SSR Transponder Procedures: Mode C Operation, Emergency Codes, and ACAS/TCAS

010-06-08Pablo Asensio Martínez2026-03-272 min

The transponder (SSR) is vital for identification, altitude reporting, and anti-collision systems.

Transponder Operation

  • Continuous Operation: If the equipment is operative, the pilot shall operate the transponder (including Mode C) at all times during flight, regardless of whether in controlled airspace or not, unless otherwise instructed by ATC.
  • Mode C: Provides pressure altitude information (reference 1013.2 hPa).
    • Resolution: Increments of 100 ft.
    • Tolerance: ± 300 ft regarding the reported altitude.
    • Essential for ATC and for TCAS operation.

Emergency Codes (Mode A)

  • 7700: General Emergency ("Going to Heaven").
  • 7600: Radio Communication Initial Failure ("Radio Fix").
  • 7500: Unlawful Interference / Hijack ("Man with a knife").

Transponder Failure

  • Before departure: Inform ATS, insert 'N' in the flight plan (item 10).
  • The flight may continue if permission is granted or for repair to the nearest suitable aerodrome, following ATC conditions.

ACAS / TCAS (Airborne Collision Avoidance System)

Last-resort system to prevent collisions, independent of ATC separation.

Advisories

  1. TA (Traffic Advisory):
    • Alert ("Traffic, Traffic"). Aids situational awareness and visual search.
    • Do not maneuver based solely on a TA.
  2. RA (Resolution Advisory):
    • Immediate vertical command (e.g., "Climb, Climb", "Descend, Descend").
    • Pilot Action:
      1. Respond immediately.
      2. Follow the RA even if it contradicts ATC instructions.
      3. Do not maneuver in the opposite sense.
      4. Limit the maneuver to the minimum necessary to comply with the RA (do not exaggerate the maneuver).
      5. Notify ATC as soon as possible of the deviation: "TCAS RA".
      6. Return to ATC clearance promptly when the conflict is resolved ("Clear of Conflict").

Responsibility

  • Following an RA takes precedence over any ATC instruction.
  • The controller is not responsible for separation during an RA maneuver until the pilot reports "Clear of Conflict" and resumes their clearance.