When you unfold a sectional aeronautical chart, the most immediately obvious features are the clusters of information surrounding airport symbols. These are called airport data blocks, and they are the FAA's way of communicating an enormous amount of operationally critical information in a very small space. A pilot who can read a data block fluently can determine, at a glance, whether an airport has a control tower, what frequency to use, how long the longest runway is, whether it has fuel, and dozens of other details — all without ever pulling out a separate reference.
For the private pilot knowledge test and for real-world flight planning, airport data blocks on sectional charts are one of the most heavily tested and most practically useful skills you can develop. This article walks through every component of a standard data block, explains what each element means and why it matters, and highlights the traps that appear on FAA exams.
The Airport Symbol Itself
Before diving into the data block, understand the symbol the block surrounds. Airports are depicted as one of several symbol types based on their operational characteristics. A circle with a filled dot in the center generally indicates a private airport. A circle alone (or with tick marks) indicates a public-use airport without a control tower — these are shown in magenta (purple-pink). An airport with an operating control tower is shown in blue. This color difference is the single fastest piece of information on the chart: blue means controlled, magenta means uncontrolled. Hard services airports (those with hard-surface runways) are depicted differently from soft- or unpaved-surface airports, and seaplane bases and heliports have their own distinct symbols. Recognizing the symbol type instantly tells you the class and nature of the airport before you read a single number.
Breaking Down the Data Block: Line by Line
The data block is arranged in a standard, consistent layout. Once you learn the pattern, it becomes second nature. Here is how the block is organized from top to bottom:
Airport Name and Elevation
The top of the data block gives the airport name in capital letters. Directly below the name is the field elevation, expressed in feet Mean Sea Level (MSL). For example, an elevation reading of 1,342 means the airport lies 1,342 feet above mean sea level. This number is critical for density altitude calculations, terrain clearance planning, and setting your altimeter during the traffic pattern. For controlled airports with an operating ATIS, you will also check elevation against the altimeter setting you receive, but the charted elevation is your starting reference.
Control Tower Frequency (CT)
For towered airports shown in blue, the next line typically shows the control tower frequency preceded by the abbreviation CT. For example, CT-118.3 means the tower operates on 118.3 MHz. This is the primary contact frequency for arriving and departing aircraft when the tower is operating. Note that this frequency may not be in use 24 hours a day — some towers have limited hours, and the data block may note the hours of operation or you may need to consult the Chart Supplement (formerly the Airport/Facility Directory) for specifics.
ATIS/ASOS/AWOS Frequency
Immediately following the tower frequency, you may see a frequency labeled ATIS, ASOS, or AWOS. ATIS stands for Automatic Terminal Information Service, a recorded weather and airport information broadcast that is updated whenever there is a change in the weather or a new hourly weather sequence is received, rather than on a strictly fixed hourly schedule. ASOS and AWOS are automated surface observation systems. Having this frequency charted allows a pilot to obtain weather before entering the traffic pattern, which is both a courtesy and a safety practice strongly encouraged by the AIM.
UNICOM Frequency
At non-towered airports, the data block shows a UNICOM frequency. UNICOM (Universal Communications) is a non-government air-to-ground radio communication service. Pilots use UNICOM to request airport advisories and to announce their position in the traffic pattern using self-announce procedures. The most common UNICOM frequency is 122.8 MHz, though 122.7, 122.725, 122.975, and others are also used. If no UNICOM is available, CTAF (Common Traffic Advisory Frequency) announcements are still expected, and the CTAF will be charted — sometimes it is the same frequency as UNICOM.
Rotating Beacon and Lighting
The star symbol (✦) next to an airport name on the chart indicates the presence of a rotating airport beacon. For civilian land airports, the beacon alternates between white and green. Military airports use white-white-green (two whites alternating with one green). The beacon is especially important for night operations: it helps you visually locate the airport from the air. Additionally, a capital letter L in the data block indicates that the runway has pilot-controlled or standard lighting available during the hours of darkness. If the L is inside parentheses — (L) — it means lighting is available but only during limited hours or on request. No L means no runway lighting.
