Narrative:

In holding in sector 53 for phl due to low ceilings/low visibility. Air carrier X was placed in hold and originally assigned 11000 ft. Air carrier Y, a B737, was placed in the same holding pattern and assigned 12000 ft. Air carrier X was subsequently given descent clearance to 10000 ft. Aircraft did not acknowledge the clearance. Data block on radar scope was amended to show assigned altitude of 10000 ft. Air carrier X was twice again given descent clearance to 10000 ft. Pilot did not acknowledge descent clearance. Controller began to assume that his frequency was not working. After a switch over to the back-up frequency, he was able to re-establish communication with aircraft on frequency. Note that the data block for air carrier X still reflected an assigned altitude of 10000 ft. Because of a low pressure system moving through the area, causing pressure to fall rapidly, the actual altitude on air carrier X showed 10800 ft. Controller, thinking the aircraft was in a descent, assigned 11000 ft to air carrier Y. Conflict alert activated after separation was already lost. Aircraft were approximately 3 1/2 mi in trail, same direction, in the same holding pattern. Moral of the story: follow through! After air carrier X did not acknowledge descent clearance, data block should have been amended to reflect aircraft level 11000 ft. Air carrier Y would not have been given clearance lower.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.