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.