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Transponder

Introduction:

  • Pilots should be aware that proper application of transponder operating procedures will provide both VFR and IFR aircraft with a higher degree of safety while operating on the ground and airborne
    • Transponders with altitude reporting mode turned ON (Mode C or S) substantially increase the capability of surveillance systems to see an aircraft, thus providing the Air Traffic Controller increased situational awareness and the ability to identify potential traffic conflicts
    • 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
  • 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

Transponder Operation:

  • Transponder Code Designation:

    • 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
  • Automatic Altitude Reporting (Mode C):

    • Some transponders are equipped with a Mode C automatic altitude reporting capability
      • This system converts aircraft altitude in 100' 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
    • 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
    • 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
  • Transponder IDENT Feature:

    • The transponder must be operated only as specified by ATC
    • Activate the "IDENT" feature only upon request of the ATC controller
  • Code Changes:

    • 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 non-discrete 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, see more special codes below
      • Only non-discrete Code 7500 will be decoded as the hijack code
    • Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations
    • Military pilots operating VFR or IFR within restricted/warning areas should adjust their trans- ponders to Code 4000 unless another code has been assigned by ATC
  • Mode C Transponder Requirements:

    • 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
    • In general, the CFRs require aircraft to be equipped with Mode C transponders when operating:
      • At or above 10,000' MSL over the 48 contiguous states or the District of Columbia, excluding that airspace below 2,500 AGL
      • Within 30 miles of a Class B airspace primary airport (mode C veil), below 10,000' 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' MSL, whichever is lower)
      • Within and above all Class C airspace, up to 10,000 feet MSL
      • 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
    • 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
    • Pilots must 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
    • 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
  • Transponder Operation Under Visual Flight Rules (VFR):

    • Unless otherwise instructed by an ATC facility, adjust transponder to reply on Mode 3/A Code 1200 regardless of altitude
      • Aircraft not in contact with an ATC facility may squawk 1255 in lieu of 1200 while en route to, from, or within the designated fire fighting area(s)
      • VFR aircraft which fly authorized SAR missions for the USAF or USCG may be advised to squawk 1277 in lieu of 1200 while en route to, from, or within the designated search area
      • Gliders not in contact with an ATC facility should squawk 1202 in lieu of 1200
    • 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
  • 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
  • Distinguishes aircraft from one another on secondary surveillance radar
    • Air Traffic Control Radar Beacon System (ATCRBS) tracks transponders
  • 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
  • Limited to "line of sight"
  • If operating at an airport with Airport Surface Detection Equipment - Model X (ASDE-X), transponders should be transmitting "on" with altitude reporting continuously while moving on the airport surface as equipped
  • Transfers altitude in 100' increments
  • If deactivation is required, ATC will report "Stop altitude squawk, altitude differs (number of feet) feet"
  • Transponders are preset to 29.92; having the wrong altimeter setting will not affect altitude readout
  • IDENT Feature:
    • IDENT should only be activated upon request by ATC
    • Avoid code changes through the 7700, 7600, 7500 codes
  • Two Modes:
    • Mode 3/A, identification only (known as mode 3 for military, A for civilian)
    • Mode C, identification, and altitude

NOTE:
Mode C requires an encoder to broadcast altitude to ATC. The encoder may be either a separate unit ("blind encoder") or integrated into the aircraft altimeter ("encoding altimeter")

  • Broadcasts aircraft's pressure altitude (29.92) when operating in Mode C
    • Understand your system! Some aircraft will read altitude based off your pitot system which, in a failure, may be inaccurate
  • Uses 4 codes (0 through 7) to set a code that is manually selected by the pilot, assigned by ATC
  • Replies to ATC radar with up to 4096 discrete codes
  • An "IDENT" button is used to flash the aircraft location on ATC's radar screen
    • ATC: "[Callsign], IDENT"
    • ATC: "[Callsign], radar contact, altimeter 29.92"

Reserved Codes:

Transponder Codes
Figure 1: Transponder Codes

Required to be used when:

  • At or above 10,000' MSL over the 48 contiguous states or the District of Columbia, excluding that airspace below 2,500' AGL;
  • Within 30 miles of a Class B airspace primary airport, below 10,000' 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 Alpha airspace and/or outside of a Class Bravo airspace and below the ceiling of the Class Bravo airspace (or 10,000' MSL, whichever is lower);
  • Within and above all Class C airspace, up to 10,000' MSL;
  • Within 10 miles of certain designated airports, excluding that airspace which is both outside the Class Delta surface area and below 1,200' AGL
  • Balloons, gliders, and aircraft not equipped with an engine driven electrical system are excepted from this requirement

Aircraft Conflict/Mode C Intruder Alert:

  • Issued if the controller observes another aircraft which places it in an unsafe proximity
  • When feasible, the controller will offer the pilot an alternative course of action

Emergency Operation:

  • Triggers an alarm or special indicator at all control positions
  • Understand you may not be in radar coverage; therefore continue squawking and establish radio communications as soon as possible

Switchology:

