SSR Transponder Failure in Controlled Airspace

Causes fall into three groups:

1. Operating/setup errors - wrong mode (standby instead of altitude/on), code not entered correctly, altitude source (blind encoder) failed so the unit is in a non-replying state, squelch/ suppression issue, or the unit is in a test/diagnostic mode. Check: mode selector, code display, encoder status, any fault annunciation.
2. Antenna and cabling - antenna disconnected, corroded connector, coax fault, or the antenna selector on the wrong antenna. A transponder with a failed antenna can show a lit display and still not reply. Check: antenna selector, visible condition where accessible, and whether the second transponder shares the antenna (it does not, typically, but the selector may).
3. Unit failure - transmitter or receiver module failure, power supply fault, or an internal fault that the display does not annunciate.

Practical checks in flight: confirm mode and code, cycle power fully (not just standby), check the encoder/altitude source, check the antenna selector, and try the second transponder even if deferred - a deferred item can be re-tested, and if it works you have a legal and safe solution. If none of that restores replies, treat the unit as unserviceable.

Obligations and options:

• Tell ATC clearly that the transponder is unserviceable and that the second unit is also unavailable. Do not leave the controller guessing.
• The clearance is not automatically cancelled by a transponder failure, but the basis of separation changes. In a radar environment the controller may no longer be able to provide the same service, and may impose procedural separation, a level restriction, or a route restriction. You must comply with whatever new clearance is issued.
• Squawk 7600 if the failure is such that communication is also affected; here communication is good, so the controller may instruct a specific code or ask you to squawk ident periodically to see if replies return. Follow their instruction.
• Continue to destination is normally acceptable in good visibility with no traffic nearby, provided ATC agrees and the airspace allows it. The alternative is to divert to an airport where the aircraft can be fixed, which costs fuel and time.
• Consider the airspace: if the destination is in airspace where a transponder is mandatory, the failure may require a specific clearance or a different routing. ATC will advise.
• Report the failure for maintenance and, if required, as an occurrence.

The judgement is: a transponder failure is not an emergency, but it degrades ATC's ability to separate you, so the correct posture is full disclosure, compliance with any new clearance, and a plan to fix it at the first suitable airport.

In a TMA where SSR is the primary separation tool, the loss of transponder is far more significant:

• Radar separation may be lost. The controller may be unable to separate you from other traffic using SSR-derived information, and may have to revert to procedural separation, which is slower and may not be possible in dense traffic.
• You may be refused entry to, or asked to leave, the TMA, or given a level/route restriction until you are outside the affected airspace.
• Traffic avoidance becomes see-and-avoid to a greater degree, and the controller will likely keep you clear of other aircraft rather than integrate you.
• The option to continue to destination may not exist; a diversion to a non-TMA airport or a hold outside the TMA may be required.
• TCAS also depends on the transponder; with no replies, other aircraft will not receive your altitude and TCAS coordination is degraded, which raises the collision risk in dense traffic.

The lesson is that the same failure has a different severity depending on the airspace and the separation method in use, and the decision must be made against that context, not against the failure alone.