What is the Mode C Veil and where does it appear on a sectional chart?

The Mode C Veil is a 30 nautical mile radius circle drawn around certain major airports, depicted on sectional charts as a magenta dashed ring, indicating the airspace where an altitude-encoding (Mode C) transponder is required. Under 14 CFR 91.215, all aircraft operating within this veil, from the surface up to 10,000 feet MSL, must have an operating transponder with automatic altitude reporting capability. Recognizing this ring on a sectional chart is essential for flight planning because it extends well beyond the core Class B airspace it surrounds.

How do you identify the Mode C Veil ring on a sectional chart?

On a sectional chart, the Mode C Veil is shown as a thick magenta dashed circle centered on the primary Class B airport, and it has a radius of 30 nautical miles from that airport's reference point. It is distinct from the solid magenta lines depicting Class B airspace floors and ceilings, so pilots and drone operators must take care not to confuse the two features. When in doubt, the chart legend confirms that the magenta dashed line specifically represents the Mode C transponder requirement boundary.

Why does the Mode C Veil matter for Part 107 drone pilots studying for the FAA Part 107 Remote Pilot Knowledge Test?

The FAA Part 107 Remote Pilot Knowledge Test covers airspace rules that apply to small unmanned aircraft systems, and the Mode C Veil is a tested concept because drone pilots flying within 30 NM of a major Class B airport must understand the transponder environment around them even though most drones do not carry transponders themselves. While Part 107 drones are not required to have Mode C transponders, the veil defines an area of dense manned aircraft traffic where situational awareness and proper airspace authorization are especially critical. Misidentifying or ignoring the Mode C Veil ring on a sectional chart could lead a remote pilot to underestimate traffic complexity and airspace restrictions in that area.

Mode C Veil Depiction on Sectional Charts

When you unfold a sectional aeronautical chart and study the airspace around a major metropolitan airport, one of the most important — yet often overlooked — features you will encounter is the Mode C Veil. For Part 107 Remote Pilot Certificate holders and applicants, understanding exactly what this veil is, where it appears, and how it is depicted on the chart is not just an exam topic; it directly governs where and how you can legally fly your small unmanned aircraft system (sUAS).

The term "Mode C" refers to the transponder reply mode that encodes an aircraft's pressure altitude, allowing air traffic control (ATC) radar to see both the identity and the altitude of an aircraft simultaneously. The Mode C Veil is a circular area, centered on certain primary airports, within which the FAA requires all aircraft — including manned aircraft operating under 14 CFR Part 91 — to be equipped with an altitude-encoding transponder. As a drone pilot, you do not use a transponder, but the airspace rules that flow from the Mode C Veil still affect your operations in important ways.

What the Mode C Veil Is and Why It Exists

The Mode C Veil is defined in 14 CFR §91.215 and applies within a 30 nautical mile (NM) radius of the primary airport's airport reference point (ARP) at each Class B airspace location listed in 14 CFR Part 91, Appendix D. The purpose is straightforward: the airspace surrounding major airports is the most congested in the national airspace system. By requiring every aircraft inside this ring to squawk altitude, ATC can build a complete three-dimensional radar picture of traffic, dramatically reducing the risk of mid-air collisions and airspace incursions. The rule applies from the surface up through 10,000 feet MSL within the veil, with only narrow exceptions for aircraft without an electrical system or those granted a specific ATC deviation authorization.

It is important to understand that the Mode C Veil is not the same as Class B airspace itself. Class B airspace is the complex, layered "upside-down wedding cake" structure that immediately surrounds the primary airport and typically extends from the surface to 10,000 feet MSL. The Mode C Veil extends much farther out — a full 30 NM from the primary airport's reference point — and it overlaps not only Class B airspace but also Class C, Class D, Class E, and even some Class G airspace that lies within that radius.

How the Mode C Veil Is Depicted on Sectional Charts

On a sectional aeronautical chart, the Mode C Veil is represented by a faint, magenta (magenta/dark pink) dashed circle drawn around the primary Class B airport. This circle has a radius of 30 NM from the airport's reference point. It is deliberately rendered in a lighter, less prominent style than airspace boundary lines so that it does not clutter the chart, but once you know what to look for, it stands out clearly.

Here is how to distinguish it from other chart symbols:

Class B airspace boundaries are depicted with solid, bold blue lines forming the characteristic layered rings and sectors of the wedding cake structure.
Class C airspace boundaries are shown with solid magenta lines, typically forming an inner circle and an outer shelf area; the exact configuration can vary by location, so always confirm boundaries against the chart legend and any published exceptions.
Class D airspace boundaries appear as dashed blue circles around airports with operating control towers.
The Mode C Veil is a large dashed magenta circle — much larger than a Class C ring — centered on the primary Class B airport. Its 30 NM radius makes it one of the most expansive single features on the chart in that region.

In practical terms, if you look at a sectional chart for a major hub like Chicago O'Hare (ORD), Los Angeles International (LAX), or Hartsfield-Jackson Atlanta (ATL), you will see the solid blue Class B structure near the center and then, considerably farther out, the wide dashed magenta ring that marks the Mode C Veil boundary.

