Foreflight course confusion

[DrPappy]

I use Foreflight and have noticed that their magnetic heading is different than my calculated magnetic heading. Anyone versed in Foreflight, please help! It has been awhile since I have done these calculations manually, so maybe I'm screwing something up.

When I select a route by entering it into my Navlog view or by using the ruler in Foreflight, the resulting course shown says it is "M" (which I assume means magnetic). For a simple example, let's plot a course between two navaids, from TNV to IDU (the V212 airway) in Foreflight's NavLog view. When you do so, under the "CRS" (course) in Foreflight it shows 230 M. However if you read the sectional, the V212 airway shows a course of 225 (magnetic). I checked the route with Skyvector.com, which also shows a course of 225. The magnetic course in Foreflight is apparently off by 5 degrees in this case.

However, if you plot a course in Foreflight in the same vicinity as before directly along the lines of latitude or longitude, which should be at 086 and 176 Magnetic (090 and 180 True, minus 4 degrees east variation), Foreflight shows 87 M and 177 M. So in this second example, Foreflight is almost right for magnetic course, where in the previous example it was off by 5 degrees.

My brain is about to explode!

 

[chartbundle]

Sounds like Foreflight is giving you the actual magnetic course and the Sectional(and IFR chart) is giving what you would set on the OBS to fly from that VOR. The current magnetic declination in that area is 4 degrees East. The VOR is set at 8 degrees East. This 4 degree difference is almost exactly correct for the 5 degree difference you see.

If you were to take a fancy paper sectional, and a fancy plastic plotter you'd find the actual magnetic course would be the same as what Foreflight is giving you.

Also, for fun, take the course line from TNV to IDU in SkyVector and drag the end just slightly off the VOR, you'll see it switch to actual magnetic declination(instead of the VOR) and change to about 230.

 

[DrPappy]

Sounds like Foreflight is giving you the actual magnetic course and the Sectional(and IFR chart) is giving what you would set on the OBS to fly from that VOR. The current magnetic declination in that area is 4 degrees East. The VOR is set at 8 degrees East. This 4 degree difference is almost exactly correct for the 5 degree difference you see.

If you were to take a fancy paper sectional, and a fancy plastic plotter you'd find the actual magnetic course would be the same as what Foreflight is giving you.

Also, for fun, take the course line from TNV to IDU in SkyVector and drag the end just slightly off the VOR, you'll see it switch to actual magnetic declination(instead of the VOR) and change to about 230.

THANKS a bunch chartbundle! I just got my BFR after a 20 year (!) hiatus and it sounds like I need to do some more refreshing on my sectional usage. So the VOR's are not set to the current magnetic variation, that's interesting.

 

[muecb]

Foreflight will adjust the magnetic heading based on its winds aloft model. If you put an altitude like 80,000 in the planner, it will not include winds aloft and you should see the magnetic headings that you're expecting.

 

[DrPappy]

Foreflight will adjust the magnetic heading based on its winds aloft model. If you put an altitude like 80,000 in the planner, it will not include winds aloft and you should see the magnetic headings that you're expecting.

Yeah EB, I thought that was the problem too, so I set the date for several days ahead so Foreflight doesn't take winds into account. Unfortunately it still showed the same "errors." See chartbundle's post above for the reason it was doing this. If I am interpreting it correctly, the VOR's have outdated magnetic variation numbers.

 

[chartbundle]

THANKS a bunch chartbundle! I just got my BFR after a 20 year (!) hiatus and it sounds like I need to do some more refreshing on my sectional usage. So the VOR's are not set to the current magnetic variation, that's interesting.

Correct, they are set when they are built, and sometimes when they have some kinds of maintenance done. Most of the places you can lookup VOR information will show the variation and when it was last updated.

For example, for Mission Bay VOR: Variation: 15E (1965)

 

[DrPappy]

I used to fly in the days when VORs were the height of technology, so I understand why it is more convenient to have the VOR radials set to magnetic. But by not updating the VORs to the current magnetic variation, it puts another wrinkle in the myriad of navigation information to know. If they weren't going to keep them current it would seem like they would have used true courses in the first place.