  • OFF: disconnects power to the unit
  • SBY: standby, applies electrical power but does not emit a signal
  • ON: operation in Mode 3/A (identification only)
    • Should be delayed as long as possible prior to takeoff
  • ALT: altitude, sets Mode 3 and Mode C
  • TST: test, confirms operation of the transponder

  • The transponder should be operated in "ON" while on the surface, but not switched to "ALT" until ready to begin the takeoff roll

Altitude reporting:

  • No person may operate an automatic pressure altitude reporting equipment if:
    • ATC directs deactivation
    • As installed, that equipment was tested and calibrated to transmit altitude data corresponding within 125' (95% probability) referenced to 29.92

Deviations:

  • Requests must be made to the ATC facility having jurisdiction over that airspace for operation of an aircraft:
    • With an operating transponder, but without Mode C, requested any time
    • 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, requested any time
    • That is not equipped with a transponder; requests must be made at least 1 hour before proposed operation

Inspections/Tests:

  • Must be tested and inspected within the previous 24 months
  • Inspected after installation or maintenance where data correspondence error could be introduced
  • Tests conducted by:
    • A certificated repair station
      • A radio rating, class III
      • A limited radio rating appropriate to the transponder
      • A limited rating appropriate to the test to be performed
    • A holder of a continuous airworthiness maintenance program as provided in Part 121 or 135.411
  • The manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by that manufacturer

Transponder Landing System (TLS):

  • The TLS is designed to provide approach guidance utilizing existing airborne ILS localizer, glide slope, and transponder equipment
  • The TLS detects the aircraft's position by interrogating its transponder
  • Next, it then broadcasts ILS frequency signals to guide the aircraft along the desired approach path
  • Ground equipment consists of: [Figure 2]
    • Transponder interrogator,
    • Sensor arrays to detect lateral and vertical position, and
    • ILS frequency transmitters
  • TLS instrument approach procedures are designated Special Instrument Approach Procedures
    • Special aircrew training is required
  • TLS ground equipment provides approach guidance for only one aircraft at a time
  • Even though the TLS signal is received using the ILS receiver, no fixed course or glidepath is generated
  • The concept of operation is very similar to an air traffic controller providing radar vectors, and just as with radar vectors, the guidance is valid only for the intended aircraft
  • The TLS ground equipment tracks one aircraft, based on its transponder code, and provides correction signals to course and glidepath based on the position of the tracked aircraft
  • Flying the TLS corrections computed for another aircraft will not provide guidance relative to the approach; therefore, aircrews must not use the TLS signal for navigation unless they have received approach clearance and completed the required coordination with the TLS ground equipment operator
  • Navigation fixes based on conventional NAVAIDs or GPS are provided in the special instrument approach procedure to allow aircrews to verify the TLS guidance
  • Transponder and ADS-B operations on the ground:

    • Civil and military aircraft should operate with the transponder in the altitude reporting mode (consult the aircraft’s flight manual to determine the specific transponder position to enable altitude reporting) and ADS-B Out transmissions enabled (if equipped) at all airports, any time the aircraft is positioned on any portion of an airport movement area. This includes all defined taxiways and runways. Pilots must pay particular attention to ATIS and airport diagram notations, General Notes (included on airport charts), and comply with directions pertaining to transponder and ADS-B usage. Generally, these directions are:
      • Departures. Select the transponder mode which allows altitude reporting and enable ADS-B (if equipped) during pushback or taxi-out from parking spot. Select TA or TA/RA (if equipped with TCAS) when taking the active runway
      • Arrivals. Maintain transponder to the altitude reporting mode or if TCAS-equipped (TA or TA/RA), select the transponder to altitude reporting mode. Maintain ADS-B Out transmissions (if equipped) after clearing the active runway. Select STBY or OFF for transponder and ADS-B (if equipped) upon arriving at the aircraft’s parking spot or gate
  • Transponder and ADS-B Operations in the Air:

    • EACH PILOT OPERATING AN AIRCRAFT EQUIPPED WITH AN OPERABLE ATC TRANSPONDER, MAINTAINED IN ACCORDANCE WITH 14 CFR SECTION 91.413 OR ADS-B TRANSMITTER, MUST OPERATE THE TRANSPONDER/TRANSMITTER, INCLUDING MODE C/S IF INSTALLED, ON THE APPROPRIATE MODE 3/A CODE OR AS ASSIGNED BY ATC. EACH PERSON OPERATING AN AIRCRAFT EQUIPPED WITH ADS-B OUT MUST OPERATE THIS EQUIPMENT IN THE TRANSMIT MODE AT ALL TIMES WHILE AIRBORNE UNLESS OTHERWISE REQUESTED BY ATC
  • 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
  • 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
  • It should be noted by all users of ATC transponders that the coverage they can expect is limited to "line of sight" with ground radar and ADS−B radio sites
    • Low altitude or aircraft antenna shielding by the aircraft itself may result in reduced range or loss of aircraft contact
    • Surveillance coverage can be improved by climbing to a higher altitude
  • 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
  • NOTE:
    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
    Advanced Navigation & Positioning Corporation, Transponder Landing System Ground Equipment
    Figure 2: Advanced Navigation & Positioning Corporation, Transponder Landing System Ground Equipment

    References: