At time 2118 PST, position 150 miles south of Karachi on ATS route G216, aircraft reported navigation issue and requested heading. Aircraft was instructed to maintain present heading. At the same time, aircraft was observed turning right and losing altitude (rate of descent 15,000 feet). Several calls were given to the aircraft, but no reply was received. At time 2121 PST, position 155 miles west of Karachi, aircraft disappeared from radar.
