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

> Study SSR transponder operating rules, emergency squawk codes, transponder failure handling, and the role of ACAS/TCAS advisories.

- **Subject:** 010 — Air Law
- **Section:** 010-06 — Aircraft Operations
- **Topic code:** 010-06-08
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/air-law/aircraft-operations/secondary-surveillance-radar-transponder-operating-procedures-icao-doc-8168/

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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.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.