going from runway 23 to runway 24 because of the change in variation over a period of years. If you lay out a course on a sectional using a plotter for a pure VFR flight using the published variation data on the sectional or for individual airports, you should be close enough.

[BrianW 124]

1 hour ago, JRBarrett said:

In this case the variation references the co-located BPI VOR, which has not had its magnetic north zero degree radial bearing changed since 1975, when the variation was 16E.

This is not unusual. There are dozens of US VORs which are calibrated to very old variation data. Many, like ABQ (Albuquerque) were last done in 1965.

There are many good reasons why this is rarely changed once set. With a VOR, the important thing is not whether a specific radial magnetic bearing corresponds to 2017 variation. The important thing is the path a given radial takes over the underlying terrain - and that never changes, no matter how long ago the zero degree reference was set.

Recalibrating a VOR north reference is an extremely expensive and time-consuming process. The FAA flight testing required to re-certify the VOR could take many weeks, which is why it is rarely done.

I did know about the VOR calibration sometimes being way off, but I didn’t realize its influence on the co-located airport and deviation outside of the navaids.  I guess it’s something to keep in mind for those that like to reset their DG to the runway heading before takeoff.

EDIT:  I wonder if addon airport developers keep this in mind too?

46 minutes ago, yankeegolf3 said:

Your explanation makes sense but if I use VFR navigation techniques (not supposed to rely on VORs), I could end up making a compass heading error, even with current documents....

Just use the isogonic lines on the chart for deviation, the FAA uses a forecast model now so they’re usually not more than a couple of degrees off.

[HighBypass 2,339]

1 hour ago, Cactus521 said:

....Why hasn't aviation switched over to GPS navigation, what are the shortfalls?...

someone or some event flicking the off switch... 

Radio navigation is less high tech and doesn't rely on functioning orbital satellites.

[skelsey 765]

51 minutes ago, yankeegolf3 said:

if I use VFR navigation techniques (not supposed to rely on VORs), I could end up making a compass heading error, even with current documents....

If you were using VFR navigation techniques, you'd be using a VFR chart with current magnetic variation data; not an IFR chart/variation data associated with radio navaids. (Edit: Jim got there ahead of me).

1 hour ago, Cactus521 said:

Will aviation ever entirely switch to GPS navigation?

The short answer: yes, undoubtedly (though I would replace "GPS" with "PBN" -- the actual sensors used - GPS, VOR, INS etc - will become increasingly less important and all that will matter is that whatever equipment it is is capable of performing to a certain specification). Very many countries now mandate at least B-RNAV on airways, so in a sense we're already there from a GA point of view in the UK at least: no RNAV5 capabiltiy (which, in GA land essentially means an approved, panel-mounted GPS), no airways access.

There are many advantages in terms of efficiency: if we could all fly accurate RNP tracks enroute then airways could be spaced much more closely, more efficient and direct routes could be used, etc etc. However, the stumbling block as always is cost, particularly for the GA end of the market -- the airliners are already well ahead of the game.

A hand-held/portable GPS will not cut it for IFR navigation: only a properly certified, panel-mounted device with RAIM prediction and error correction and/or some form of augmentation capability (i.e. GBAS or, more likely going in to the future, SBAS) is going to cut it. Many new light aircraft these days are coming with integrated G1000 type avionics which takes care of it. However, it's a heck of an expense for a 50 year old C152. There's enough pushback over here in the UK (at least) over mandatory Mode S and mandatory 8.33 kHz which is costing a lot of light aircraft owners large sums of money in avionics upgrades. There's also the fact that particular where precision approaches are concerned, nothing has yet trumped a CAT III capable ILS for accuracy and reliability, but when SBAS and 3D LPV procedures become more widespread I don't doubt that will start to change also.

In the end the VORs and NDBs will go, I'm certain of that. It's a question of when, rather than if.

[skelsey 765]

1 minute ago, HighBypass said:

someone or some event flicking the off switch... 

Edit to add:

This is less of an issue these days: the US government says that it has abandoned Selective Availability (and that the newer satellites supposedly don't even have the capability). However, whether you trust the US government or not on that, it is more accurate nowadays to talk of a non-specific "GNSS" (Global Navigation Satellite System). The Russian GLONASS system is fully operational, Galileo (the EU version) is more or less fully operational and you have countries like India, Japan, France etc launching their own systems as well. Modern receivers are/should be capable of integrating all the systems together, so even if the US NAVSTAR GPS (or any of the other systems) were to be 'switched off', there shouldn't be a problem.

[John_Cillis 2,406]

I guess there are risks to the gps system way up there in space.  Navaids will probably remain as they are trusted and ground based.  In P3d I navigate via the GPS since it's just a simulation, no worries about an outage.  I only use VOR's on an ILS approach.  GPS to get me there, VOR to land.

John

[JRBarrett 3,034]

2 minutes ago, BrianW said:

I did know about the VOR calibration sometimes being way off, but I didn’t realize its influence on the co-located airport and deviation outside of the navaids.  I guess it’s something to keep in mind for those that like to reset their DG to the runway heading before takeoff.

 

Just use the isogonic lines on the chart for deviation, the FAA uses a forecast model now so they’re usually not more than a couple of degrees off.

The variation shown for an airport is normally separate and apart from the variation of a nearby navaid - but it can be confusing when an airport and navaid have the same identifier. The FAA changed a bunch of VOR identifiers a few years ago to prevent this very thing. Just in my local area, Elmira VOR changed from ELM to ULW. Binghamton changed from BGM to CFB, and Williamsport changed from IPT to FQM.

Still, there are some VORs associated with very large airports that still carry the same identifier as the host airport. JFK, ATL and ABQ come to mind.

Usually (but not always) the published variation for an airport will be reasonably current - within the past 5 years or so.

[BrianW 124]

That's what I thought, but in this case the Foreflight airport diagram shows the deviation as 16 E, and both Foreflight and Skyvector list the runway headings based on this deviation from true. 

[JRBarrett 3,034]

Just now, BrianW said:

That's what I thought, but in this case the Foreflight airport diagram shows the deviation as 16 E, and both Foreflight and Skyvector list the runway headings based on this deviation from true. 

BPI is a small airport in a very rural part of Wyoming. It doesn't even have an official FAA airport diagram, so the airport diagram shown in Foreflight is not official, but a Foreflight-generated map.

The published variation for the airport appears to be 11 degrees, which corresponds to the current sectional. That's the variation to use if you are laying out a VFR dead reckoning flight to or from this airport. 

[scott967 62]

Haven' looked at it lately, but FAA practice was to use "epochs" which are refigured every 5 years (and then a model is created to allow calculating expected variation for any given date).  FAA analyzes the change of the upcoming epoch when doing chart revisions, and if the difference is larger than a certain amount from the epoch used in the current chart, it updates the chart accordingly (I think the delta is as large as 10 degrees).  So for example the localizer course shown on the chart is generally not going to be a compass heading.  Same thing with VOR, with the added complication as described before the problem of "rezeroing" and certifying the VOR.

scott s.

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