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This is a detail from the San Francisco sectional aeronautical chart (click the map for a larger view). Federal Aviation Administration

How to Read a Pilot’s Map of the Sky

The first time I saw an aeronautical chart, best I can recall, was at the little airport café in Half Moon Bay, California, while waiting for a table. The coastal mountains and cities scattered around San Francisco Bay were easily recognizable. But superimposed on that familiar landscape were cryptic numbers, strange symbols, and overlapping circles that hinted at an entirely different world in the skies above.

“It looks pretty complicated, doesn’t it?” says Brendan Quinn-Narkin, a commercial pilot and certified flight instructor in Northern California. Quinn-Narkin sells instructional videos for pilots online, and he recently volunteered to help me make sense of these maps, which pilots use for planning and navigation.

In the United States, aeronautical charts are published by the FAA, the Federal Aviation Administration. The one above is similar to the one I saw in Half Moon Bay. It’s used for flying under visual flight rules (commonly referred to as VFR—if you’re going to be a pilot, you’d better get comfortable with acronyms). “Basically that means you have to be able to see outside,” Quinn-Narkin says. “You can’t be in a cloud or fog that would obstruct your view. You’re expected to be able to see other airplanes and avoid them.”

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Airline pilots and private pilots certified to fly by instruments alone use a different set of charts that look like they were drawn by and for robots (more on these later). The gallery above highlights a variety of aeronautical charts, including one showing the restricted airspace above the White House and the complicated runway map for Dallas Fort Worth International Airport.

VFR charts like the one at the top of this post are a kind of hybrid, representing both what a pilot would see outside the cockpit with his own eyes as well as more abstract layers of information that come from instrument readings and the rules of aviation. The FAA publishes a set of 54 of these “sectional charts” that cover the entire country.

Features on the ground are represented without much detail. The urban centers of the Bay Area are uniform yellow splotches. The hilly topography is shown in shaded relief, but with muted colors.

What stands out most on these maps, to me anyway, are all the colored circles and other shapes. These, Quinn-Narkin explains, indicate different types of airspace, mostly around airports, where pilots have to follow extra rules or procedures.

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This part of the legend for the San Francisco sectional chart explains how different classes of airspace are represented. Federal Aviation Administration

For example, the nation’s busiest airports are surrounded by what’s called class B airspace. To enter it, pilots have to request clearance from air traffic control. On the map at the very top, the concentric blue circles around San Francisco airport represent class B airspace.

But, unlike the map, the airspace itself is three dimensional. It has the shape of an upside-down wedding cake (picture a giant, round, six-tiered cake with the skinny end sitting on SFO and the fat end sticking up 10,000 feet in the air—that’s the class B airspace). Planes coming in for a landing or gaining altitude as they take off will pass through this space, so, for safety reasons, it’s carefully controlled.

On the map, the tiers get flattened into a set of concentric circles. When you’re far out, the class B airspace only extends from 8,000 feet to 10,000 feet. That’s indicated by the 100/80 figure in the bottom right corner of the map. Most numbers on aeronautical charts are truncated to save space, so you have to add two zeros to get the right value: 80 = 8,000, 100 = 10,000, and so on. As you get closer to SFO, the floor of the restricted airspace gets progressively lower, until finally at the airport itself it goes all the way from the ground to 10,000 feet.

The Bay Area’s other two major airports, Oakland and San Jose, are surrounded by slightly less restrictive class C airspace, as indicated by the magenta circles (well, almost circles) and other shapes around them. “You need to be on the radio with air traffic control, but you don’t need to get permission to enter,” Quinn-Narkin says of the class C designation. “You also need to have a transponder in your aircraft that tells the controller what altitude you’re at and your location.” Smaller airports in more remote areas are surrounded by class E airspace, indicated by thick, shaded magenta lines.

A lot of the text in this part of the map has to do with information about the many airports in the region. The detail below, for example, zooms in on Reid Hillview airport near San Jose, which Quinn Narkin has often used. The little white lines inside the round blue symbol show the actual orientation of the two runways.

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This section of the map shows San Jose’s Mineta airport and the smaller Reid Hillview airport just to the southeast. Federal Aviation Administration

The fact that the symbol is blue, not magenta, means that there is a control tower here. The radio frequency for the control tower is given right under the airport’s name, where it says CT-119.8. The other numbers give radio frequencies for other communication and information channels, along with other details. 135 is the elevation above sea level. 31 is the length of the runway (you have to add two zeroes, so really it’s 3,100 feet).

The map also depicts potential obstructions and other things to steer clear of. The large blue 48 just to the east of Reid Hillview airport indicates the lowest safe flying altitude in that area (4,800 feet). San Francisco’s tall buildings and bridge towers are represented by blue symbols that resemble mountains in the map detail below, which comes from a more detailed “terminal area chart.”

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Golden Gate Park, Alcatraz, and other San Francisco landmarks are visible in this more detailed chart of the area around San Francisco International Airport. Federal Aviation Administration

The FAA isn’t just looking out for pilots, though. Notes on the sectional chart warn pilots to watch out for endangered California Condors around Pinnacles National Monument and not to fly below 1,000 feet over Monterey Bay Marine Sanctuary.

If you look closely at the map at the very top of this post, you’ll see light blue lines crisscrossing it. These are routes used by commercial pilots and others certified to fly using instruments alone. The FAA produces an entirely different set of aeronautical maps for flights under instrument flight rules. These IFR charts (see the example below) are crisscrossed by lines indicating routes that are anchored by a network of navigational radio beacons on the ground. The VFR charts also include some of the prominent routes, which a pilot could use as a point of reference when talking to air traffic control or even decide to follow, Quinn-Narkin says.

The IFR charts are fascinating too, no doubt. They’re part of the reason planes can cross the country in the middle of the night and don’t get lost flying through clouds. But they’re somehow less inviting than the VFR charts. They look like something the Cylons of Battlestar Galactica would use. Virtually all traces of the terrain below and its human inhabitants have been erased.

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This is an instrument flight rules chart for the area around Oakland, California. Federal Aviation Administration

–Greg Miller