Runway Length
The next number in the data block represents the length of the longest runway at the airport, expressed in hundreds of feet. So a value of 35 means the longest runway is 3,500 feet. A value of 60 means 6,000 feet. This is a truncated representation — always mentally add two zeros. Knowing the runway length is essential for performance planning: your aircraft's Pilot Operating Handbook (POH) specifies the minimum runway lengths needed for normal and short-field takeoff and landing under various conditions.
Fuel Availability
The letters 100, 100LL, JET A, or combinations thereof may appear in the data block or be abbreviated. On sectional charts, fuel availability is usually shown by the presence of specific service notations. The lack of any fuel notation is a significant planning consideration, particularly for cross-country flights. The Chart Supplement provides more granular fuel information, including operating hours of the FBO. Running out of fuel is the top cause of preventable accidents that a fuel stop can avert, so always verify fuel availability before a long leg.
Special Use and Other Notations
The data block may also include notation for military airports (a different symbol with additional identifiers), seaplane bases (anchor symbol), or airports with special restrictions. Some data blocks include a small flag or other icon indicating military operations. An R for restrictions or a note pointing to restricted airspace nearby may also appear adjacent to the symbol.
Why Accurate Data Block Reading Matters
Consider this scenario: you are planning a cross-country flight and need to select a fuel stop. By reading data blocks, you can quickly identify which airports have the longest runways (reducing performance risk), which have lighting for a potential after-dark arrival, which have fuel, and which have ATIS so you can get weather in advance. All of this happens during preflight planning without opening a single additional document.
Misreading a data block — especially confusing runway length (in hundreds of feet) for actual feet, or missing the absence of a lighting indicator — can lead to an aircraft landing at an airport too short for its performance requirements, or arriving at an unlighted airport at night without preparation. These are not minor inconveniences; they represent genuine safety risks.
Key Numbers and Rules
- Blue airport symbol: operating control tower. Magenta airport symbol: non-towered airport.
- Runway length: expressed in hundreds of feet. Always multiply by 100.
- Rotating beacon: depicted as a star (✦) next to the airport name. Green and white = civil airport. Green, white, white = military airport.
- L in data block: lighting available. (L) in parentheses = limited lighting hours.
- CT: control tower frequency. UNICOM 122.8 MHz is the most common non-towered frequency.
- Elevation: always MSL in feet.
- ATIS, ASOS, AWOS: automated weather broadcasts; frequency charted in data block when available.
Common Test Traps
- Runway length in hundreds: A data block showing 50 means 5,000 feet, NOT 50 feet. Exam questions often present this value and ask for the correct runway length — always add the two zeros.
- Color confusion: Students sometimes mix up blue and magenta. Remember: blue = tower, magenta = no tower. The color is one of the most testable facts about airport symbols.
- Lighting parentheses: The difference between L and (L) is operationally significant. Exams may ask whether lighting is continuously available — parentheses mean it is NOT continuously available.
- Elevation is MSL, not AGL: Airport elevation in the data block is above mean sea level. Traffic pattern altitude is typically 1,000 feet AGL above field elevation — you must add these together to get your pattern altitude on the altimeter.
- UNICOM vs. CTAF: UNICOM is a two-way frequency used for advisories and services. CTAF is the designated frequency for traffic pattern self-announce procedures. At many airports they are the same frequency, but they are not the same concept. Knowing this distinction avoids confusion on both the knowledge test and in real operations.
Mastering airport data blocks is one of the most efficient investments a student pilot can make. The information density is high, the format is standardized, and the payoff — safe, well-planned cross-country flying — is immediate. Spend time with a real sectional chart pointing at data blocks and reading each element aloud until the pattern is automatic. That habit will serve you on the knowledge test and every flight thereafter.