TCAS RAs Generated due to Transponder Testing on the Ground

TCAS RAs Generated due to Transponder Testing on the Ground

Background information

  • Airborne Collision Avoidance System (ACAS) II interrogates, within its range, all Mode S and Mode A/C SSR transponders squawking altitude. That includes ground-based transponders operated for testing or maintenance.
  • If these transponders respond with an altitude report close to that of aircraft flying in the vicinity, their TCAS II Traffic Display will show a ‘ghost’ target and, more seriously, could generate TAs/RAs against such targets.
  • Recently, events have been reported in which RAs were generated by transponders that were being tested on the ground.
  • These unnecessary RAs were disruptive to the flight crew and air traffic control.

EUROCONTROL Mode S & ACAS programme advice

  • TAs/RAs due to transponder testing on the ground are disruptive and potentially hazardous and must be prevented.
  • To avoid these TAs/RAs special caution and appropriate procedures are required during transponder testing and maintenance.
  • In order to prevent the transmission of a virtual altitude which could then be mistakenly used by airborne systems, the followings steps are recommended:
    • Use effective screening or absorption devices on the antennas or physically connect the ramp test set to the antenna system.
    • Where possible, perform the testing inside a hangar to take advantage of any shielding properties it may provide.
    • Manually set the altitude to a high value (e.g. over 60,000 feet) or unrealistically low (e.g. negative 2000).
    • Select the transponder(s) to ‘OFF’ or ‘Standby’ when testing is complete.
  • The simulation of TCAS operations must not be carried out by the radiation from an antenna located on, or remotely based from a workshop.

Additional information and advice

Your support is required

  • Note the subject and investigate the relevance for your operational environment.
  • National Regulators: develop national guidance for transponder testing on the ground to prevent the interference with TCAS II operations.
  • Maintenance/test facilities: ensure that steps are taken along the recommendations in this Safety Warning Message and any national requirements when transponders are tested on the ground.
  • ANSPs/airlines: report occurrences of RAs generated by transponder testing on the ground to national regulatory authorities.

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© European Organisation for Safety of Air Navigation (EUROCONTROL) February 2008. 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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