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Color Code

If you can break the color code you can 'see' a lot more than the airspace on a sectional chart.If you can break the color code you can 'see' a lot more than the airspace on a sectional chart.

Remember that the boundary between Class G and Class E airspace is an imaginary surface that hovers above the Earth's surface. Most of the time the boundary hovers at 1200 feet AGL. Then in other locations (inside the magenta shading) the boundary is at 700 feet AGL. These boundaries are imaginary, but if you could see them they would have the same features as the terrain that is under them. So the key to 'seeing' the shape of the boundary is to be able to 'see' the shape of the Earth. The key to 'seeing' the shape of the Earth is being able to interpret the sectional chart's color codes.

In aviation we use charts -- not maps...

  • Maps do not depict the Earth's terrain features. A road map does not show river valleys, ridges, canyons, and mountains -- a map shows the Earth as if it were flat.
  • Charts use 'contour lines' and different colors to indicate how high above sea level that the Earth's surface is at every location.
The difference between a map and chart is topography -- an illustration of the Earth's terrain features and elevations.

...or in this case, me. I live in a part of the country that is relatively flat; in fact, within a 200-mile radius of my home airport the terrain's elevation is never higher than 1,000 feet above sea level. That means that the sectional chart of my home area has only one 'background color.' The color used is a pale green, but this color is not meant to indicate what the Earth looks like from above looking down. The Earth is actually many more colors than pale green. The pale green color indicates that the terrain is within the first 1,000 feet of sea level. Any terrain that sticks up above 1,000 feet MSL would be shown with a slightly darker shade of green and the boundary between the pale and dark green colors would be a contour line.

Since my area is so flat it is easy to take that pale green color for granted and forget that it is actually a code-color for the elevation in my part of the country. To really be able to translate the color code into a 3-D mental image of what the terrain really looks like, I have found that looking at charts that have a wide range of elevation helps. There is no better example of wide ranging elevations than the Big Island of Hawaii.

Figure 1
Figure 1 (above) is the southern part of the Island of Hawaii shown on a sectional chart. This chart has all the colors. The shore of the island is, of course, at sea level, but within just 12 miles the terrain climbs to over 13,000 feet -- that is extremely steep, and extremely tall... but then, it's a volcano. It starts at sea level with the pale greens, then darker greens, then on to shades of tan, and finally at the highest altitude browns and dark browns.

But this island/volcano actually has more 1,000-foot levels of elevation than there are colors.

Look carefully at the higher elevations -- darker shades -- and you will see contour lines within the shade indicating additional 1,000-foot intervals. Count up all the colors and contour lines and you will see 13 divisions -- that's 13 different 1,000-foot levels.

Now to really get the picture, one of my colleagues took a Big Island sectional chart and cut out all the different colors and contour lines. Then each color and contour was glued on a different level of cardboard stacked one on top of the other.

Figure 2
The effect turns the color code into a real volcano that you can actually see (Figure 2). Pilots must develop the ability to 'see' the volcano or -- any other terrain feature -- using just the color codes. When I lay a sectional chart across a table and look across the different colors, I start to see the mountains and ridges popping off the table. I see the valleys, cliffs, and rifts dipping down below the table. I take the flat chart and, by using the colors, I see the Earth's elevations in my imagination. A map could never do that!

BOTTOM LINE: When I 'see' the Earth as it truly is -- I can then construct in my mind how the airspace looks because the Class G/E boundary is a mirror image of the terrain beneath it. As you can see flying requires a really good imagination.

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About This Author:
Paul A. Craig is a Gold Seal Multiengine and Instrument Flight Instructor. He currently holds a total of 11 Flight Certificates including his ATP. Craig is a previous winner of the North Carolina and Tennessee Flight Instructor of the Year award, the NCVT Outstanding Teacher award and has served as the regional representative of the National Air and Space Museum. Craig is an FAA Aviation Safety Counselor and the author of eight books, including Pilot In Command, The Killing Zone: How and Why Pilots Die, and Controlling Pilot Error: Situational Awareness (all from McGraw Hill).
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