What do the solid blue lines on a sectional chart mean?

Solid blue lines on a sectional aeronautical chart depict the lateral boundaries of Class B airspace, which surrounds the nation's busiest airports. Each enclosed area, or 'shelf,' represents a distinct segment of the airspace with its own ceiling and floor altitudes. Class B airspace requires an ATC clearance before entry, as outlined in 14 CFR Part 91.

How do you read the ceiling and floor fractions labeled on Class B airspace shelves on a sectional chart?

Each Class B shelf segment is labeled with a fraction where the top number represents the ceiling in hundreds of feet MSL and the bottom number represents the floor in hundreds of feet MSL — for example, '80/30' means the shelf extends from 3,000 feet MSL up to 8,000 feet MSL. The letters 'SFC' in place of the bottom number indicate that the airspace extends all the way down to the surface. The Pilot's Handbook of Aeronautical Knowledge (PHAK) and the Aeronautical Information Manual (AIM) both explain this labeling convention for identifying where authorization or clearance is required.

What's the difference between Class B airspace and Class C airspace as shown on a sectional chart?

Class B airspace is depicted with solid blue lines and typically features multiple shelf segments stacked at different altitudes around major hub airports, while Class C airspace is shown with solid magenta lines and generally has just two layers — a surface area and a shelf — surrounding airports with a moderate level of operations. Entry into Class B requires an explicit ATC clearance, whereas entry into Class C requires only that two-way radio communication be established with ATC. Both types of airspace, their dimensions, and their requirements are described in 14 CFR Part 91 and the Aeronautical Information Manual.

Class B Airspace Shelf Depiction on Sectional Charts

If you have ever spread out a VFR sectional chart over a large metropolitan area, you have probably noticed a series of concentric, irregularly shaped rings drawn in bold blue ink surrounding the busiest airports. Those rings represent Class B airspace — the most controlled airspace category in the National Airspace System (NAS) — and learning to decode every line, label, and number on that depiction is one of the most heavily tested skills on both the FAA Private Pilot Knowledge Test and the Part 107 Remote Pilot Certificate exam. For drone operators, the stakes are especially concrete: flying a small unmanned aircraft system (sUAS) in Class B airspace without proper authorization violates 14 CFR Part 107 and can carry serious civil penalties.

This article walks you through exactly how Class B airspace is drawn on a sectional chart, how to read the altitude labels, how the shelf structure works in three dimensions, and how to apply that knowledge when planning a flight or a UAS operation near a major airport.

What Class B Airspace Is and Why It Has a Shelf Structure

Class B airspace surrounds the nation's busiest commercial airports — think Atlanta Hartsfield-Jackson, Chicago O'Hare, Los Angeles International, and similar hubs. Its purpose, as described in the Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25), is to provide an orderly flow of IFR traffic in and out of these airports while keeping VFR traffic separated through positive ATC control. Every pilot (or remote pilot) inside Class B airspace must receive an explicit clearance or, in the case of sUAS operators, an authorization through the appropriate FAA channel.

Unlike Class C or Class D airspace, which are relatively simple cylinders, Class B airspace is shaped like an upside-down wedding cake — or, more precisely, a series of concentric shelves that get wider as you move away from the primary airport and that start at progressively higher floor altitudes. This tiered design exists because arriving and departing airliners are at different altitudes depending on how far they are from the runway. An aircraft on a 20-mile final approach is at a much higher altitude than one on a 5-mile final, so the outer shelves only need to extend down to the altitude where those aircraft actually operate.

How Class B Airspace Is Drawn on the Sectional Chart

On a standard VFR sectional chart, Class B airspace boundaries are depicted with solid, thick blue lines. Each closed polygon or arc represents one shelf of the airspace structure. The innermost ring is usually the core shelf, which extends from the surface up to the ceiling of the Class B. Moving outward, each successive ring represents a shelf that begins at a higher floor altitude than the one before it.

Inside or adjacent to each shelf segment, you will find an altitude label written as a fraction. For example, you might see 100/30 printed inside a blue boundary. The top number is the ceiling expressed in hundreds of feet MSL, and the bottom number is the floor also in hundreds of feet MSL. So 100/30 means the shelf extends from 3,000 feet MSL up to 10,000 feet MSL. If the floor of a shelf is the surface, the label will show SFC instead of a number — for instance, 100/SFC indicates the shelf runs from the surface to 10,000 feet MSL. This is almost always the innermost shelf, directly above the primary airport.

The ceiling of all shelves within a single Class B airspace is usually the same number — only the floor changes as you move outward. This gives you the wedding-cake appearance when viewed in cross-section: the top of the cake is flat (common ceiling), while the steps going down form the different shelf floors.

