ACFT Y WAS GIVEN A DSCNT CLRNC TO STAY ABOVE ACFT X; WHO WAS HOLDING. NO ACKNOWLEDGEMENT WAS RECEIVED FROM ACFT Y. LATER; CLRNC WAS REISSUED TO ACFT Y. ACFT Y CLBED INTO ACFT X'S ALT; LEGAL SEPARATION LOST.

Narrative

1 ACFT IN HOLDING AT FL240 IN HIGH ALT AIRSPACE (B737 FOR SFO). THIS ACFT'S DATA BLOCK WAS SUPPRESSED DUE TO CONGESTION AT THE HOLDING STACK WITH 6 OR 7 OTHER ACFT. MD80 FOR SJC WAS AT FL260 AND ISSUED DSCNT TO FL230. INSTRUCTOR QUESTIONED THE CLRNC AND THE MD80 WAS REISSUED FL250. XMISSION WAS NOT ACKNOWLEDGED. AFTER FREQ CONGESTION CLRED; THE CLRNC TO FL250 WAS REISSUED AGAIN AND MD80 BEGAN CLB BACK TO FL250. 2 SECTORS WERE COMBINED. AS IN MOST CASES; THIS INCIDENT COULD HAVE BEEN AVOIDED IN VARIOUS WAYS: 1) HIGH ALT SHOULD/COULD HAVE RERTED THE SJC ARR CLR OF THE HOLDING STACK. 2) VERBAL COORD BTWN THE RADAR ASSOCIATE POS AND THE RADAR POS RELATIVE TO THE 2 ACFT IN QUESTION PRIOR TO THE INCIDENT. 3) EXTENDING THE LEADER LINES ON THE DATA BLOCKS TO MORE EASILY SEPARATE THEM. 4) DECOMBINE POS WHEN ENTERING HOLDING OR COMPLEXITY INCREASES. 5) GREATER CTLR VIGILANCE. 6) PLTS SHOULD BE AWARE OF NORMAL SEPARATION CRITERIA AND IN CONJUNCTION WITH TCASII QUESTION CTL INSTRUCTIONS PLACING THEM IN PROX TO OTHER ACFT; IE; TEAMWORK.