If I ever see that Uncle Sam guy, he's getting an earful from me.

 

[jesse]

Foreflight is never going to give you a magnetic heading or any heading. It will give you course.

 

[dukeblue219]

Foreflight is never going to give you a magnetic heading or any heading. It will give you course.

Jesse... this is simply not true. It absolutely will take your charted course and determine the appropriate magnetic heading you should fly.

 

[flyingron]

Note well that the magnetic course changes continually over a straight line (except in degenerate cases). Even without wind you will see a few degrees change in heading while tracking a VOR radial.

 

[Pedals2Paddles]

Learn something new every day. I had no idea the VORs didn't reflect the current magnetic declination. In fact that concept never even crossed my mind.

What about things like runway headings and localizer courses?


<< Sent from my mobile device at 0agl >>

 

[dell30rb]

What about things like runway headings and localizer courses?

Magnetic. As variation changes, they will actually re-paint the numbers on the runways.

 

[MBDiagMan]

Jesse... this is simply not true. It absolutely will take your charted course and determine the appropriate magnetic heading you should fly.

Correct! You took the response right out of my mouth..... er... keyboard.

I learned a few years ago, that life is good if you stop by the wireless on the way to the runway and get the latest weather downloads.

 

[dukeblue219]

Learn something new every day. I had no idea the VORs didn't reflect the current magnetic declination. In fact that concept never even crossed my mind.

What about things like runway headings and localizer courses?

Magnetic. As variation changes, they will actually re-paint the numbers on the runways.

It's based on magnetic, but is not always up to date. It's less likely to cause an issue than a VOR radial, but still not necessarily any more accurate. In fact, some runways are deliberately off. Atlanta has the following Eastbound runways:

9L (95deg magnetic)
9R (95deg magnetic)
8L (95deg magnetic)
8R (95deg magnetic)
10 (95deg magnetic)

The "8" runways are a full 15 degrees off what you might expect.

 

[MAKG1]

Foreflight will adjust the magnetic heading based on its winds aloft model. If you put an altitude like 80,000 in the planner, it will not include winds aloft and you should see the magnetic headings that you're expecting.

Only if the forecasted winds aloft are correct.

And if the Easter Bunny says so (it's almost as likely).

 

[BigBadLou]

Great information, I too did not know that the VORs are not adjusted to magnetic variation changes over time. I assumed (incorrectly) that they are adjusted, just like runway designations.

DrPappy, Navasota for the win! I assume you are local to TNV or IDU? Are you in Brenham for lunch today? First VFR weekend this year, should be taking advantage of that!

 

[denverpilot]

Only if the forecasted winds aloft are correct.



And if the Easter Bunny says so (it's almost as likely).

They were within 3 knots at both altitudes I tried last night. And one hell of a headwind. Sigh.

 

[Henning]

Wait, you are mixing Course and Heading, that may be the problem. IIRC Foreflight will give your Course as corrected for set and drift, ie wind correction applied.

 

[muecb]

Only if the forecasted winds aloft are correct.

And if the Easter Bunny says so (it's almost as likely).

Not sure what you thought I said, but I explained how to remove the winds aloft calculation, not rely on it for accurate headings.

 

[Mikey1719]

Not sure what you thought I said, but I explained how to remove the winds aloft calculation, not rely on it for accurate headings.

Thank you VERY much DB! I couldn't figure out how to get MC instead of MH on Foreflight. What a great tip!

 

[RussR]

Great information, I too did not know that the VORs are not adjusted to magnetic variation changes over time. I assumed (incorrectly) that they are adjusted, just like runway designations.

They ARE adjusted, just not every time the magnetic variation changes. The "standard" is to change them when they're off by 3 degrees or more, but even that is often not warranted.

Think of all the work that has to occur to change the alignment of a VOR. There's the physical technician work, of course, but also EVERY approach procedure, SID, STAR, airway, Sectional chart, IFR enroute chart (high and low), etc. that uses that VOR also has to be changed. That's a lot of man-hours in updating procedures, revising charts, updating databases, etc. And for what?

