ATC Loss of Transponder Signal

ATC Loss of Transponder Signal

What we have been Informed

After experiencing a Airborne Collision Avoidance System (ACAS) fault, with associated drill “TCAS MODE……STBY”, the crew of an aircraft set the Mode Selector Switch to STBY on the ATC/TCAS panel. As, on this panel, the STBY position sets both TCAS and ATC Transponder to standby this caused a temporary loss of secondary radar information and obstructs the automatic update of the flight data. On this ATC/TCAS panel, the TCAS mode only is set to standby when the mode selector is on the XPNDR position.

During this time the Tower Controller tried to contact the ACC/APP Centre. Although several attempts were made the calls were not answered. Unknown to the ACC/APP controller the aircraft was climbing in conflict with another departing aircraft. As the aircraft was not transponding, no TCAS or Short Term Conflict Alert (STCA) alerts were triggered and the minimum separation reduced to 3.7 nm and 0 ft.

The TCAS Panel

The airline concerned by the event is fitted with the following TCAS panel:

TCAS Panel

Mode Selector

Mode Selector

The Aircraft Operator Took the Following Actions

  • A crew notice was issued to all the fleet pilots to point out the anomaly and reiterate the correct procedure.
  • The airline has featured the incident in their Safety Journal for the purposes of lesson learning.
  • The airline safety services and fleet technical pilots are reviewing how to permanently reflect this information in operating manuals.
  • The airline has contacted the aircraft manufacturer to determine a way forward.

The Aircraft Manufacturer

  • The aircraft manufacturer is still investigating this subject. Several technical solutions are under study but their feasibility is still to be validated by the design office.

The Air Navigation Service Provider took the following Action

  • Jointly with the airline, presented this incident to 100 safety managers at the Flight Safety Committee.
  • Priority telephone lines will be installed between the Tower and ACC/APP Centre.
  • The incident was featured in the safety publications.

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© European Organisation for Safety of Air Navigation (EUROCONTROL) August 2006. This alert is published by EUROCONTROL for information purposes. It may be copied in whole or in part, provided that EUROCONTROL is mentioned as the source and to the extent justified by the non-commercial use (not for sale). The information in this document may not be modified without prior written permission from EUROCONTROL. The use of the document is at the user’s sole risk and responsibility. EUROCONTROL expressly disclaim any and all warranties with respect to any content within the alert, express or implied.


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