Narrative:

Traffic and complexity was heavy around the etg and psb area. Military returning aircraft at FL270-290 wbound over psb. At least 4-6 aircraft had to be moved, descended to insure separation was insured with the military aircraft, this traffic situation caused a lot of coordination. Air carrier X was descended from FL290 to FL240 to insure separation with several aircraft. Air carrier was given direct to hdr. Air carrier Y was eastbound at FL290 and descended to FL250. Air carrier X was level at FL240 and air carrier Y was level FL250. Air carrier Y was issued traffic 3 O'clock, 15 mi, nebound at FL260 and told to maintain FL250. On the tape air carrier Y read back FL230. I thought I heard air carrier Y read back FL250. Air carrier Y was switched to ZNY frequency, air carrier X was already on ZNY's frequency. The aircraft had already passed when air carrier Y descended to FL230. The technique of restating an altitude to maintain was done to insure that something like this doesn't happen. In the future a courtesy TA might not be given, but only if the targets are going to merge. By trying to be too conscientious and doing more work than necessary may have allowed this to happen. Only corrective action is to try and listen better, and try to have the phraseology changed so there are no altitudes given in a TA. Words to use instead of the altitude would be above or below.

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Original NASA ASRS Text

Title: ACR Y UNAUTH DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACR X. PLTDEV. SYS ERROR.

Narrative: TFC AND COMPLEXITY WAS HVY AROUND THE ETG AND PSB AREA. MIL RETURNING ACFT AT FL270-290 WBOUND OVER PSB. AT LEAST 4-6 ACFT HAD TO BE MOVED, DSNDED TO INSURE SEPARATION WAS INSURED WITH THE MIL ACFT, THIS TFC SITUATION CAUSED A LOT OF COORD. ACR X WAS DSNDED FROM FL290 TO FL240 TO INSURE SEPARATION WITH SEVERAL ACFT. ACR WAS GIVEN DIRECT TO HDR. ACR Y WAS EBOUND AT FL290 AND DSNDED TO FL250. ACR X WAS LEVEL AT FL240 AND ACR Y WAS LEVEL FL250. ACR Y WAS ISSUED TFC 3 O'CLOCK, 15 MI, NEBOUND AT FL260 AND TOLD TO MAINTAIN FL250. ON THE TAPE ACR Y READ BACK FL230. I THOUGHT I HEARD ACR Y READ BACK FL250. ACR Y WAS SWITCHED TO ZNY FREQ, ACR X WAS ALREADY ON ZNY'S FREQ. THE ACFT HAD ALREADY PASSED WHEN ACR Y DSNDED TO FL230. THE TECHNIQUE OF RESTATING AN ALT TO MAINTAIN WAS DONE TO INSURE THAT SOMETHING LIKE THIS DOESN'T HAPPEN. IN THE FUTURE A COURTESY TA MIGHT NOT BE GIVEN, BUT ONLY IF THE TARGETS ARE GOING TO MERGE. BY TRYING TO BE TOO CONSCIENTIOUS AND DOING MORE WORK THAN NECESSARY MAY HAVE ALLOWED THIS TO HAPPEN. ONLY CORRECTIVE ACTION IS TO TRY AND LISTEN BETTER, AND TRY TO HAVE THE PHRASEOLOGY CHANGED SO THERE ARE NO ALTS GIVEN IN A TA. WORDS TO USE INSTEAD OF THE ALT WOULD BE ABOVE OR BELOW.

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.