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Trivia Testers

Why is there a difference between the magnetic variation for an airport, and the VOR located at that same airport?

Not Architectural, Neurological, Silvicultural, or Ceramic
What aviation related item should come to mind when you see the terms "columns", "dendrites", "needles", and "plates"?

  1. descriptions of different lightning shapes
  2. types of mountain ranges
  3. various possible snowflake shapes
  4. cloud formations

Click to Enlarge Answer: C. These are some of the possible shapes that ice crystals take when snowflakes form. For any of you rock hounds out there, you might've thought they could have also been mineralogical. Certain minerals, due to their internal atomic structure, align themselves in a much more limited variety of patterns that are usually particular to that one mineral (although the basic rules that govern these assemblies are currently not understood in useful detail). With freezing water vapor however, there is a great variety in snow crystal formation (but the physics behind the morphology of snowflakes is also not too well understood, either). There are other additional terms to describe snowflake shapes, as well. There are "prisms" (hollow and solid), so-called "stars", and even "cups". Which one depends upon the temperature as well as the moisture content of the air at the time.

The Longer Runway
What is the significance of a runway that is 8069 feet in length?

  1. aside from the fact that this is the minimum short field landing distance of a B-747-400 at max gross weight, none at all
  2. That is the cutoff point for a different runway symbol on all aeronautical charts.
  3. This is the maximum length for a single runway to be depicted on a sectional chart inside a circle that is just under three-sixteenths of an inch wide.
  4. When NOAA standards for symbology were redefined in 1965, this was the minimum length for the top five percent of runway lengths in the US, and this determined how large the aerodrome symbol would be.

Answer: C. If you knew that it's based on what fits in those little bitty circular runway symbols, you were on to something there. Choice B is close. (Also, choices A and D are total fiction.) As you know, different aerodrome symbols are used to depict runways on various aeronautical charts, such as Terminal Area Charts, and Sectionals. Several different symbols are used to show hard-surfaced runways. First, regarding sectional charts, here are the gory details, according to the National Aeronautical Charting Office, or NACO (whose 300 or so employees were transferred from NOAA to the FAA on October 1, 2000). When a runway is between 7970 and 8069 feet in length, they say that cartographers round it to 8000. This equates to a line that is 0.192 inches in length on the sectional's scale. In this case, a circular aerodrome symbol is used. (They're pretty good about showing the relative orientation of the predominant runway, and some effort is made to proportionally shorten the line a bit for runways that are between 1500 and 8069 feet long.) NACO says that if the runway is between 8070 and 8169 feet in length (inclusive, incidentally), they round to 8100, which equates to a line 0.1944 inches in length on the sectional's scale. Since this line is too long to fit into the largest circular aerodrome symbol NACO has available (which happens to be 0.193656 of an inch, which when multiplied by the sectional's scale factor of half a million, comes out to 8069 feet), they place a box around the actual runway pattern forming a representative enclosed shape. So when a runway is 8070 feet or longer, it's going to actually look like a smaller version of the runway, right? Well, almost. Actually, for airports with multiple runways, if the maximum distance between the two furthest points between the respective opposite ends of any two runways exceeds 8069 feet (even if the longest runway length is shy of 8069), then they do it that way, too. And for Terminal Area Charts, outlined boxes showing the actual runways are used for any airport with a runway that is just 1500 feet long, or longer. (Why isn't the TAC chart cutoff 4035 feet, since it shows things at twice the scale of a sectional? Probably because for local flights where pilotage and perspective matter more, they want us to have a better idea of what to look for.) This figure below is fairly representative of what you will see on a sectional:

Dance Of the Magnets
Why is there a difference between the magnetic variation for an airport, and the VOR located at that same airport?

  1. Magnetic variation changes over time and remains constant to within one one-hundredth of a degree for any given location, whereas the radials from a VOR might vary by ten times that over a single day.
  2. Because of the obliquity of the ecliptic (the tilt of the earth's axis), there is a very slow inductive coupling between the earth's magnetic field and the solar wind, which causes the earth's magnetic poles to precess about a common point over a cycle of roughly 26,000 years. This affects the earth's magnetic field, and it is the reason that we have to paint new numbers on our runways every few decades. However, if a VOR could last 26,000 years, it wouldn't change one bit.
  3. When a navaid is first installed, the antenna is physically oriented to True North. A potentiometer adjustment is then made to slave the navaid with Magnetic North. This action matches the isogonic line making it agree with a magnetic compass. Initially, these values are the same, but the magnetic variation of the earth changes at a rate of 50.27 seconds of arc per year. Navaids are commissioned and remain online, 24/7. Although periodic maintenance is performed as needed, a re-slaving to match the isogonic value requires a total navaid shut down, re-alignment, and a re-certification flight check. Only when the navaid is out of tolerance by at least +/-6 degrees will a re-slaving procedure be initiated allowing the navaid and airport magnetic variation to match again.
  4. Baloney. (the same for choice A, and choice B - at least as far as the solar wind and the obliquity jazz) But this precession is real, and because of it, in about 12,000 years, the Nothern Hemisphere will have summer in December and winter in June (if we still have months with those names then). But it has nothing to do with the earth's magnetic field. No, the magnetic poles wander mostly because of one thing, which is the "secular" variation due to the chaotic churning and shifting in the earth's molten core. (There's also a secodary artifact due to continental drift.) Yes, it's true about the six degrees, the re-slaving, and the re-certification flight check. Actually, choice C came from the FAA's own Aviation System Standards, and NACO's FAQ page. But they didn't get the story exactly straight. The change in magnetic variation at all locations on earth (or any two chosen at random) is not the same single value. Yes, it's got to do with the earth's molten innards. However, the variation in the earth's magnetic field is not predictable (at least yet, and certainly not to the degree of precision which the FAA's number implies). There may be some short-term zonal periodicity, or it may be predictable in one area, more or less, over a few years, but certainly not in all locations, indefinitely! The rate of change in other words, is changing, and that is at any one spot! Geologists and the like actually have different words for what we think of as magnetic variation. To them, it's magnetic declination. The actual "dip" angle (which results in the compass errors that so plague us) is known as magnetic inclination. Have a look at these charts, and then tell me it's always 50.27 arc-seconds per year!

Answer: It's D. The contour interval for the isopleths on the second chart (which by the way is at http://geomag.usgs.gov/MagCharts/wmm-gif/WMM-00Ds.gif), and which shows the annual rate of change in what we know as magnetic variation, is one minute of arc per year.

Main Field Annual Change
Click to Enlarge Click to Enlarge

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