Atc Transponder and Altitude Reporting Equipment and Use

Section 91.215 — ATC transponder and altitude reporting equipment — establishes where Mode C transponders are required.

Paragraph (b) — Transponder + altitude reporting required in:

• (b)(1) — Class A airspace (FL180 and above)
• (b)(2) — Class B airspace AND in all airspace within 30 NM of a Class B primary airport from the surface up to 10,000 MSL (the Mode C veil)
• (b)(3) — Class C airspace
• (b)(4) — Above 10,000 MSL (excluding airspace at and below 2,500 AGL)

The Mode C veil (paragraph b(2)): The 30 NM circle around every Class B primary airport, surface to 10,000 MSL. Within the veil, Mode C transponder is mandatory regardless of underlying airspace class.

The 10,000 MSL exception (paragraph b(4)): Mode C is required above 10,000 MSL, EXCEPT at or below 2,500 AGL — this allows flights in mountainous terrain at MSL altitudes above 10,000 but close to the surface (e.g., flying a Cessna over Colorado Rockies at 12,000 MSL where the terrain is at 9,500 MSL).

Paragraph (d) — Transponder operation: When operating in airspace where a transponder is required, the transponder shall be ON with appropriate altitude reporting equipment ENABLED.

Paragraph (e) — Removal/inoperative: If the transponder fails or is removed, the pilot may operate in or transit transponder-required airspace ONLY with prior ATC permission. The pilot calls ahead, ATC may approve a routing that doesn't require the transponder.

Mode C is altitude-reporting: The Mode C function provides automatic altitude reporting to ATC. A Mode A-only transponder is not sufficient for § 91.215 compliance. Mode S (which subsumes Mode C functionality) also satisfies the rule.

Reference: AIM 4-1-20 on Transponder Operation.

Where the Transponder Is Required — § 91.215 Decoded

Most pilots can recite "transponder and Mode C" without being able to draw the boundaries on a sectional. § 91.215(b) is a list of airspace volumes, not a single rule — and the one that bites VFR pilots is not the airspace itself, it's the invisible cylinder around it. The 30 nautical mile Mode C veil is drawn from the primary Class B airport, not from the lateral edge of the Class B shelves. You can be nowhere near the magenta ring on the chart and still be inside the veil.

Here's the full list, as the reg actually reads.

One more piece every modern pilot has to carry: § 91.225 (ADS-B Out) now applies in the same airspace as Mode C. The shorthand pilots actually use on the ramp is "Mode C veil equals ADS-B veil." If you need the transponder, you need ADS-B Out. There is no longer a clean "transponder-only" airspace for the airplanes most of us fly.

The exception line in § 91.215(c) matters too. If your transponder is inop — or your airplane doesn't have one — you can still get into the airspace, but only with prior ATC authorization. Call the controlling facility at least one hour before the proposed operation. They don't have to say yes.

What an Examiner Asks About § 91.215

This reg is a checkride favorite because it has a clean diagram and a hidden trap. Examiners want to see that you can read a sectional and tell them, in plain language, whether your transponder has to be on, off, or even installed.

• "What's the Mode C veil?" A 30 NM cylinder around every Class B primary airport, surface to 10,000 MSL. Inside it, you need a Mode C transponder and ADS-B Out per § 91.225. The radius is measured from the airport, not from the edge of the Class B.
• "You're at 9,500 MSL over the Rockies, not near any Class B or C. Do you need a transponder?" Maybe not. 91.215(b)(5) exempts airspace at and below 2,500 AGL even above 10,000 MSL — so if the terrain underneath you is 8,000 feet, you're inside the carve-out. The reg is about airspace, not just altitude.
• "When is the next transponder inspection due?" 24 calendar months from the last one — meaning it expires at the end of the month, not 730 days later. The requirement lives in § 91.413; 91.215 just requires the equipment.
• "Your transponder fails enroute. Can you keep going?" Outside the airspace listed in (b), yes. To enter or stay in any of it, you need ATC authorization per § 91.215(c) — and they'll usually accommodate you if traffic permits.

Transiting Near a Class B, Under § 91.215

You're flying a VFR cross-country from a small field northeast of a Class B. The route takes you 25 NM from the primary airport at 4,500 MSL — well clear of the Class B shelves on the sectional, well below the 10,000 MSL ceiling. The magenta ring is nowhere near your pencil line. Do you need the transponder on?

Yes. You're inside the 30 NM Mode C veil even though you're not entering the Class B itself. § 91.215(b)(2) doesn't care that you're under a shelf or outside the lateral limits — it draws a 30 NM cylinder from the surface to 10,000 MSL around the primary airport, and that cylinder catches you. ADS-B Out has to be on for the same reason per § 91.225.

Operationally, here's how that flight runs. Transponder on ALT before takeoff, code 1200 for VFR. Pull up the Class B approach frequency before you launch — even if you have no intention of asking for a clearance, monitoring it gives you traffic awareness from the controllers who own the airspace above and around you. If they have time, they'll often issue traffic alerts to you on guard or on the approach freq even without a flight following request. Better yet, ask for VFR flight following before you depart: they'll assign you a discrete squawk, identify you on radar, and call traffic. It's free, it's optional, and it turns a stressful transit into a routine one.

The trap is the pilot who looks at the sectional, sees the Class B is "over there," and assumes the rule lives with the airspace. The veil is its own rule, with its own cylinder. The chart shows it as a thin solid magenta line — easy to miss, expensive to violate.