Given compass error, the radial you are tracking only has a passing relationship with your compass heading anyway. Even in no wind with a perfectly aligned VOR the two are not likely to be the same. Add any amount of wind, and the difference grows. So, it doesn't make much difference if the value is exactly on, or 6 degrees off. You fly whatever you need to to maintain the radial.

So we're faced with a situation that has limited impact to actual flight operations, but costs a lot to "fix". It's no surprise that many VORs are "off" by many degrees.

I imagine in this era of federal budget deficits and such, nobody would be excited about the government spending more money to make sure those VORs are up-to-date.

Magnetic variation changes yearly. In some parts of the country it's quick, some parts it's not (like that 1965 example). But, for example, in Alaska, they'd be changing VORs every few years if they kept it within 1 degree.

 

[midlifeflyer]

Great information, I too did not know that the VORs are not adjusted to magnetic variation changes over time.

They are. But over a long time and not on a regular schedule. Changing a VOR declination isn't as easy as grabbing a bucket of paint and painting new numbers on a runway .

I've seen reports of a decade or more and with the beginnings of what may eventually be a full phase out, they may simply not bother any more.

 

[Turningfinal]

It's based on magnetic, but is not always up to date. It's less likely to cause an issue than a VOR radial, but still not necessarily any more accurate. In fact, some runways are deliberately off. Atlanta has the following Eastbound runways:

9L (95deg magnetic)
9R (95deg magnetic)
8L (95deg magnetic)
8R (95deg magnetic)
10 (95deg magnetic)

The "8" runways are a full 15 degrees off what you might expect.

The "8s" are 15 degrees off only because they have 5 parallel runways.

The same system is in use in LAX, DEN, ORD, etc. When you have more than 3 parallel runways (L, C, R) you need to change numbers.

 

[Lindberg]

Magnetic. As variation changes, they will actually re-paint the numbers on the runways.

And sometimes they'll repaint the runway numbers and leave the signs. It can be confusing for a transient sitting at runway 35 when tower clears you to takeoff runway 36.

 

[arnoha]

Sounds like Foreflight is giving you the actual magnetic course and the Sectional(and IFR chart) is giving what you would set on the OBS to fly from that VOR. The current magnetic declination in that area is 4 degrees East. The VOR is set at 8 degrees East. This 4 degree difference is almost exactly correct for the 5 degree difference you see.

If you were to take a fancy paper sectional, and a fancy plastic plotter you'd find the actual magnetic course would be the same as what Foreflight is giving you.

Also, for fun, take the course line from TNV to IDU in SkyVector and drag the end just slightly off the VOR, you'll see it switch to actual magnetic declination(instead of the VOR) and change to about 230.

This, but there's also a smaller piece in there, too, that can affect larger distances. FF (and nearly all navigators) calculate great circle courses. That can change the round-off even for smaller distances.

 

[ChrisK]

https://support.foreflight.com/hc/e...airway-heading-differ-from-my-NavLog-heading-

 

[mkosmo]

The "8s" are 15 degrees off only because they have 5 parallel runways.

The same system is in use in LAX, DEN, ORD, etc. When you have more than 3 parallel runways (L, C, R) you need to change numbers.

R, CR, C, CL, and L aren't clear enough?!

 

[John Collins]

I used to fly in the days when VORs were the height of technology, so I understand why it is more convenient to have the VOR radials set to magnetic. But by not updating the VORs to the current magnetic variation, it puts another wrinkle in the myriad of navigation information to know. If they weren't going to keep them current it would seem like they would have used true courses in the first place.

If I ever see that Uncle Sam guy, he's getting an earful from me.

Many VOR's were last aligned more than 30 years ago. At the time they were installed, the magnetic heading used to track a given radial was generally correct. Now 30 years later, you still set your OBS on your VOR to the old number, but the magnetic heading you use to follow the course under no winds conditions will have varied by the difference in the current variation as compared with the original variation. If you keep the CDI needle centered, you will fly over precisely the same route you would have 30 years earlier.

There is little to no point in realigning many VOR systems. Heck, many on this forum were totally unaware that this occurred, yet somehow managed to follow the Victor airways because they set the OBS to the value on the chart.