Transponder codes: 1200, 7500, 7600, 7700 and where Mode C and ADS-B are required

Mode A, Mode C, Mode S and ADS-B Out

A Mode A transponder replies to radar with the four-digit code you set, one of 4,096 possible codes. 14 CFR 91.215(b) AIM 4-1-20 Mode C adds automatic pressure-altitude reporting, transmitting your altitude in 100-foot increments; a Mode S transponder also replies to Mode A interrogations with the code ATC specified. 14 CFR 91.215(b) Altitude encoders are preset to standard pressure and ATC applies the local altimeter correction, so a wrong altimeter setting does not change the altitude ATC sees — but it does change the altitude you actually fly. AIM 4-1-20

ADS-B Out is a separate requirement from the transponder. In Class A it must meet the 1090 MHz extended-squitter standard (TSO-C166b or C166c); below 18,000 feet MSL a 978 MHz Universal Access Transceiver (TSO-C154c or C154d) also qualifies. 14 CFR 91.225(a) 14 CFR 91.225(b) If your aircraft has ADS-B Out, you must keep it transmitting at all times unless ATC directs otherwise or an FAA-authorized sensitive government mission applies. 14 CFR 91.225(f)

Where Mode C and ADS-B Out are required

• In Class A, Class B and Class C airspace. 14 CFR 91.215(b)(1) 14 CFR 91.225(a) 14 CFR 91.225(d)(1)
• Within 30 nautical miles of the airports listed in Part 91 Appendix D, section 1 — generally the Class B primary airports — from the surface up to 10,000 feet MSL, commonly called the Mode C veil. 14 CFR 91.215(b)(2) 14 CFR 91.225(d)(2) AIM 3-2-3
• Above the ceiling and within the lateral boundaries of Class B or Class C airspace, up to 10,000 feet MSL. 14 CFR 91.215(b)(4) 14 CFR 91.225(d)(3)
• In the 48 contiguous states and the District of Columbia at and above 10,000 feet MSL, excluding the airspace at and below 2,500 feet above the surface. 14 CFR 91.215(b)(5)(i) 14 CFR 91.225(d)(4)
• ADS-B Out only: in Class E airspace at and above 3,000 feet MSL over the Gulf, from the U.S. coastline out to 12 nautical miles; the transponder rule does not include this airspace. 14 CFR 91.225(d)(5) AIM 4-1-20
• Exception: aircraft never certificated with an engine-driven electrical system, balloons and gliders may fly in the Mode C veil without them if they stay outside Class A, B and C airspace and below the Class B or C ceiling or 10,000 feet MSL, whichever is lower. 14 CFR 91.215(b)(3) 14 CFR 91.225(e)

Using the transponder correctly

• If your aircraft has an operable transponder, you must operate it, including Mode C, in all controlled airspace and in the airspace listed above; the AIM also asks you to keep it on in uncontrolled airspace when practicable. 14 CFR 91.215(c) AIM 4-1-20
• When changing codes, avoid passing through 7500, 7600 or 7700: to go from 2700 to 7200, select 2200 first, then 7200. AIM 4-1-20
• Press IDENT only when ATC asks you to. AIM 4-1-20
• On the airport movement area, keep the transponder in its altitude-reporting mode and ADS-B Out transmitting; select standby or off at the parking spot. AIM 4-1-20
• With an inoperative transponder you may ask ATC for a deviation at any time; to fly in the required airspace with no transponder installed, ask at least one hour before the flight. 14 CFR 91.215(d)

The three emergency codes in practice

For 7700, squawk the code with Mode C on and immediately establish communication with ATC; keep squawking it, because you may not be inside radar coverage. AIM 6-2-2 For 7600, set the code as soon as you lose two-way radio capability. Under VFR in Class D you may still land if the weather is at or above basic VFR minimums, you keep visual contact with the tower, and you receive a clearance to land, for example by light signal. AIM 6-4-2 14 CFR 91.129(d)(2) AIM 4-3-13 For 7500, ATC will ask you to verify the code; if you are not being hijacked, say so in the clear. Refuse any assignment of 7500 in any other situation. AIM 6-3-4

Common mistakes

• Rolling the knob through 7500, 7600 or 7700 on the way to a new code. Those codes trigger alarms at ATC facilities even for a moment, so change digits in an order that avoids them. AIM 4-1-20
• Assuming Class D requires a transponder. Class D requires two-way radio communication; you need Mode C and ADS-B Out there only when the airport also lies in one of the areas above, such as a Mode C veil. AIM 3-2-5 14 CFR 91.215(b)
• Thinking the requirement ends at the edge of the Class B or C airspace. It continues above the ceiling within the lateral limits up to 10,000 feet MSL, and out to 30 nautical miles of the Class B primary airports. 14 CFR 91.215(b)(2) 14 CFR 91.215(b)(4)
• Forgetting the 2,500-foot exclusion. Above 10,000 feet MSL the requirement does not apply at and below 2,500 feet above the surface, which matters over mountains. 14 CFR 91.215(b)(5)(i) 14 CFR 91.225(d)(4)

Quick answers

Learn it properly

• Lesson: Transponder Codes and Flight Following
• Lesson: Light Gun Signals and Lost Communication
• Lesson: Busy Airports: Class B, C and D
• Lesson: Avionics: Glass Cockpit, Transponder and GPS
• Regulation: 14 CFR 91.215, full text and study notes
• Regulation: 14 CFR 91.225 on the eCFR ↗

Sources

14 CFR (2025 annual edition, as hosted on this site; current text on the eCFR) and the Aeronautical Information Manual (Basic, with Changes 1 and 2). Checked 2026-09-27.

AIM 4-1-20 AIM 6-3-4 AIM 6-4-2 AIM 6-2-2 14 CFR 91.215(b) 14 CFR 91.225(d) 14 CFR 91.225(a) 14 CFR 91.225(b) 14 CFR 91.225(f) 14 CFR 91.215(b)(1) 14 CFR 91.225(d)(1) 14 CFR 91.215(b)(2) 14 CFR 91.225(d)(2) AIM 3-2-3 14 CFR 91.215(b)(4) 14 CFR 91.225(d)(3) 14 CFR 91.215(b)(5)(i) 14 CFR 91.225(d)(4) 14 CFR 91.225(d)(5) 14 CFR 91.215(b)(3) 14 CFR 91.225(e) 14 CFR 91.215(c) 14 CFR 91.215(d) 14 CFR 91.129(d)(2) AIM 4-3-13 AIM 3-2-5