A VFR sectional chart is one of the densest information graphics in ordinary use. It has to show terrain, obstacles, airports, radio navigation, five classes of airspace stacked on top of each other, military training areas, and enough ground detail to navigate by looking out of the window, all on a sheet of paper a pilot can fold onto a kneeboard.
It manages this through a rigorous colour and symbol grammar. Learn the grammar and the chart stops being decoration. Below is enough to read one confidently, whether you are flying, spotting, or just wondering why the traffic over your house always turns at the same place.
This is a general explanation for interest and education. The chart layers in the app, and everything here, are reference material only. Never use them for navigation. The FAA publishes a free Aeronautical Chart User's Guide that is the authoritative reference for every symbol on a sectional, and it is the right place to settle any detail.
Scale, coverage and expiry
Sectionals are drawn at 1:500,000. One inch on the chart is about 6.9 nautical miles on the ground, close enough that pilots measure with a plotter and get useful answers. Each sheet covers a named region and is titled for a major city on it.
Where a Class B airport makes that scale too crowded, the FAA also publishes a Terminal Area Chart at 1:250,000: twice the scale, the same grammar, far more room. If you are trying to understand traffic around a big airport and the sectional feels cramped, the TAC is what you want.
Charts expire. Every sheet is stamped with an edition and an expiry, and airspace, frequencies and obstacles all change between editions. Two things are never on a sectional at all: temporary flight restrictions and NOTAMs. A chart cannot tell you about a stadium TFR or a closed runway, because those are published separately and change by the hour.
The colour system: blue means towered
This is the single most useful rule on the chart. Blue relates to airports and airspace with an operating control tower; magenta relates to airports and airspace without one. Once that clicks, the whole sheet reorganises itself.
So an airport symbol drawn in blue has a tower. The same symbol in magenta does not. Traffic there self-announces on a common frequency instead. The rule extends upward into the airspace boundaries drawn around them.
Airport symbols
Airports are drawn as circles, with detail added by rule:
- An airport with hard-surfaced runways long enough to matter is drawn with its runways in true configuration once they exceed 8,069 feet, so the shape on the chart really is the shape on the ground. Shorter hard-surfaced fields get a circle with the runway pattern shown inside.
- A plain open circle means the field has no hard-surfaced runway meeting those criteria: grass, gravel, or dirt.
- Tick marks radiating from the circle mean fuel is available during normal business hours.
- Private and restricted fields are marked as such, and heliports, seaplane bases and ultralight areas each have their own symbol.
The data block
Beside each airport sits a compact block of text. Reading left to right and
top to bottom, it typically gives the airport name, its identifier, the control
tower frequency if there is one (prefixed CT), the common traffic
advisory frequency if there is not (marked with a circled C), the
field elevation in feet above mean sea level, a lighting indicator, and the
length of the longest runway in hundreds of feet.
That last convention catches everyone once. A runway shown as 72
is 7,200 feet long, not 72. And the elevation is MSL, not height above the
surrounding ground, which is exactly the number you need when you are flying an
altimeter.
The lighting indicator deserves a note: an asterisk beside it warns that the lighting is limited, part-time, or pilot-activated rather than simply on from dusk to dawn. The Chart Supplement (the companion volume of airport data) has the details the chart has no room for.
Airspace, drawn in four line styles
American controlled airspace comes in classes, and the chart distinguishes them by line style and colour rather than by labels:
| Class | How it is drawn | Roughly what it is |
|---|---|---|
| B | Solid blue lines | The busiest airports. Stacked shelves like an upside-down wedding cake; entry requires an explicit clearance. |
| C | Solid magenta lines | Busy airports with radar service. Two-way radio contact required before entering. |
| D | Dashed blue lines | Airports with an operating control tower but less traffic. Usually a small cylinder around the field. |
| E (surface) | Dashed magenta lines | Controlled airspace reaching all the way to the ground at a non-towered field, normally where there are instrument approaches. |
| E (700 ft AGL) | Soft-edged magenta band | Inside the shaded band, controlled airspace begins 700 feet above the ground rather than higher up. |
| G | Not drawn | Uncontrolled airspace: whatever is left underneath everything else. |
There is also a soft-edged blue band, which marks places where the floor of Class E differs from the usual value away from airports. It appears far less often than the magenta one, and the chart legend states precisely which side of it means what, a good example of a symbol worth checking rather than remembering.
