An aircraft without transponder – Guidance for Controllers

An aircraft without transponder – Guidance for Controllers


This article describes the best practices to be used by air traffic controllers to maximize the chance that an aircraft without transponder is detected as soon as possible and to mitigate its impact on the provision of air traffic service.

Suggested actions

The advice given in this section is derived from experience, best practices and common sense. It is not intended to replace or supersede local instructions and procedures.

The following actions usually help the controller to adequately handle a situation where an aircraft is experiencing a transponder failure:

  • Quick discovery of the situation is essential. It gives more time to develop a plan and reduces the chance of an incident happening.
    • Regular scanning – this is something a controller normally does;
    • Lost track tool may be very helpful for the initial discovery; this feature is present under different names in different ATM systems but the common idea is to alert the controller that a correlated track has been lost. One of the possible reasons for this is transponder failure.
    • Uncorrelated and primary tracks within (or about to enter) the area of responsibility should be carefully considered.
    • Flight plan track tool (if available) may help controller identify a lost track and anticipate flight's current position. However, the precision is much lower, as this is a predicted position based on flight plan data.
  • Assumptions should be avoided. Filling information gaps with something plausible may lead to unexpected results. All possible scenarios should be considered unless there is a reasonable certain information
  • Correct situation assessment is also very important. The controller should determine the subtype of the situation which is critical for choosing the correct following actions. A list of examples is given below:
    • An (expected) aircraft with a transponder failure; this is a relatively routine situation compared to other options;
    • An (expected) aircraft with a transponder failure and no radio contact (due to communication failure, improper handover, etc.) resulting in the controller being unable to obtain information from or issue instructions to the aircraft;
    • Strayed or lost aircraft, i.e. an aircraft that is generally not supposed to be there; this situation may develop into (or may already have become) an unusual or emergency situation;
    • Intruder or military aircraft (especially near or above international waters or areas with military conflicts); this situation may develop into e.g. air policing mission.
  • Clarification – if there is radio contact with the aircraft the controller should try to determine the failure type (mode A, C or S) together with the pilot, e.g.
    • Verify the level of the aircraft;
    • Instruct the pilot to reset the transponder code;
    • Confirm or correct the Mode S flight ID.
  • The controller should try to obtain additional information – from neighbouring ATS units or military units; although these should normally take the initiative and advise the downstream unit, the controller should consider the probability that the previous unit may have not noticed the situation;
  • Informing all concerned (next ATS sectors or units, military authorities, the supervisor, etc.) as described in local instructions helps them plan for the situation in advance.
  • Transponder validation procedures (e.g. level verification on initial contact) are helpful for discovering a transponder failure.
  • Lateral separation should be considered. This is especially true if no reliable information about the aircraft level can be obtained. In such cases usually the safest option is to consider the traffic with transponder failure to be at all levels and provide lateral separation.
  • Traffic information may be useful in some cases and will most likely make the flight crew spend extra effort on “see and avoid”, especially if there is no radio contact with the aircraft experiencing transponder failure. If used, the transponder failure and relevant details should be clearly stated (e.g. so that the pilots do not rely on TCAS readings in case of Mode C or total failure);
  • Estimates and position reports can be used for the provision of ATS to the aircraft experiencing transponder failure even if no surveillance information is available (application of procedural separation may be necessary).

Factors to be considered

The following factors should be considered by the controllers having aircraft with no or corrupt transponder reading within their area of responsibility:

  • A total loss of transponder is often combined with radio communication loss (e.g. due to severe electrical system failure or the previous ATS unit not transferring the aircraft on the next frequency).
  • As mentioned before, different procedures apply based on the situation subtype (e.g. in case an intruder/unknown/military aircraft is involved). Wrong assumptions may lead to unpredictable results. It is therefore crucial that any uncertainties are clearly stated as such when communicating with other parties (ATS units, military authorities, etc.).
  • Ghost tracks (either primary or secondary) may distract the controller. If they happen frequently the controllers may consider a “real” track to be false or they may spend valuable time waiting to determine whether a track is “real” or a “ghost”.
  • Meteorological conditions affect the “see and avoid” barrier practiced by pilots. They also impact the pilots’ ability to see and recognize the traffic referred to in the traffic information broadcast made by the controller.
  • If vectoring to provide lateral separation is chosen, the controller should consider the possibility that the aircraft being vectored may lose visual contact with the aircraft experiencing transponder failure. Furthermore, if the situation deteriorates, i.e. positional information is also lost, there may be no way to determine the relative position of the aircraft.
  • A military aircraft (intruder) might consider vectoring of another aircraft towards or behind it as a sign of aggression; it is therefore essential to perform a correct situation assessment, as described in the previous section.
  • If using pilot’s report to confirm the level of an unknown aircraft the controller should consider the fact that this information depends on various factors and may be very far from the truth.
  • Communication loss may be a side effect of a total transponder failure.
  • The lost track tool may produce false alarms. If these happen often, the controllers might not be able to detect a “real” situation, especially in high density traffic. Therefore, careful examination of all lost track tool warnings is recommended as well as proper personnel training (given the fact that this tool is not supposed to activate often, it is likely that some controllers are not familiar enough with it).

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