14 CFR 91.215 — ATC Transponder and Altitude Reporting Equipment (Explained)

What does 14 CFR 91.215 require?

14 CFR 91.215(a) requires that any U.S.-registered civil aircraft operating in the airspace listed in paragraph (b) be equipped with a transponder meeting TSO-C74b, TSO-C74c, or TSO-C112 standards. TSO-C74c and TSO-C112 include altitude reporting capability — commonly called Mode C. The transponder must be operable, not merely installed. An installed but malfunctioning unit does not satisfy the requirement unless ATC authorizes a deviation under paragraph (d).

Section 91.215 is a VFR and IFR rule. It applies regardless of flight rules — you need a functioning Mode C transponder any time you enter the listed airspace, even on a clear-day VFR cross-country.

Where is a Mode C transponder required?

14 CFR 91.215(b) identifies five airspace categories where an operable transponder with altitude reporting is mandatory:

The table above covers the most operationally significant paragraphs. Note that paragraph (b)(3) carves out a limited exception for aircraft without engine-driven electrical systems — addressed below.

What is the 30-nm Mode C veil?

The Mode C veil is created by 14 CFR 91.215(b)(2) , which requires transponder and altitude-reporting equipment in all airspace within 30 nautical miles of each airport listed in Part 91 Appendix D, Section 1, from the surface upward to 10,000 feet MSL. Appendix D, Section 1 lists all Class B airports — every major commercial hub in the United States is on this list.

The veil extends well beyond the charted Class B airspace boundaries. An aircraft flying VFR at 3,000 feet MSL, 25 nm from a Class B airport but outside the charted Class B lateral limits, is still inside the Mode C veil and must have an operable Mode C transponder. The practical takeaway: within 30 nm of any Class B airport and below 10,000 feet MSL, you need Mode C even in Class E or G airspace.

How does the 10,000-foot MSL rule work — and what is the 2,500-foot AGL exception?

14 CFR 91.215(b)(5) extends the Mode C requirement nationwide to all aircraft operating at and above 10,000 feet MSL, but explicitly excludes airspace at and below 2,500 feet above the surface. This exception matters primarily in mountainous terrain. An aircraft cruising at 10,500 feet MSL over a plateau at 8,500 feet MSL is only 2,000 feet AGL — below the 2,500-foot AGL floor — and is not subject to this particular paragraph.

The exception does not, however, override the Mode C veil or Class airspace requirements. If that same aircraft is within 30 nm of a Class B airport, it is inside the veil and paragraph (b)(2) still applies regardless of AGL.

What is the transponder operation requirement under paragraph (c)?

14 CFR 91.215(c) requires that any person operating an aircraft equipped with an operable ATC transponder operate the transponder — including Mode C equipment if installed — and reply on the appropriate code or as assigned by ATC. This applies when operating in any of the airspace listed in paragraph (b) or in controlled airspace. The AIM expands on transponder operation procedures in AIM 4-1-20 , which addresses code assignment, squawking 1200 in uncontrolled airspace, and Mode C operation throughout flight.

The practical effect: if you have an operable Mode C transponder installed, you must run it in the covered airspace. You cannot selectively disable altitude reporting while leaving the transponder on transponder-only mode unless ATC directs otherwise.

What is the deviation authority under paragraph (d)?

14 CFR 91.215(d) authorizes ATC facilities to grant deviations from the transponder requirements on a case-by-case basis. The requesting pilot must contact the ATC facility having jurisdiction over the affected airspace. The advance-notice requirement differs based on the aircraft's situation:

1. 1
   Transponder inoperative (installed but failed): request may be made at any time — no advance notice required.
2. 2
   Mode C inoperative but transponder otherwise functional: request may be made at any time.
3. 3
   No transponder installed: request must be made at least one hour before the proposed operation.

Deviation authority is not a blanket waiver — each flight or operation requires its own request, and ATC may decline based on traffic. If you depart with an inoperative transponder, coordinate with ATC before entering any affected airspace, not after.

Are any aircraft exempt from the Mode C requirement?

14 CFR 91.215(b)(3) provides a limited exemption for aircraft that have no engine-driven electrical system — primarily unpowered gliders and balloons. These aircraft may operate within the 30-nm Mode C veil provided they remain outside Class A, B, and C airspace and stay below the ceiling of the overlying Class B or C airspace or below 10,000 feet MSL, whichever is lower.

This exemption is narrow. A powered glider with an engine-driven electrical system does not qualify. The exemption also does not apply inside the lateral limits of Class B or C airspace — a glider cleared into the Class B core must still have an operable transponder, or the controlling facility must authorize a deviation.

How does 91.215 relate to ADS-B Out under 91.225?

14 CFR 91.225 mandates ADS-B Out equipment in airspace that substantially overlaps the areas covered by 91.215 — Class A, B, and C airspace; within 30 nm of Class B airports from the surface to 10,000 feet MSL; and at and above 10,000 feet MSL in Class E airspace over the 48 contiguous states. ADS-B Out has been required since January 1, 2020.

The two rules are distinct. Mode C transponders respond to radar interrogation (secondary surveillance radar); ADS-B Out continuously broadcasts GPS-derived position and identification without interrogation. Meeting the transponder requirement under 91.215 does not satisfy 91.225 — ADS-B Out equipment meeting TSO-C166b, TSO-C166c, or TSO-C154c is separately required. In practice, most modern avionics installations combine a Mode S transponder with an ADS-B Out transmitter in a single unit, satisfying both regulations simultaneously.