Why the Mode C Veil Matters for Part 107 Drone Pilots

Under 14 CFR Part 107, sUAS operations are governed by rules that interact with the existing airspace structure, including the Mode C Veil. Here are the key reasons every remote pilot must understand this feature:

Airspace authorization requirements: Flying a drone inside Class B, Class C, or Class D airspace, or within the lateral boundaries of a Class E airspace that extends to the surface, requires prior authorization — either through the FAA's Low Altitude Authorization and Notification Capability (LAANC) system or a formal Airspace Authorization from the FAA's DroneZone portal. While the Mode C Veil itself does not automatically trigger an authorization requirement for drones (the authorization requirement is tied to specific airspace classes), it is a strong visual indicator that you are in a high-traffic area where authorization is almost certainly needed for some or all of your intended flight altitudes.

Situational awareness: The Mode C Veil tells you that you are within 30 NM of a major commercial airport. Even at locations within the veil where Class G or Class E airspace reaches the surface — and where a drone flight might be technically permissible without formal authorization — the volume of manned aircraft traffic is likely to be elevated. Remote pilots operating in these areas must exercise heightened vigilance for manned aircraft traffic and must ensure compliance with all applicable airspace authorizations.

Chart reading proficiency: The FAA Part 107 Aeronautical Knowledge Test regularly asks candidates to interpret sectional chart excerpts. Being able to quickly identify the Mode C Veil ring, distinguish it from Class C airspace circles, and correctly state its radius (30 NM) are directly tested skills.

Key Numbers and Rules

Radius: 30 nautical miles from the primary Class B airport's reference point.
Altitude applicability (for manned aircraft): Surface up to 10,000 feet MSL, as specified in 14 CFR §91.215.
Chart depiction: Large dashed magenta circle on the sectional chart.
Regulatory basis: 14 CFR §91.215 (transponder and altitude reporting equipment requirements); airports listed in 14 CFR Part 91, Appendix D.
Not the same as Class B airspace: The veil extends far beyond the Class B boundaries — it can encompass Class C, D, E, and G airspace.
Part 107 relevance: Does not itself impose a drone authorization requirement, but signals proximity to major airports and heavily trafficked airspace where other rules apply.

Reading the Chart: A Practical Walkthrough

When you receive a sectional chart excerpt on the knowledge test — or when you are preflight planning a real drone operation — use this step-by-step approach to identify the Mode C Veil:

Locate the primary airport symbol. Major Class B airports are depicted with a blue airport symbol and labeled with their three-letter identifier. Look for the large, layered solid-blue rings of the Class B structure immediately around it.
Scan outward from the airport. Moving your eye outward from the Class B structure, look for a large dashed magenta circle. It will be much farther from the airport than the Class B or Class C rings.
Verify the color and line style. The Mode C Veil is magenta (pinkish-purple) and dashed. Class D airspace is also dashed but blue. Class C is solid magenta. The large dashed magenta ring is the Mode C Veil.
Check the legend. The sectional chart legend, found in the margin area, depicts the Mode C Veil symbol and labels it clearly. When in doubt on a test or in the field, consult the legend.
Measure if needed. If you have a sectional plotter or a distance tool, you can confirm the boundary is approximately 30 NM from the primary airport. This is useful when planning a drone flight near the edge of the veil.

Common Test Traps

Confusing the Mode C Veil with Class C airspace. Both use magenta, but Class C boundaries are solid lines forming a smaller circular structure (often an inner circle plus an outer shelf). The Mode C Veil is a single large dashed circle. Mixing these up is one of the most common sectional chart errors on the knowledge test.
Believing the veil creates a drone no-fly zone. The Mode C Veil does not, by itself, prohibit drone operations. What matters for Part 107 operators is the specific airspace class at the intended flight location and altitude. You must determine which airspace class governs the surface (or your operating altitude) and then check authorization requirements for that class.
Forgetting the 30 NM number. Some students confuse the 30 NM radius of the Mode C Veil with the 5 NM or 3 NM distances associated with notification requirements under older rules. Under current Part 107, the 30 NM figure is specifically associated with the Mode C Veil definition in §91.215.
Assuming the veil applies only below the Class B floor. For manned aircraft, the transponder requirement within the veil applies from the surface up to 10,000 feet MSL regardless of whether the aircraft is inside or below the Class B shelves. Candidates sometimes think it only applies within the actual Class B structure.
Overlooking the veil on a busy chart. Sectional charts contain enormous amounts of information, and the faint dashed magenta ring can be easy to miss when you are focused on airspace class labels and airport symbols. Train yourself to identify the Mode C Veil circle as one of the first things you look for when studying any chart excerpt featuring a major airport.

Mastering Mode C Veil recognition on sectional charts is a foundational skill for any Part 107 remote pilot. It connects regulatory knowledge (14 CFR §91.215), chart reading proficiency, and practical situational awareness into a single, testable concept. Study the chart legend, practice identifying the dashed magenta ring on real sectional excerpts, and always cross-reference it with the underlying airspace classes at your planned flight location before every operation.