Operation without a Transponder or with a Dysfunctional Transponder


Operation of an aircraft without a transponder, or with a dysfunctional one, constitutes a single threat with the potential to “pass” through all the existing safety barriers up to “see and avoid”. Transponder failure can occur due to a number of reasons. These can include incorrect input data, electrical faults, and simple SSR-transponder communication problems, such as a bit flip (a random and unintended change of an information bit value from 0 to 1 or vice-versa).

Transponder Failure Types

The most common transponder failure types classified by feature are:

  • Mode A code only (Aircraft Identifier);
  • Mode C information only (Altitude data);
  • Mode S 24-bit address only, which may result in unidentified aircraft being present on the situational display or a wrong surveillance track to flight plan correlation;
  • Total failure (A, C & S), which may result in the aircraft disappearing from the controller's situational display.
  • Partial loss of transponder functionality (e.g. operational at reduced power limiting transponder detection by ATC radar and/or other aircraft).

The most common transponder failures classified by the severity of the failure are:

  • Total loss (feature not available);
  • Corrupted (feature operating but wrong data output);
  • Intermittent (feature working with interruptions);
  • Duplicated (two or more aircraft transmitting the same information).

The transponder failure can be characterised by a feature failure and a severity element. Of course, some combinations are inappropriate (e.g. duplicated mode C).

Possible Outcomes and Impact on Operations

Depending on the circumstances, the following may occur following a transponder failure:

Typical Scenarios

A total loss of transponder information may happen due to a number of factors, e.g.:

  • Poor surveillance coverage (e.g. due to terrain, low altitude or surveillance sensor failure);
  • Transponder technical failure;
  • Other avionics failure (e.g. a restart of an aircraft system leading to the transponder being switched to standby mode);
  • Flight crew not turning the transponder on;
  • Flight crew accidentally switching the transponder to standby mode;
  • Flight crew switching transponder to standby mode after miscommunication with the controller.

Intermittent or corrupted mode C signal may be caused by various technical faults of the on-board equipment or by radar detection failures. Duplicate Mode S addresses although unlikely, may happen due to:

  • Technical fault in the avionics;
  • Transponder being transferred from one aircraft to another.
  • Incorrect Mode S address of newly delivered or registered aircraft (due to block allocation of addresses to aircraft within a state);

ATC system failures may sometimes result in outcomes similar to transponder failure, e.g.:

  • Flight level data of surveillance tracks being swapped (e.g. if an aircraft is right above/below another);
  • A surveillance track being dropped (e.g. if an aircraft is right above/below another).

Prevention Barriers

Prevention barriers are designed to reduce the probability of occurrence of the event (operation without or with a dysfunctional transponder).

Following the company SOPs and the aircraft/avionics manufacturer’s service bulletins to timely address any problems. Moreover, the aircraft operator should follow the maintenance schedule established by the manufacturers in order to reduce the probability of equipment failures, including transponder-related ones.

Repairable mitigation barriers

Mitigation barriers are designed to reduce the impact of the event after it has already happened. A mitigation barrier is called “repairable” when a failure has reduced the effectiveness of a barrier in the system, but certain actions may still be able to restore the effectiveness of this barrier. The main repairable mitigation barriers types are:

  • Application of transponder code validation procedures on first contact;
  • More effective flight plan data;
  • Tactical conflict management:
    • Regular air situation scanning by ATCO;
    • Use of primary radar data, if available;
    • Detection of transponder failure by flight crew;
    • Built-in alerting functions for the transponder failure at the flight deck
    • Built-in alerting function in the automated ATM system issuing warning when under specific circumstances the surveillance track data, and respectively, the track to flight plan correlation is is lost.

The total loss of transponder information may sometimes be mitigated by the availability of primary radar data. However, if false primary tracks appear too often, a “real” track may be considered to be fake. This is especially true when no other data is available (e.g. information from the previous sector).

New and Existing Mitigation Barriers

The following generic barriers could mitigate the effects of operation without or with dysfunctional transponder:

  • Design and strategic planning:
    • Airspace design – e.g. using de-conflicted RNAV routes;
    • Procedure design taking account of potential transponder malfunction;
    • Appropriate ATC system design and calibration;
  • Sector capacity planning reducing the probability of very high ATC workload periods;
  • Use of voice reporting by flight crews;
  • Tactical conflict management:
    • Alert for change in track status;
    • Use of voice reporting;
  • ATC collision avoidance - collision avoidance via procedural control;
  • Crew collision avoidance - see and avoid practiced by flight crew.

Accidents and Incidents

The following events in the SKYbrary database include transponder operation as a contributory factor:

Related Articles

Further Reading


SKYbrary Partners:

Safety knowledge contributed by: