AIM 4-1-20

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4-1-20. Transponder Operation

a. General

1. Pilots should be aware that proper application of transponder operating procedures will provide both VFR and IFR aircraft with a higher degree of safety in the environment where high-speed closure rates are possible. Transponders substantially increase the capability of radar to see an aircraft and the Mode C feature enables the controller to quickly determine where potential traffic conflicts may exist. Even VFR pilots who are not in contact with ATC will be afforded greater protection from IFR aircraft and VFR aircraft which are receiving traffic advisories. Nevertheless, pilots should never relax their visual scanning vigilance for other aircraft.
2. Air Traffic Control Radar Beacon System (ATCRBS) is similar to and compatible with military coded radar beacon equipment. Civil Mode A is identical to military Mode 3.
4. A pilot on an IFR flight who elects to cancel the IFR flight plan prior to reaching destination, should adjust the transponder according to VFR operations.
5. If entering a U.S. OFFSHORE AIRSPACE AREA from outside the U.S., the pilot should advise on first radio contact with a U.S. radar ATC facility that such equipment is available by adding "transponder" to the aircraft identification.
6. It should be noted by all users of ATC transponders that the coverage they can expect is limited to "line of sight." Low altitude or aircraft antenna shielding by the aircraft itself may result in reduced range. Range can be improved by climbing to a higher altitude. It may be possible to minimize antenna shielding by locating the antenna where dead spots are only noticed during abnormal flight attitudes.
7. If operating at an airport with Airport Surface Detection Equipment - Model X (ASDE-X), transponders should be transmitting (on position) continuously with altitude reporting while moving on the airport surface.
8. Aircraft equipped with ADS-B (1090 ES or UAT) must operate the equipment in the transmit mode (on position) at all times while on any airport surface.

Pilots of aircraft equipped with ADS-B should refer to AIM, Automatic Dependant Surveillance - Broadcast Services, Paragraph 4-5-7, for a complete description of operating limitations and procedures.

b. Transponder Code Designation

1. For ATC to utilize one or a combination of the 4096 discrete codes FOUR DIGIT CODE DESIGNATION will be used, e.g., code 2100 will be expressed as TWO ONE ZERO ZERO. Due to the operational characteristics of the rapidly expanding automated ATC system, THE LAST TWO DIGITS OF THE SELECTED TRANSPONDER CODE SHOULD ALWAYS READ "00" UNLESS SPECIFICALLY REQUESTED BY ATC TO BE OTHERWISE.

c. Automatic Altitude Reporting (Mode C)

1. Some transponders are equipped with a Mode C automatic altitude reporting capability. This system converts aircraft altitude in 100 foot increments to coded digital information which is transmitted together with Mode C framing pulses to the interrogating radar facility. The manner in which transponder panels are designed differs, therefore, a pilot should be thoroughly familiar with the operation of the transponder so that ATC may realize its full capabilities.
2'. Adjust transponder to reply on the Mode A/3 code specified by ATC and, if equipped, to reply on Mode C with altitude reporting capability activated unless deactivation is directed by ATC or unless the installed aircraft equipment has not been tested and calibrated as required by 14 CFR Section 91.217. If deactivation is required by ATC, turn off the altitude reporting feature of your transponder. An instruction by ATC to "STOP ALTITUDE SQUAWK, ALTITUDE DIFFERS (number of feet) FEET," may be an indication that your transponder is transmitting incorrect altitude information or that you have an incorrect altimeter setting. While an incorrect altimeter setting has no effect on the Mode C altitude information transmitted by your transponder (transponders are preset at 29.92), it would cause you to fly at an actual altitude different from your assigned altitude. When a controller indicates that an altitude readout is invalid, the pilot should initiate a check to verify that the aircraft altimeter is set correctly.
3. Pilots of aircraft with operating Mode C altitude reporting transponders should report exact altitude or flight level to the nearest hundred foot increment when establishing initial contact with an ATC facility. Exact altitude or flight level reports on initial contact provide ATC with information that is required prior to using Mode C altitude information for separation purposes. This will significantly reduce altitude verification requests.

d. Transponder IDENT Feature

1. The transponder shall be operated only as specified by ATC. Activate the "IDENT" feature only upon request of the ATC controller.

