Defining Sector Magnetic Variation

[Rob Killins 897126]

The instructions for defining the sector magnetic variation I find a bit vague based on the information I have.

The eighth line is the magnetic variation for the sector. This is used to rotate the display on the scope so

that runways and aircraft ground tracks are aligned relative to magnetic north.

(VRC docs)

 

Referring to my airport charts, information referencing variation indicate a 5 degree E variation in 2003, with an annual rate of change of 8' west. What value would I enter in line 8 of the INFO section to best reflect the magnetic variation?

[David Zhong]

If the annual rate of change is 8 minutes, then you can work out the current magnetic variation:

 

5 - (8/60)*11 = 3.53 degrees East

 

However, most flight sims still use the base 2003 data, so you'd probably be best just to use 5 degrees.

[Rob Killins 897126]

Thank you David, that was very helpful. Would I use 5? Or -5?

[David Zhong]

I can't remember which sign convention is used. You can try both and measure a known bearing (e.g. runway heading) to see if it matches.

[Alexander Cohrs]

West is minus, East is plus. So in your case, it would be +5.

[Bradley Grafelman]

West is minus, East is plus. So in your case, it would be +5.

Based on the wording above, it sounds like the opposite is true. If magnetic variation is 5 degrees east, you would subtract 5 degrees from true heading to arrive at magnetic heading.

[Alexander Cohrs]

Both is true, but it's different questions

 

A western variation has a negative sign and an eastern variation has a positive sign. That's what I was referring to.

 

When you want to convert from true course to magnetic course, you have to subtract the variation from the true course - but with the correct sign!

 

For example:

True course 180, Variation 5 W: 180 - (-5) = 180+5=185

True course 180, Variation 5 E: 180 - (+5) = 180-5=175

 

I think that the sectorfile only asks for the variation itself to have the correct info for the maths, so in this case, if VRC or Euroscope simply wants to know the variation at the given sector, and the variation is 5E, you would need to enter 5 (+5) there.

 

Alex

[Evgeny Vygornitsky]

"West is Best, East is least"; that is to say, add W declinations when going True headings to Magnetic Comp[Mod - Happy Thoughts], and subtract E ones.

http://en.wikipedia.org/wiki/Magnetic_declination

[Alexander Cohrs]

Which is also correct but does not answer the original question. Still a Western Variation is -5 and an Eastern variation is +5. Only when calculating from True course to magnetic course you have a sign change, as it's TC - VAR = MC. If you variation is positive, like in the given example with 5 E, the result is a lower number, as - (+5) results in -5.

[Bradley Grafelman]

Only when calculating from True course to magnetic course you have a sign change

... which appears to be exactly what VRC is doing with the value given based on the docs, hence why ZLA's sector file has a negative value due to the easterly magnetic variation. (Can't speak for ES - haven't worked on a sector file for that client... or even used the client, for that matter. )

[Alexander Cohrs]

Haha, so at least we are on the same track now and can blame VRC and Euroscope for our discussion

 

Honestly, if the sector file just asks for the variation itself, the correct entry in the setctorfile should be -5 for West and +5 for East. The calculations thereafter are a complete different thing.

 

I am mainly insisting on that difference between positive and negative sign on the one hand and the subtraction process on the other hand because only if you get that right on your head, you are also able to calculate the other way from Magnetic Course to True Course - in that case not subtracting, but adding, and again with the correct signs.

[Dhruv Kalra]

Alex, you have it backwards. If the variation is 5E, you code your sector file as -5.

[Alexander Cohrs]

Guys, I really start feeling desperate. The problem of this whole thread is that we are discussing different things, and my problem is that it seems that I am not able to make my point clear enough so that you understand what I am talking about. Maybe it's also a language issue, would be a bit easier in my native language. But I make another try:

 

I am not talking about what the VRC sector file demands, because I honestly don't know it how it is coded. I am also not talking about what happens when you do calculations from True Course to Magnetic Course or vice versa, as you run into a change of signs as explained earlier.

 

All I want to tell you guys is that per definition, 5W is a synonym to -5 and 5E is a synonym to +5. Again, when calculating with that, you may run into a change of signs and have the opposite effect, but that's not my point. It's just, if somebody asks you: "What is the variation at your home airport?", you could either answer "The variation is 5 West" or you could answer "The variation is minus 5". It's the same!

 

And as I see that you simply don't believe me, I have taken the time to find the relevant parts in two different PPL books. As both are german, I translate it for you, sorry in case it sounds a bit strange, blame it just on my translation.

 

Let's start with "Engine powered flight compact" by Winfried K[Mod - Happy Thoughts]era, that book definitely is one of the benchmarks in PPL lessons, page 95:

 

According to the orientation of the comp[Mod - Happy Thoughts] courses, a western variation is negative (...) Variation West (negative) (...) Variation East (positive)

 

My other source is the PPL course by "Civil Aviation Training", book "General Navigation", page 62 on the iPad edition:

 

The sign is East or +, if the magnetic pole is located right from the geographic pole (...) The sign is West or -, if the magnetic pole is located left from the geographic pole

 

Alex

[Bradley Grafelman]

The problem of this whole thread is that we are discussing different things

True. Some of us are on-topic... you seem to be on a different topic. Simplest way to end the on-topic discussion:

 

East = negative

West = positive

 

Period. Full stop. QED.

[Alexander Cohrs]

Okay, then go with it. It still remains wrong in real world, but obviously it is like that in the sector file.

[David Zhong]

It's not right or wrong to use a particular sign convention. All you are doing is giving meaning to 'positive' or 'negative' in the real world.

 

In aviation, when we refer to vertical distances, we use a positive-up convention, which means that Amsterdam-Schipol has an elevation of about -10 feet, while Melbourne Airport has an elevation of around 450 feet. In bathymetry (submarines, etc.) vertical distances use a position-down convention, which means that positive numbers are used below sea level.