B747 CARGO FLT; DURING A TEMPORARY LOSS OF ACFT CTL IN TURB; DSNDS THROUGH ITS ASSIGNED ALT OF FL290 IN PROX OF ANOTHER ACR FLT AT FL280 NEAR SUC VORTAC; FO.

Narrative

APCHING SUC VOR (JAPAN) FROM THE SW (7-8 DME); DSNDING TO FL290; AT FL300 WE DECREASED RATE OF DSCNT TO ABOUT 1500 FPM. TCASII CALLED 'TFC; TFC.' NO ACFT SYMBOL APPEARED (OFF SCALE ON 6 MI SCOPE). WE WERE DSNDING IN CLOUD AND DID NOT VISUALLY ACQUIRE THE TFC AT THIS TIME. NEXT TCASII CALL WAS 'MONITOR VERT SPD;' COMMANDING ABOUT 500 FPM DOWN. TARGET WAS 1 O'CLOCK POS ON EDGE OF SCOPE -13 ALT. DISCONNECTED AUTOPLT TO ADJUST VERT SPD. ACFT WAS LEVELED OFF AND THEN CONTINUED TO FL290 REMAINING CLR OF RED AREA ON VSI. VISUALLY ACQUIRED THE TFC AT ABOUT FL295. WHILE LEVELING THE ACFT AT FL290; WE ENCOUNTERED MODERATE TO SEVERE TURB AND VERT SPD INCREASED RAPIDLY DOWN. WE ARRESTED THE DSCNT AT FL280 AND IMMEDIATELY BEGAN TO RECOVER TO FL290. TFC AT FL280 PASSED APPROX 1000-1500 FT HORIZLY R AND 200 FT VERTLY BELOW US. NO ADDITIONAL TCASII CALLS TO CHANGE THE INITIAL MONITOR VERT SPD CALL. TCASII CALLED 'CLR OF TFC' AFTER THE OTHER ACFT PASSED OUR R SIDE. DISCUSSION: INITIAL RESPONSE MAY HAVE BEEN TOO MUCH. IN RETROSPECT; REMAINING IN AUTOPLT AND REDUCING VERT SPD SHOULD HAVE RESOLVED THE RA. AUTOPLT ELEVATOR CHANNEL IS VERY EFFECTIVE AND MAY HAVE ELIMINATED THE EXCESSIVE RATE OF DSCNT THAT PRODUCED THE ALT DEV. CONTRIBUTING FACTORS WERE A MOONLESS NIGHT AND ATTEMPTING TO KEEP TFC IN SIGHT. THE SIT DEVELOPED VERY RAPIDLY AND THE ENTIRE EXCURSION FROM FL290 DOWN TO FL280 AND BACK TO FL290 WAS COMPLETED IN LESS THAN 1 MIN. HAND FLYING THE ACFT IN TURB ALSO CONTRIBUTED.