e. Code Changes

1. When making routine code changes, pilots should avoid inadvertent selection of Codes 7500, 7600 or 7700 thereby causing momentary false alarms at automated ground facilities. For example, when switching from Code 2700 to Code 7200, switch first to 2200 then to 7200, NOT to 7700 and then 7200. This procedure applies to nondiscrete Code 7500 and all discrete codes in the 7600 and 7700 series (i.e., 7600-7677, 7700-7777) which will trigger special indicators in automated facilities. Only nondiscrete Code 7500 will be decoded as the hijack code.
2. Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations.
3. Military pilots operating VFR or IFR within restricted/warning areas should adjust their transponders to Code 4000 unless another code has been assigned by ATC.

f. Mode C Transponder Requirements

1. Specific details concerning requirements to carry and operate Mode C transponders, as well as exceptions and ATC authorized deviations from the requirements are found in 14 CFR Section 91.215 and 14 CFR Section 99.12.
2. In general, the CFRs require aircraft to be equipped with Mode C transponders when operating:
(a) 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;
(b) Within 30 miles of a Class B airspace primary airport, below 10,000 feet MSL. Balloons, gliders, and aircraft not equipped with an engine driven electrical system are excepted from the above requirements when operating below the floor of Class A airspace and/or; outside of a Class B airspace and below the ceiling of the Class B airspace (or 10,000 feet MSL, whichever is lower);
(c) Within and above all Class C airspace, up to 10,000 feet MSL;
(d) Within 10 miles of certain designated airports, excluding that airspace which is both outside the Class D surface area and below 1,200 feet AGL. Balloons, gliders and aircraft not equipped with an engine driven electrical system are excepted from this requirement.
3. 14 CFR Section 99.12 requires all aircraft flying into, within, or across the contiguous U.S. ADIZ be equipped with a Mode C or Mode S transponder. Balloons, gliders and aircraft not equipped with an engine driven electrical system are excepted from this requirement.
4. Pilots shall ensure that their aircraft transponder is operating on an appropriate ATC assigned VFR/IFR code and Mode C when operating in such airspace. If in doubt about the operational status of either feature of your transponder while airborne, contact the nearest ATC facility or FSS and they will advise you what facility you should contact for determining the status of your equipment.
5. In-flight requests for "immediate" deviation from the transponder requirement may be approved by controllers only when the flight will continue IFR or when weather conditions prevent VFR descent and continued VFR flight in airspace not affected by the CFRs. All other requests for deviation should be made by contacting the nearest Flight Service or Air Traffic facility in person or by telephone. The nearest ARTCC will normally be the controlling agency and is responsible for coordinating requests involving deviations in other ARTCC areas.

g. Transponder Operation Under Visual Flight Rules (VFR)

1. Unless otherwise instructed by an ATC facility, adjust transponder to reply on Mode 3/A Code 1200 regardless of altitude.
2. Adjust transponder to reply on Mode C, with altitude reporting capability activated if the aircraft is so equipped, unless deactivation is directed by ATC or unless the installed equipment has not been tested and calibrated as required by 14 CFR Section 91.217. If deactivation is required and your transponder is so designed, turn off the altitude reporting switch and continue to transmit Mode C framing pulses. If this capability does not exist, turn off Mode C.

h. Radar Beacon Phraseology

Air traffic controllers, both civil and military, will use the following phraseology when referring to operation of the Air Traffic Control Radar Beacon System (ATCRBS). Instructions by ATC refer only to Mode A/3 or Mode C operation and do not affect the operation of the transponder on other Modes.

1. SQUAWK (number). Operate radar beacon transponder on designated code in Mode A/3.
2. IDENT. Engage the "IDENT" feature (military I/P) of the transponder.
3. SQUAWK (number) and IDENT. Operate transponder on specified code in Mode A/3 and engage the "IDENT" (military I/P) feature.
4. SQUAWK STANDBY. Switch transponder to standby position.
5. SQUAWK LOW/NORMAL. Operate transponder on low or normal sensitivity as specified. Transponder is operated in "NORMAL" position unless ATC specifies "LOW" ("ON" is used instead of "NORMAL" as a master control label on some types of transponders.)
6. SQUAWK ALTITUDE. Activate Mode C with automatic altitude reporting.
7. 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.
8. STOP SQUAWK (mode in use). Switch off specified mode. (Used for military aircraft when the controller is unaware of military service requirements for the aircraft to continue operation on another Mode.)
9. STOP SQUAWK. Switch off transponder.
10. SQUAWK MAYDAY. Operate transponder in the emergency position (Mode A Code 7700 for civil transponder. Mode 3 Code 7700 and emergency feature for military transponder.)
11. SQUAWK VFR. Operate radar beacon transponder on Code 1200 in the Mode A/3, or other appropriate VFR code.