What do DPEs ask about 91.215 during the oral exam?

Transponder requirements are a reliable oral exam topic — both because the airspace rules overlap in ways that catch candidates off guard and because the deviation authority provision tests whether you know what to do when equipment fails. Examiners frequently probe the distinction between the Mode C veil and the Class B airspace itself, and the AGL exception to the 10,000-foot rule.

• "You're flying VFR at 4,500 feet MSL, 20 nm from a Class B airport but well outside the Class B boundaries. Do you need a transponder?"
• "Your Mode C fails on the ground before departure. What are your options?"
• "You're at 10,200 feet MSL over mountainous terrain at 8,500 feet elevation. Do the transponder requirements apply?"
• "Explain the difference between what 91.215 requires and what 91.225 requires."
• "A balloon pilot wants to fly through the Mode C veil. Is that legal, and under what conditions?"
• "If you need a deviation to fly without a transponder, who do you call and when?"

Practice Questions

1. You are flying VFR at 8,000 feet MSL, 28 nm from a Class B airport listed in Appendix D. You are outside all Class B and C airspace boundaries. Is an operable Mode C transponder required? Cite the specific paragraph that answers this.

2. During preflight, your altitude-reporting function fails but the transponder still transmits a squawk code. You need to transit the Mode C veil to reach your destination. What does 14 CFR 91.215(d) require you to do, and when must you make the request?

3. A DPE asks: "You're cruising at 10,500 feet MSL over terrain that sits at 8,200 feet elevation. Does the 10,000-foot MSL transponder rule in 91.215(b)(5) apply to you?" Walk through the AGL exception in your answer.

4. Your aircraft has no transponder installed. ATC has granted a deviation. En route, you wish to transit through Class C airspace. Does the deviation from 91.215(d) automatically authorize you to enter Class C? Why or why not?

5. Compare the equipment requirements under 14 CFR 91.215 and 14 CFR 91.225. What does each regulation require, and can a single avionics unit satisfy both?

Frequently Asked Questions

Q: Where is a Mode C transponder required under 14 CFR 91.215?

Mode C is required in Class A, B, and C airspace; within 30 nm of any Class B airport listed in 14 CFR Part 91 Appendix D from the surface to 10,000 feet MSL; above the ceiling and within the lateral limits of Class B or C airspace up to 10,000 MSL; and nationwide at and above 10,000 feet MSL except at and below 2,500 feet AGL.

Q: What is the 30-nm Mode C veil?

The Mode C veil is the airspace within 30 nautical miles of each airport listed in 14 CFR 91.215(b)(2) and Appendix D, Section 1 (all Class B airports), from the surface up to 10,000 feet MSL. Any aircraft operating inside this ring must have an operable transponder with altitude-reporting capability.

Q: Does the 10,000-foot MSL transponder rule apply below 2,500 feet AGL?

No. Under 14 CFR 91.215(b)(5) , the nationwide 10,000-foot MSL requirement specifically excludes airspace at and below 2,500 feet above the surface. An aircraft flying at 10,500 feet MSL over terrain at 9,000 feet (500 feet AGL) would be below 2,500 AGL and exempt from this particular paragraph.

Q: Can you fly with an inoperative transponder?

Yes, with ATC authorization. Under 14 CFR 91.215(d) , you may request a deviation from the ATC facility having jurisdiction over the affected airspace. If the transponder is inoperative (but installed), the request may be made at any time with no advance notice requirement.

Q: How much advance notice is required to fly without a transponder?

At least one hour before the proposed operation if no transponder is installed at all. If a transponder is installed but inoperative, or if Mode C is inoperative, the request may be made at any time without the one-hour advance requirement under 14 CFR 91.215(d) .

Q: Does 14 CFR 91.215 require ADS-B Out?

No. Section 91.215 covers Mode C transponders only. ADS-B Out is separately mandated by 14 CFR 91.225 , which applies in largely the same airspace but requires different equipment. Satisfying 91.215 does not automatically satisfy 91.225.

Q: Are gliders and balloons exempt from the Mode C veil?

Partially. Under 14 CFR 91.215(b)(3) , aircraft without engine-driven electrical systems may operate within the 30-nm Mode C veil provided they stay outside Class A, B, and C airspace and remain below the floor of the overlying Class B or C airspace or 10,000 feet MSL, whichever is lower.

Q: What equipment standard must a Mode C transponder meet?

Under 14 CFR 91.215(a) , the transponder must meet TSO-C74b (Mode A, no altitude), TSO-C74c (Mode A with altitude reporting), or TSO-C112 (Mode S) performance standards. Mode C altitude reporting requires a pressure altitude source — the transponder reports uncorrected pressure altitude, not altimeter-corrected altitude.

Sources

• 14 CFR 91.215 — ATC Transponder and Altitude Reporting Equipment (Cornell LII)
• 14 CFR 91.215 — ATC Transponder and Altitude Reporting Equipment (eCFR)
• 14 CFR 91.225 — ADS-B Out Equipment (Cornell LII)
• 14 CFR Part 91 Appendix D — Airports / Airspace Where Mode C Is Required
• AIM 4-1-20 — Transponder and ADS-B Operation
• FAA Instrument Rating ACS (FAA-S-ACS-8C)

This article was researched from FAA primary sources (14 CFR Part 91 via Cornell LII / eCFR, Aeronautical Information Manual, Instrument Rating ACS) and citing current regulatory text — drafted by MockDPE. Last updated: May 2026. If you spot an inaccuracy, email corrections@mockdpe.org.