Narrative:

Holding at ard on the dylin arrival to ewr at 10000 ft, we were cleared to 8000 ft. At 9500 ft, we noted an aircraft only 500 ft below us on TCASII, and asked the controller about him. He seemed a little confused and asked us to maintain 9000 ft initially, but then had us climb back to 10000 ft and the other aircraft descend to 8000 ft. I think the controller must have mistakenly cleared us to 8000 ft initially, instead of the other aircraft. The controller made a couple of calls to both aircraft to ascertain their altitudes/identify. We probably were within approximately 1 mi of each other at virtually the same altitude at our closest point of approach. We never got a TCASII alert. I later confirmed TCASII was in TA/RA mode. Factors: we were virtually on top of the other aircraft in the holding pattern, which may have led the controller to confuse our call sign. As we stepped down in altitude in the holding pattern, I had reported out of the altitude to the new altitude, ie...leaving 11000 ft for 10000 ft. This time I said, '...descending to 8000 ft.' had I said out of 10000 ft for 8000 ft, it may have alerted the controller to the error. We complied with the clearance to descend and then maintain 9000 ft despite seeing the other aircraft on TCASII, because we assumed the controller understood the situation, and maybe the other aircraft was heading away or slow to descend out of 9000 ft. We never got a TCASII alert, and so, never felt a sense of urgency. In retrospect, I feel I should have had my copilot (PF) aggressively stop the descent at 9500 ft until the situation was resolved.

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Original NASA ASRS Text

Title: B737-800 CREW AND UNIDENTED ACFT IN HOLDING PATTERN HAD LESS THAN LEGAL SEPARATION IN ZDC CLASS E.

Narrative: HOLDING AT ARD ON THE DYLIN ARR TO EWR AT 10000 FT, WE WERE CLRED TO 8000 FT. AT 9500 FT, WE NOTED AN ACFT ONLY 500 FT BELOW US ON TCASII, AND ASKED THE CTLR ABOUT HIM. HE SEEMED A LITTLE CONFUSED AND ASKED US TO MAINTAIN 9000 FT INITIALLY, BUT THEN HAD US CLB BACK TO 10000 FT AND THE OTHER ACFT DSND TO 8000 FT. I THINK THE CTLR MUST HAVE MISTAKENLY CLRED US TO 8000 FT INITIALLY, INSTEAD OF THE OTHER ACFT. THE CTLR MADE A COUPLE OF CALLS TO BOTH ACFT TO ASCERTAIN THEIR ALTS/IDENT. WE PROBABLY WERE WITHIN APPROX 1 MI OF EACH OTHER AT VIRTUALLY THE SAME ALT AT OUR CLOSEST POINT OF APCH. WE NEVER GOT A TCASII ALERT. I LATER CONFIRMED TCASII WAS IN TA/RA MODE. FACTORS: WE WERE VIRTUALLY ON TOP OF THE OTHER ACFT IN THE HOLDING PATTERN, WHICH MAY HAVE LED THE CTLR TO CONFUSE OUR CALL SIGN. AS WE STEPPED DOWN IN ALT IN THE HOLDING PATTERN, I HAD RPTED OUT OF THE ALT TO THE NEW ALT, IE...LEAVING 11000 FT FOR 10000 FT. THIS TIME I SAID, '...DSNDING TO 8000 FT.' HAD I SAID OUT OF 10000 FT FOR 8000 FT, IT MAY HAVE ALERTED THE CTLR TO THE ERROR. WE COMPLIED WITH THE CLRNC TO DSND AND THEN MAINTAIN 9000 FT DESPITE SEEING THE OTHER ACFT ON TCASII, BECAUSE WE ASSUMED THE CTLR UNDERSTOOD THE SIT, AND MAYBE THE OTHER ACFT WAS HEADING AWAY OR SLOW TO DSND OUT OF 9000 FT. WE NEVER GOT A TCASII ALERT, AND SO, NEVER FELT A SENSE OF URGENCY. IN RETROSPECT, I FEEL I SHOULD HAVE HAD MY COPLT (PF) AGGRESSIVELY STOP THE DSCNT AT 9500 FT UNTIL THE SIT WAS RESOLVED.

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.