SSR transponder operating procedures

Obligation to operate

A pilot of an aircraft fitted with a serviceable SSR transponder must operate it at all times during flight, regardless of whether the aircraft is within or outside airspace where SSR is used for ATS purposes SERA.13001@2025-05-01.

The IDENT feature must not be activated unless ATS specifically requests it SERA.13001@2025-05-01.

Exemption for limited electrical power. Except in airspace where the competent authority has designated mandatory transponder operation, aircraft — typically sailplanes — whose electrical supply covers only on-board communication, navigation and surveillance equipment but is insufficient for continuous transponder operation are exempt from the always-on requirement. Aircraft with electrically powered engines are excluded from this exemption, because their certification provides sufficient electrical capacity for both propulsion and supporting systems [SERA.13001@2025-05-01; GM2 SERA.13001@2026-07-18].

Formation flights. Aircraft joining a formation are expected to continue operating the transponder until the formation is established. Once established, all aircraft except the lead should be instructed to squawk standby GM1 SERA.13001@2026-07-18.

Mode A code selection

Outside of emergency, code selection follows this priority order SERA.13005@2025-05-01:

1. The code assigned by the ATS unit.
2. If no ATS code instruction has been issued, code 2000 (or another code as prescribed by the competent authority).
3. When not receiving air traffic services, code 7000 to improve detection of the aircraft, unless the competent authority prescribes otherwise.

Special contingency codes are required in the following situations SERA.13005@2025-05-01:

| Situation | Code | |---|---| | Emergency | 7700 | | Radio communication failure | 7600 | | Unlawful interference (hijack) | 7500 |

For Code 7500, the regulation specifies "attempt to select" rather than a firm obligation to select — the attempt itself fulfils the requirement SERA.13005@2025-05-01.

If ATC has already assigned a specific code when an emergency occurs, the pilot may still select 7700 whenever there is specific reason to believe that doing so is the best course of action SERA.13005@2025-05-01. For unlawful interference, if circumstances warrant, 7700 should be selected instead of 7500 SERA.13005@2025-05-01.

Code 7500 — confirmation procedure. If ATC requests confirmation that 7500 has been selected, the pilot should either confirm or not reply at all. ATC treats silence as confirmation that the selection is intentional and not an inadvertent false code GM1 SERA.13005@2026-07-18.

Discrepancy between assigned and displayed code. When a controller observes that the displayed code differs from what has been assigned, the pilot will be asked to verify the setting on the transponder controls and, if necessary, reselect the correct code, then confirm the displayed setting to ATC AMC1 SERA.13005@2026-07-18. If the discrepancy persists, the pilot may be asked to stop transponder operation, and subsequent ATS units will be notified accordingly SERA.13005@2025-05-01.

Mode C — pressure-altitude transmission

If the aircraft carries serviceable Mode C equipment, the pilot must operate it continuously unless ATC directs otherwise SERA.13010@2025-05-01. Unless otherwise prescribed by the competent authority, verification shall be effected at least once by each suitably equipped ATS unit on initial contact with the aircraft concerned or, if this is not feasible, as soon as possible thereafter SERA.13010@2025-05-01.

If the displayed altitude is outside tolerance, the pilot is asked to check the pressure setting and confirm the aircraft's level. If a discrepancy continues after confirmation, ATC may, depending on circumstances: request a switch to an alternative transponder; ask the pilot to stop Mode C altitude transmission (provided position and identity are not lost); or — to preserve position and identity — allow the erroneous Mode C to continue and, when so prescribed by the local instructions, override the displayed level with the reported one GM1 SERA.13010@2026-07-18.

A specific hazard applies: ACAS accepts Mode C replies even when they are erroneous, meaning it can issue Resolution Advisories based on false altitude data. When the measure described in (b)(1) — requesting the pilot to select an alternative transponder — cannot be implemented, controllers should take into account the likelihood of generating ACAS RA in the provision of ATS GM1 SERA.13010@2026-07-18.

Mode S and ADS-B — aircraft identification

An aircraft equipped with Mode S or ADS-B that has an aircraft identification feature must transmit the identification exactly as specified in the flight plan. Where no flight plan has been filed, the aircraft registration is used, unless the competent authority has approved an alternative SERA.13015@2025-05-01. The transmitted identification should conform to the format of Item 7 of the ICAO flight plan form AMC1 SERA.13015@2026-07-18.

If a discrepancy is observed on the situation display, the pilot will be asked to confirm and, if necessary, re-enter the correct identification. If the discrepancy persists even after the pilot confirms the correct setting is entered, ATC informs the pilot, corrects the label where possible, and notifies downstream units SERA.13015@2025-05-01.

ADS-B emergency transmissions. ADS-B-equipped aircraft may transmit specific emergency or urgency categories: emergency, communication failure, unlawful interference, minimum fuel or energy, and medical GM1 SERA.13015@2026-07-18. Some first-generation ADS-B avionics lack these individual categories and can only transmit a general emergency alert regardless of which Mode A code the pilot selects GM2 SERA.13015@2026-07-18.

Phraseology note

Standard SSR phraseology — phrases such as "squawk [code]", "squawk standby", "squawk IDENT", and "confirm squawk" — derives from the ICAO procedures framework and is not covered in detail by the EU SERA corpus. This material is treated as ICAO-framework content.

This block connects to your school's own Operations Manual (OM-A/B/C/D). In the full product it shows, cited to your manual, how YOUR organisation implements the regulation above — private to your school. (Demo placeholder.)

The following cases are labeled illustrative context only; they are not regulatory material and do not establish requirements.

These examples are drawn from live ADS-B surveillance detections of military traffic, showing the Mode A squawk codes and altitudes that appear on situation displays in practice.

CASE-1 An EC45 helicopter (G72234 / reg. 12-72234) detected at 1,450 ft was squawking code 1200 CASE-1. In US airspace, 1200 is the standard VFR code — the local equivalent of the European default code 7000. This illustrates that default VFR squawks vary by competent authority; European PPL holders operating abroad must verify the locally prescribed default.

CASE-2 A T-6 Texan II trainer (CASH31 / reg. 00-3595) at 15,350 ft was also squawking 1200 CASE-2. The same VFR default is in use, consistent with operating in uncontrolled or non-SSR airspace without a discrete ATC assignment.

CASE-3 A C-5M Galaxy heavy transport (RCH2154 / reg. 85-0005) at 33,025 ft was squawking code 3303 CASE-3. A four-digit code outside the emergency range (7700/7600/7500) and outside the default ranges (2000, 7000, 1200) indicates an ATC-assigned discrete code — consistent with the requirement that pilots set the code as instructed by the ATS unit when receiving an air traffic service.

Taken together, the cases illustrate that Mode A squawk codes visible to ATC and on ADS-B displays directly reflect the pilot's transponder setting, making correct code discipline immediately observable by controllers — and that discrepancies are detected in real time.