Already a member? Please login below for an enhanced experience. Not a member? Join today

Transponder RequirementsTransponder Requirements

Transponder Requirements

The information below assumes that your airplane isn't a balloon or glider.

Modes and Codes

Mode A. Sometimes referred to as mode 3/A. Civil Mode A is identical to military Mode 3. Mode A responds to an ATC interrogation signal with the transponder code set by the pilot.

Mode C. Refers to aircraft equipped with an altitude encoder and altimeter. With Mode C, ATC will actually see the flight level altitude on their radar screen if the transponder is operating in the Mode C or "ALT" (altitude) Mode.

Mode S. Mode S is a possible platform for a variety of other applications, such as Traffic Information Service (TIS), Graphic Weather Service, and Automatic Dependent Surveillance-Broadcast (ADS-B). Under ADS-B, each aircraft periodically broadcasts its identification, position, and altitude. Overall, Mode S provides improved surveillance quality, discrete aircraft addressing function, and digital capability. Mode S is not required for general aviation aircraft.

Squawk Codes

  • 1200 Visual Flight Rules (VFR)
  • 1202 Gliders 
  • 7500 Hijack
  • 7600 Communications failure
  • 7700 emergency
  • 7777 military intercept code

Transponder Equipment Suffixes

Suffix Equipment Capability
/X No transponder
/T Transponder with no Mode C
/U Transponder with Mode C
/D No transponder
/B Transponder with no Mode C
/A Transponder with Mode C
/M No transponder
/N Transponder with no Mode C
/P Transponder with Mode C
/Y LORAN, VOR/DME, or INS with no transponder
/C LORAN, VOR/DME, or INS, transponder with no Mode C
/I LORAN, VOR/DME, or INS, transponder with Mode C
  ADVANCED RNAV WITH TRANSPONDER AND MODE C (If an aircraft is unable to operate with a transponder and/or Mode C, it will revert to the appropriate code listed above under Area Navigation.)
/E Flight Management System (FMS) with DME/DME and IRU position updating
/F FMS with DME/DME position updating
/G Global Navigation Satellite System (GNSS), including GPS or Wide Area Augmentation System (WAAS), with en route and terminal capability.
/R Required Navigational Performance (RNP). The aircraft meets the RNP type prescribed for the route segment(s), route(s) and/or area concerned.
  Reduced Vertical Separation Minimum (RVSM). Prior to conducting RVSM operations within the U.S., the operator must obtain authorization from the FAA or from the responsible authority, as appropriate.
/J /E with RVSM
/K /F with RVSM
/L /G with RVSM
/Q /R with RVSM

All Controlled Airspace

According to the AIM, Section 4-1-19: In all cases, while in controlled airspace, each pilot operating an aircraft equipped with an operable ATC transponder maintained in accordance with 14 CFR section 91.413 shall operate the transponder, including Mode C if installed, on the appropriate code or as assigned by ATC.

Other Airspace Requirements

The following areas require the operation of a Mode C transponder:

  • Operations within Class A, Class B, and Class C airspace.
  • Operations within 30 nautical miles of the primary airport within Class B airspace from the surface to 10,000 feet msl (see airports listed below).
  • Operations above the ceiling and within the lateral boundaries of Class B and C airspace.
  • Operations above 10,000 feet msl in the contiguous 48 states, excluding the airspace at and below 2,500 feet agl.
  • The AIM states in Section 4-1-19(a)(3) that for airborne operations in Class G airspace, the transponder should be operating unless otherwise requested by ATC.

Above 10,000 Feet

All aircraft are required to be equipped with a Mode C transponder when flying at or above 10,000 feet msl, over the 48 contiguous states or the District of Columbia, excluding that airspace below 2,500 feet agl.

Into and Out of the United States

According to 14 CFR 99.13, no person may operate an aircraft into or out of the United States, or into, within, or across an ADIZ designated in subpart B unless operating a transponder with Mode C. Certain exemptions might apply to aircraft that were not originally certified with an engine-driven electrical system; see 99.13(d).


Aircraft not originally certificated with an engine-driven electrical system or subsequently have not been certified with such a system installed, balloons, or gliders may conduct operations:

In the airspace within 30 nautical miles of the listed airports as long as operations are conducted:

  • Outside of Class A, B, and C airspace.
  • Below the altitude of the ceiling of a Class B or Class C airspace area designated for an airport, or 10,000 feet msl, whichever is lower.
  • Above 10,000 feet msl (excluding airspace above the lateral limits of Class B and C airspace).

ATC Authorized Deviations

According to 14 CFR 91.215(d), requests for deviations must be made to the ATC facility having jurisdiction over the concerned airspace within the time periods specified as follows:

  • For operation of an aircraft with an operating transponder but without operating automatic pressure altitude reporting equipment having a Mode C capability, the request may be made at any time.
  • For operation of an aircraft with an inoperative transponder to the airport of ultimate destination, including any intermediate stops, or to proceed to a place where suitable repairs can be made or both, the request may be made at any time.
  • For operation of an aircraft that is not equipped with a transponder, the request must be made at least one hour before the proposed operation.

