Transponder related between a Boeing 747-438, VH-OJB and Tupolev KGA515, Lajak, on 17 July 1999

While maintaining flight level (FL) 330, the crew of a Boeing 747 (B747) aircraft monitored the position report from the crew of a Tupulov 154 (Tu154) aircraft. This position report indicated that the Tu154 was estimating position LAJAK at 1907 UTC. The B747 was estimating LAJAK at 1906. Both aircraft were maintaining FL 330.

The crew of the B747 advised air traffic control of the conflict, who directed the crew of the Tu154 to descend to FL 290. The aircraft were vertically separated at LAJAK.

Despite the close proximity of the aircraft, the B747's traffic alert and collision avoidance system (TCAS) did not display the TU154. The crew confirmed with air traffic control that the secondary surveillance radar (SSR) transponder mode C function of the Tu154 was operating correctly. The crew of the B747 reported that their aircraft's TCAS system was functioning correctly and that they were able to monitor aircraft at other stages of the flight.

Investigation revealed that some Eastern Bloc SSR transponders may be selected by the crew to output altitude information in either feet or metres. The investigation was unable to determine if the Tu154 was equipped with this type of transponder.

The operator of the B747 issued an alert to all crews regarding the capability of Eastern Bloc transponders to output altitude information in either feet or metres and if metres was selected the aircraft may not be visible on TCAS.