Reading the Labels Step by Step

To correctly interpret a Class B label, use this sequence every time:

Identify the blue boundary line that encloses the area you are evaluating. Each distinct enclosed area is one shelf.
Find the fraction label printed inside that boundary (or nearest to it on a crowded chart). The label may appear on the boundary itself or just inside the shelf area.
Read the top number as the ceiling. Multiply by 100 to convert to feet MSL. The value 110 means 11,000 feet MSL.
Read the bottom number as the floor. Multiply by 100 to convert to feet MSL. If it reads SFC, the floor is the surface (ground level at that location, effectively 0 AGL for authorization purposes).
Determine whether your planned altitude falls between the floor and ceiling. If it does, you are in that shelf of Class B airspace and authorization or clearance is required.

A practical example: suppose you are planning a drone flight at 300 feet AGL near a large city. You look at the sectional and find that at your location, the nearest blue shelf label reads 80/20. That means the shelf extends from 2,000 feet MSL to 8,000 feet MSL. If the ground elevation at your location is 500 feet MSL, your drone at 300 feet AGL would be at approximately 800 feet MSL — well below the 2,000-foot floor. In this case, you are under the shelf and not inside Class B airspace. However, if the label instead read 80/SFC, the shelf would begin at the surface, and that same 300-foot AGL flight would be inside Class B airspace, requiring LAANC authorization or a Part 107 waiver.

Why This Matters for Part 107 Remote Pilots

Under 14 CFR § 107.41, no person may operate a small unmanned aircraft in Class B, Class C, or Class D airspace, or within the lateral boundaries of the surface area of Class E airspace designated for an airport, without prior authorization from Air Traffic Control. For Class B airspace, this authorization most commonly comes through the Low Altitude Authorization and Notification Capability (LAANC) system, which provides near-real-time automated approvals up to certain altitudes in predefined grids. Remote pilots can also request authorization through the FAA DroneZone portal.

Because Class B shelves vary significantly in their floor altitudes across the coverage area, a remote pilot might be operating in Class B at one location but completely clear of it at another location only a few miles away — even at the same altitude. This is precisely why reading the sectional chart carefully, shelf by shelf and label by label, is not a formality but a genuine operational safety skill. Misreading a 40/SFC label as a 40/20 label could be the difference between a legal, safe flight and an unauthorized incursion into busy terminal airspace.

Key Numbers and Rules

Depiction color: Class B airspace is shown with solid blue lines on sectional charts. (Class C uses solid magenta, Class D uses dashed blue — do not confuse these.)
Label format: Ceiling (hundreds of feet MSL) over floor (hundreds of feet MSL, or SFC). Always top = ceiling, bottom = floor.
SFC floor: The innermost shelf of nearly every Class B airspace extends from the surface. SFC in the denominator means your drone needs authorization starting at 0 feet AGL.
Common ceiling: Most Class B ceilings are at 10,000 feet MSL (label reads 100), though some differ; always verify on the current chart.
Authorization requirement: 14 CFR § 107.41 requires ATC authorization before any sUAS flight within Class B airspace, regardless of altitude — even hovering at 1 foot AGL inside an SFC shelf requires authorization.
LAANC grids: LAANC approval altitudes are set by UAS Facility Maps, which show the maximum altitude the FAA will approve via LAANC for each grid square. A LAANC approval of 0 feet means no automated approval is available; the remote pilot must use the DroneZone manual process.
Chart currency: Sectional charts are updated every 56 days. Always use the current edition; Class B boundaries are periodically revised.

Memory Aid

"Top over Bottom, Ceiling over Floor" — When you see a Class B fraction on the chart, the top number is the ceiling and the bottom number is the floor. Think of it literally: the ceiling is physically above the floor in a room, just as the top number is above the bottom number in the label. This simple spatial association prevents the single most common chart-reading error on the knowledge exam.

Common Test Traps

Confusing ceiling and floor in the label. Test questions will give you a label such as 60/20 and ask whether an aircraft at 4,500 feet MSL is inside the shelf. The answer is yes — 4,500 is between the floor (2,000) and ceiling (6,000). Students who misread the label will choose the wrong answer.
Forgetting to convert hundreds of feet. The label 80 means 8,000 feet MSL, not 80 feet. Always multiply by 100. An answer choice of "8 feet" is a distractor designed to catch students who forget this conversion.
Assuming SFC only applies at the airport itself. The SFC shelf often covers a significant radius around the primary airport — sometimes several miles. Do not assume you are clear of Class B just because you cannot see the runway from your location.
Mistaking Class B blue lines for Class D dashed blue lines. Class B uses solid blue; Class D uses dashed blue. On a busy chart, these can look similar. Check whether the line is continuous (solid = Class B) or broken (dashed = Class D).
Believing LAANC authorization is automatic up to 400 feet everywhere. LAANC approval altitudes vary by grid and are often 0 feet in the core Class B area directly over or near the airport. Never assume a LAANC approval will be granted; always check the UAS Facility Map before flight.