Flying into a Mode C Veil Without a Transponder

For flying into a Mode C veil without an operable transponder, the pilot needs to telephone the appropriate radar facility for the Class B airspace and ask for permission to make the flight. Upon agreeing to conditions (including direction of flight and altitude), the pilot will be given a code number that he will mention to the controller upon initial radio contact. This is the same procedure that a pilot with an inoperative transponder/encoder would use to fly in or out of the Mode C-veil airports for avionics repair.

The situation may be slightly different if the pilot is landing at a satellite Class D (towered controlled) airport within the veil but outside of Class B airspace. The approval is still given by the controlling radar facility via telephone. The radar facility may still issue the code number but may only require the pilot to contact the tower in the Class D airspace.

NOTE: You should not expect approvals at the busiest of Class B airports during their peak times or under difficult weather conditions, but if this telephone procedure can expand the utilization of your aircraft occasionally, then by all means, phone to find if you can "fit into" the system.

Transponder Tests and Inspections

According to 14 CFR 91.413, a transponder may not be used for the above purposes unless, within the preceding 24 calendar months, the ATC transponder has been tested and inspected and found to comply with appendix F of FAR Part 43.

If a transponder is installed or the maintenance of a transponder may introduce errors, the transponder must be inspected and found to comply with paragraph (c), appendix E, of FAR Part 43. The tests and inspections must be conducted by a properly equipped repair station certified in accordance with 91.415(c)(1), the holder of a continuous airworthiness maintenance program under Part 121 or 135, or the manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by the manufacturer.

ATC Transponder Phraseology

  • Squawk. Operate transponder on designated code in Mode A.
  • Ident. Engage the "IDENT" feature on the transponder.
  • Squawk and Ident. Operate transponder on designated code in Mode A and engage the "IDENT" feature.
  • Squawk Standby. Switch transponder to "STANDBY" or "SBY" position.
  • Squawk Altitude. Active Mode C with automatic altitude reporting. Typically this is the "ALT" option.
  • Stop Altitude Squawk. Turn off altitude reporting switch and continue transmitting Mode C framing pulses. If your equipment does not have this capability, turn off Mode C.
  • Stop Squawk. Switch off transponder.
  • Squawk Mayday. Operate transponder in the emergency position (7700).
  • Squawk VFR. Operate transponder on code 1200.

Resources, Additional Help

Operations within 30 nm of the following airports require an operable transponder-ON (excluding exempted aircraft):

  • Atlanta, GA (The William B. Hartsfield Atlanta International Airport)
  • Baltimore, MD (Baltimore Washington International Airport)
  • Boston, MA (General Edward Lawrence Logan International Airport)
  • Chantilly, VA (Washington Dulles International Airport)
  • Charlotte, NC (Charlotte/Douglas International Airport)
  • Chicago, IL Chicago-O'Hare International Airport)
  • Cleveland, OH (Cleveland-Hopkins International Airport)
  • Covington, KY (Cincinnati Northern Kentucky International Airport)
  • Dallas, TX (Dallas/Fort Worth Regional Airport)
  • Denver, CO (Denver International Airport)
  • Detroit, MI (Metropolitan Wayne County Airport)
  • Honolulu, HI (Honolulu International Airport)
  • Houston, TX (Houston Intercontinental Airport)
  • Kansas City, MO (Kansas City International Airport)
  • Las Vegas, NV (McCarran International Airport)
  • Los Angeles, CA (Los Angeles International Airport)
  • Memphis, TN (Memphis International Airport)
  • Miami, FL (Miami International Airport)
  • Minneapolis, MN (Minneapolis-St. Paul International Airport)
  • Newark, NJ (Newark International Airport)
  • New Orleans, LA (New Orleans International Airport-Moisant Field)
  • New York, NY (John F. Kennedy International Airport)
  • New York, NY (LaGuardia Airport)
  • Orlando, FL (Orlando International Airport)
  • Philadelphia, PA (Philadelphia International Airport)
  • Phoenix, AZ (Phoenix Sky Harbor International Airport)
  • Pittsburgh, PA (Greater Pittsburgh International Airport)
  • St. Louis, MO (Lambert-St. Louis International Airport)
  • Salt Lake City, UT (Salt Lake City International Airport)
  • San Diego, CA (San Diego International Airport)
  • San Francisco, CA (San Francisco International Airport)
  • Seattle, WA (Seattle-Tacoma International Airport)
  • Tampa, FL (Tampa International Airport)
  • Washington, DC (Washington National Airport)

And, of course, you can always call AOPA for help or advice with any aeronautical problem:

AOPA Pilot Information Center
800/USA-AOPA (800/872-2672)

Updated Tuesday, October 24, 2006 11:09:27 AM