IFR charts in magnetic?

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flyboy595

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Flyboy595
Hey guys,
Reading the instrument oral exam guide, and the question "What are Enroute Low-Altitude Charts?" came up. As part of the answer it says "all courses are magnetic and distances are nautical miles". Does this mean when i take a course off a line that i drew on the chart, i'm reading magnetic, not true? That doesn't seem to make sense to me, as sectionals are true.
 
They're probably referring to the victor airways, as the courses coming from those are magnetic.

Otherwise it's the same as a sectional using a plotter, you would use the lines of variation and the Lat/Long.
 
kevmor99 said:
They're probably referring to the victor airways, as the courses coming from those are magnetic.

Otherwise it's the same as a sectional using a plotter, you would use the lines of variation and the Lat/Long.
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Thats what I figured, they just worded it kind of weird. Thanks!
 
flyboy595 said:
Hey guys,
Reading the instrument oral exam guide, and the question "What are Enroute Low-Altitude Charts?" came up. As part of the answer it says "all courses are magnetic and distances are nautical miles". Does this mean when i take a course off a line that i drew on the chart, i'm reading magnetic, not true? That doesn't seem to make sense to me, as sectionals are true.
Click to expand...

Well, in reference to what? The longitude lines are true North and that's how the map is aligned, but for practical purposes you need to do your directional takeoffs in alignment with a magnetic reference, such as a VOR or an airway.
 
flyboy595 said:
Hey guys,
Reading the instrument oral exam guide, and the question "What are Enroute Low-Altitude Charts?" came up. As part of the answer it says "all courses are magnetic and distances are nautical miles". Does this mean when i take a course off a line that i drew on the chart, i'm reading magnetic, not true? That doesn't seem to make sense to me, as sectionals are true.
Click to expand...

All the heading values listed on the chart are magnetic and all the distance values list are NM.
 
Actually the answer is wrong but it's closer than anything else (in the idiotic FAA test writer's weasel wording: the best answer) is magnetic.

The truth is the airway numbers are neither true NOR magnetic, but they are VOR redials which are close but not necessarily equal to the magnetic bearing from the station.
 
flyingron said:
Actually the answer is wrong but it's closer than anything else (in the idiotic FAA test writer's weasel wording: the best answer) is magnetic.

The truth is the airway numbers are neither true NOR magnetic, but they are VOR redials which are close but not necessarily equal to the magnetic bearing from the station.
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But if a drew a line on the chart and used a plotter to get course, it'd still be true, correct?
 
flyboy595 said:
But if a drew a line on the chart and used a plotter to get course, it'd still be true, correct?
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Correct. The orientation of the lat/longs are true, as is the orientation of the chart itself.
 
flyboy595 said:
But if a drew a line on the chart and used a plotter to get course, it'd still be true, correct?
Click to expand...

Depends on how you used the plotter, but if you're talking about computing angles of lat/lon lines, then you will be reading true.
 
hindsight2020 said:
Correct. The orientation of the lat/longs are true, as is the orientation of the chart itself.
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The chart orientation isn't necessarily aligned with anything. Some are predominantly east/west (true) but even then they have a bit of a cant with regard to the longitude lines. There are some along the coast and the US/Canada border that are at complete oblique angles.
 
Some of the VORs are so old, they are 6-10 degrees out of alignment, due to magnetic variance


Sent from my iPad using Tapatalk
 
I seem to remember the Instrument Flying Handbook stating that all courses are magnetic and distances are in nautical miles on IFR enroute low altitude charts. Are you guys saying that's wrong? Appreciate all the background info on this topic but sometimes too much information is confusing.
 
If it said that, it's as I said either wrong or incomplete depending on how exact you want to be. A victor airway is nearly always defined by VORs. The numbers are the VOR radial which are not courses nor are they other than coincidentally magnetic. There are a few magnetic bearings from ADBs and some magnetic bearings from GPS fixes for T-airways, but most of the directional numbers are VOR radials.


In fact, your magnetic (or true) course changes continually along the airway (except in certain degenerate cases).
 
Clip4 said:
There is no zero degree radial.
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Sure there is, it's just not printed on the charts. There's a 720° and a 1080° radial as well. :D

An a more interesting (to me) factoid is that most "real" VOR courses are not straight lines although AFaIK they are drawn that way on the charts. A "perfect" VOR's electronic guidance actually follow a great circle (as does a computed GPS course) and the only time this projects a straight line on a Mercator chart is when the course runs through the Earth's true north pole.

That said this doesn't matter much as the worst case difference between a rhumb line and a GCR below 60° latitude is about half a mile on a 100 nm long course.
 
gismo said:
Sure there is, it's just not printed on the charts. There's a 720° and a 1080° radial as well. :D

An a more interesting (to me) factoid is that most "real" VOR courses are not straight lines although AFaIK they are drawn that way on the charts. A "perfect" VOR's electronic guidance actually follow a great circle (as does a computed GPS course) and the only time this projects a straight line on a Mercator chart is when the course runs through the Earth's true north pole.

That said this doesn't matter much as the worst case difference between a rhumb line and a GCR below 60° latitude is about half a mile on a 100 nm long course.
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Well mostly right, but av charts aren't mercator projections but lambert conformal conics. With the relatively short distances and large map scales, there "straight lines" on the chart are pretty much the same as the "straight lines" in reality (i.e., the VOR radial or great circle). This is because this chart allows better "angular" correctness at the expense of more distorted distance scaling.

Actually in a non-tranverse mercator, the poles is a great singularity, it's depicted as a line so even a course through the north pole is problematic. I've never seen such a mercator actually used for modern navigation. The military does make some use (on very large scale charts) of transverse mercator, but even then they limit it to strips no wider than six degrees of longitude and either the northern or southern hemisphere and switch to other projections (stenographic) at the poles.

Sorry for the pedantic comments here, but map projections is what I did for a living for 23 years.
 
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