ZDC SUPVR CTLR BECOMES DISTR AND MISINTERPS MODE C ALT DATA BLOCK INFO AND DSNDS A B734 INTO AN MD80 HOLDING AT THE SAME FIX.

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IN HOLDING IN SECTOR 53 FOR PHL DUE TO LOW CEILINGS/LOW VISIBILITY. ACR X WAS PLACED IN HOLD AND ORIGINALLY ASSIGNED 11000 FT. ACR Y; A B737; WAS PLACED IN THE SAME HOLDING PATTERN AND ASSIGNED 12000 FT. ACR X WAS SUBSEQUENTLY GIVEN DSCNT CLRNC TO 10000 FT. ACFT DID NOT ACKNOWLEDGE THE CLRNC. DATA BLOCK ON RADAR SCOPE WAS AMENDED TO SHOW ASSIGNED ALT OF 10000 FT. ACR X WAS TWICE AGAIN GIVEN DSCNT CLRNC TO 10000 FT. PLT DID NOT ACKNOWLEDGE DSCNT CLRNC. CTLR BEGAN TO ASSUME THAT HIS FREQ WAS NOT WORKING. AFTER A SWITCH OVER TO THE BACK-UP FREQ; HE WAS ABLE TO RE-ESTABLISH COM WITH ACFT ON FREQ. NOTE THAT THE DATA BLOCK FOR ACR X STILL REFLECTED AN ASSIGNED ALT OF 10000 FT. BECAUSE OF A LOW PRESSURE SYS MOVING THROUGH THE AREA; CAUSING PRESSURE TO FALL RAPIDLY; THE ACTUAL ALT ON ACR X SHOWED 10800 FT. CTLR; THINKING THE ACFT WAS IN A DSCNT; ASSIGNED 11000 FT TO ACR Y. CONFLICT ALERT ACTIVATED AFTER SEPARATION WAS ALREADY LOST. ACFT WERE APPROX 3 1/2 MI IN TRAIL; SAME DIRECTION; IN THE SAME HOLDING PATTERN. MORAL OF THE STORY: FOLLOW THROUGH! AFTER ACR X DID NOT ACKNOWLEDGE DSCNT CLRNC; DATA BLOCK SHOULD HAVE BEEN AMENDED TO REFLECT ACFT LEVEL 11000 FT. ACR Y WOULD NOT HAVE BEEN GIVEN CLRNC LOWER.