Aeronautical Information Manual (AIM) — Chapter 4 — Air Traffic Control
4-1-1 ARTCCs • 4-1-2 Towers • 4-1-3 FSS's • 4-1-4 Recording • 4-1-5 Release of IFR Aircraft • 4-1-6 Visits to Air Traffic Facilities • 4-1-7 Operation Take-off and Operation Raincheck • 4-1-8 Approach Control Service for VFR Aircraft • 4-1-9 Airports Without Operating Control Towers • 4-1-10. IFR Approaches/Ground Vehicle Ops • 4-1-11 UNICOM/MULTICOM Freqs • 4-1-12 UNICOM for ATC • 4-1-13 ATIS • 4-1-14 AFIS • 4-1-15 RADAR Traffic Service • 4-1-16 Safety Alert • 4-1-17 Radar Assistance to VFR Aircraft • 4-1-18. VFR Terminal Radar Services • 4-1-19 TEC • 4-1-20 Transponders • 4-1-21 Hazardous Area Reporting • 4-1-22 Airport Reservation Ops • 4-1-23 Requests for Waivers • 4-1-24 Weather Processor
4-2-1 General • 4-2-2 Radio Technique • 4-2-3 Contact Procedures • 4-2-4 Call Signs • 4-2-5 Leased Aircraft • 4-2-6. Ground Call Signs • 4-2-7 Phonetic Alphabet • 4-2-8 Figures • 4-2-9 Altitudes • 4-2-10 Directions • 4-2-11 Speeds • 4-2-12 Time • 4-2-13 Radio Inop Comm. • 4-2-14 VFR Comm.
4-3-1 General • 4-3-2 Towered Airports • 4-3-3 Traffic Patterns • 4-3-4 Visual Indicators at Untowered Airports • 4-3-5 Unexpected Maneuvers • 4-3-6. Use of Runways • 4-3-7 LLWS/Microbursts • 4-3-8 Braking Reports • 4-3-9 Runway Friction • 4-3-10 Intersection Takeoffs • 4-3-11 LAHSO • 4-3-12 Low Approach • 4-3-13 Light Signals • 4-3-14 Comms. • 4-3-15 Departure Delays • 4-3-16 VFR Flights • 4-3-17 VFR Helicopter Ops • 4-3-18 Taxiing • 4-3-19 Low Vis. Taxi • 4-3-20 Exiting the Runway • 4-3-21 Practice Approaches • 4-3-22 Option Approach • 4-3-23 Aircraft Lights • 4-3-24 Flight Inspection/`Flight Check' Aircraft in Terminal Areas • 4-3-25 Hand Signals • 4-3-26 Uncontrolled Airports With ASOS/AWOS
4-4-1 Clearance • 4-4-2 Clearance Prefix • 4-4-3 Clearance Items • 4-4-4 Amended Clearances • 4-4-5 CDR • 4-4-6 Special VFR • 4-4-7 Pilot Responsibility • 4-4-8 VFR-on-top • 4-4-9 VFR/IFR • 4-4-10 Adherence to Clearance • 4-4-11 IFR Separation • 4-4-12 Speed Adjustments • 4-4-13 Runway Separation • 4-4-14 Visual Separation • 4-4-15 Visual Clearing Procedures • 4-4-16 TCAS I & II • 4-4-17 TIS
4-5-1 Radar • 4-5-2 ATCRBS • AIM 4-5-34-5-3 Surveillance Radar • 4-5-4 PAR • 4-5-5 ASDE-X • 4-5-6 TIS • 4-5-7 ADS-B • 4-5-8 TIS-B • 4-5-9 FIS-B • 4-5-10 ADS-R
4-6-1 RVSM Mandate • 4-6-2 • Flight Levels • 4-6-3 Operator Approval • 4-6-4 Flight Planning into RVSM • 4-6-5 Pilot RVSM • 4-6-6 MWA • 4-6-7 Wake Turbulence • 4-6-8 Pilot/Controller Phraseology • 4-6-9 Contingency Actions • 4-6-10 Accommodation of Non-RVSM • 4-6-11 Climb/Descent to/from Flight Levels Above RVSM