The numbers stacked like a fraction
Inside Class B and Class C boundaries you will see pairs of numbers written
one above the other, such as 100 over 30. These are
altitudes in hundreds of feet MSL: a ceiling of 10,000 feet
above a floor of 3,000 feet. SFC in the lower position means the
airspace reaches the surface.
Class D is simpler: a small dashed box with a number in it gives the ceiling in hundreds of feet MSL, and a minus sign in front means "up to but not including" that altitude, because the airspace above belongs to somebody else.
This is the mechanism behind something you can watch on any live map near a big airport. Light aircraft transiting the area fly a track that looks oddly kinked, staying low over one district and climbing sharply once past it. They are threading underneath a Class B shelf, and the sectional shows exactly where the floor steps up.
Terrain, and the big blue numbers
Terrain is shown two ways at once: contour lines, and hypsometric tinting (the graded colours that go from green at low elevations through tan to brown as the ground rises). The legend gives the elevation bands.
Then there are the large bold numbers scattered across the chart, one per quadrangle, often written as a big digit and a small raised one. These are Maximum Elevation Figures, and they are the most quietly important thing on a sectional. The MEF gives the highest known terrain or obstacle within that quadrangle, in thousands and hundreds of feet above mean sea level. A quadrangle marked with a large 5 and a small 6 has something in it reaching 5,600 feet.
It is not a minimum safe altitude and it is not a clearance figure. It is a statement about the tallest thing in the box, and it exists so a pilot can glance at a chart and know instantly what the ground is capable of.
Obstacles
Towers, stacks, and anything else worth avoiding get an obstacle symbol, with a taller variant used for obstacles reaching 1,000 feet or more above the ground. The number beside one gives its top in feet MSL, with the height above ground in parentheses underneath. A radiating pattern around the symbol indicates it is lit at night. Guyed structures are a particular hazard, because the supporting wires extend well beyond the tower itself, and the chart legend is the place to check exactly how each obstacle variant is drawn.
Navigation aids and airways
A VOR is drawn as a small polygon inside a compass rose, and the rose is aligned to magnetic north rather than true north, which is why they appear slightly rotated relative to the chart grid. Beside it, a box gives the station's name, its frequency, its three-letter identifier and the identifier again in Morse code, because the way you verify you have tuned the right station is by listening to it identify itself. NDBs, VORTACs and VOR/DME stations each have their own symbol variant.
Straight blue lines running between navaids and labelled V plus a
number are Victor airways: the published low-altitude route
structure. Instrument traffic follows them, and on a live map you can watch it
happen. A line of aircraft on a sectional airway is one of the most satisfying
overlays there is.
The dashed magenta lines that wander across the whole chart, unrelated to anything else on it, are isogonic lines: contours of equal magnetic variation, the difference between true and magnetic north at that point.
Special use airspace
Hatched bands mark airspace with restrictions attached:
- Blue hatched bands enclose Prohibited (
P-), Restricted (R-), Warning (W-) and Alert (A-) areas. - Magenta hatched bands enclose Military Operations Areas, which carry names rather than numbers and can be wonderfully unserious.
Both are cross-referenced to tables printed on the chart margins, giving the altitudes involved, the times of use, and the controlling agency to call. The areas themselves are drawn but their rules are not, because there is nowhere near enough room.
Military low-level training routes appear as thin grey lines labelled
IR or VR with a route number. If you have ever watched
a fast military aircraft on a tracker follow a series of implausibly precise
dogleg turns at low level, a training route is very often why.
Putting it under live traffic
All of this becomes considerably more interesting when there are aircraft moving on top of it. Airplanes.live carries VFR sectional and IFR chart layers underneath the unfiltered live map, which turns the chart from a static reference into an explanation: why an arrival stream bends where it does, why light traffic hugs a particular valley, why nothing flies over one specific patch of ground.
Two closing cautions, both meant sincerely. The charts in the app are a reference layer and not a navigation product. They carry no NOTAMs, no TFRs and no guarantee of currency, and the terms of use say so plainly. And if the reason you are reading this is that you are learning to fly: your instructor and the FAA's own publications are the authority here, not a blog post from a flight-tracking app.
