Operational Safety Issue

EUROCONTROL SISG secretariat was warned by the Switzerland Air navigation Service Provider Skyguide of several cases of radar track loss. Through the help of some airlines, it was discovered and confirmed an anomaly due to an interface issue in the Honeywell Primus suite with Mode-S transponders. If the flight crew take longer than 5 seconds to complete a Mode A code change, the transponder will revert to “Standby” mode. This will cause the track to be dropped by radars, and the Airborne Collision Avoidance System (ACAS) II on board the aircraft will fail. The operational consequences are high with a safety critical impact:

• Temporary loss of radar contact;
• Degradation of the ACAS safety net - the aircraft's TCAS will fail, and the aircraft will not be acquired by other TCAS II equipped aircraft in the vicinity since the transponder is in “Standby” mode;

Note that this problem could occur in all airspace environments