Narrative:

Military C130 (aircraft X) sbound at FL190. Air carrier Y was wbound climbing to 17000 ft (looking for FL200). I thought I climbed air carrier Y to FL180. I called traffic and both aircraft had each other in sight. I actually had climbed air carrier Y to FL190. I missed the readback. When conflict alert went off, I verified the assigned altitude, and then descended air carrier Y back to FL180. I know I made a mistake. I wish, however, that the pilots would have questioned me as I called traffic. Supplemental information from acn 420592: the radar controller thought he had assigned FL180. When the radar controller noticed air carrier Y leave FL180 on the data block, he read the pilot the altimeter. The pilot stated he was climbing to FL190. The radar controller assigned FL180 to the pilot. Supplemental information from acn 420581: ZDC cleared us to climb to FL190 and turn to heading 290 degrees. As we passed about 17000 ft, he pointed out C130 traffic at FL190 at 2 O'clock. I saw it as it passed about our 2:30 - 3 O'clock position and watched it pass behind us on TCASII at about 3 mi. As we passed about FL183, the controller told us the local altimeter setting. I wondered why we needed that information above FL180, so I said something like, 'roger, but we're climbing to FL190.' he said something like, 'negative, your clearance is FL180!' I said 'we read back FL190, but we'll descend back to FL180.' no immediate comment from ATC. So we started a descent to FL180 as we apexed our climb at FL185. Shortly thereafter ATC cleared us to our cruise altitude of FL200. I now think I should have asked him why the C130 and us were eventually going to be at FL190 within 10 mi and/or was there going to be a conflict. That question would have cleared up the obvious flight lvl clearance discrepancy early on.

Related reports:

Google
 

Original NASA ASRS Text

Title: ZDC RADAR CTLR INCORRECTLY ASSIGNS AN ACR AN ALT CONFLICTING WITH A MIL C130. CTLR THOUGHT HE ASSIGNED AN ALT 1000 FT BELOW THE C130.

Narrative: MIL C130 (ACFT X) SBOUND AT FL190. ACR Y WAS WBOUND CLBING TO 17000 FT (LOOKING FOR FL200). I THOUGHT I CLBED ACR Y TO FL180. I CALLED TFC AND BOTH ACFT HAD EACH OTHER IN SIGHT. I ACTUALLY HAD CLBED ACR Y TO FL190. I MISSED THE READBACK. WHEN CONFLICT ALERT WENT OFF, I VERIFIED THE ASSIGNED ALT, AND THEN DSNDED ACR Y BACK TO FL180. I KNOW I MADE A MISTAKE. I WISH, HOWEVER, THAT THE PLTS WOULD HAVE QUESTIONED ME AS I CALLED TFC. SUPPLEMENTAL INFO FROM ACN 420592: THE RADAR CTLR THOUGHT HE HAD ASSIGNED FL180. WHEN THE RADAR CTLR NOTICED ACR Y LEAVE FL180 ON THE DATA BLOCK, HE READ THE PLT THE ALTIMETER. THE PLT STATED HE WAS CLBING TO FL190. THE RADAR CTLR ASSIGNED FL180 TO THE PLT. SUPPLEMENTAL INFO FROM ACN 420581: ZDC CLRED US TO CLB TO FL190 AND TURN TO HDG 290 DEGS. AS WE PASSED ABOUT 17000 FT, HE POINTED OUT C130 TFC AT FL190 AT 2 O'CLOCK. I SAW IT AS IT PASSED ABOUT OUR 2:30 - 3 O'CLOCK POS AND WATCHED IT PASS BEHIND US ON TCASII AT ABOUT 3 MI. AS WE PASSED ABOUT FL183, THE CTLR TOLD US THE LCL ALTIMETER SETTING. I WONDERED WHY WE NEEDED THAT INFO ABOVE FL180, SO I SAID SOMETHING LIKE, 'ROGER, BUT WE'RE CLBING TO FL190.' HE SAID SOMETHING LIKE, 'NEGATIVE, YOUR CLRNC IS FL180!' I SAID 'WE READ BACK FL190, BUT WE'LL DSND BACK TO FL180.' NO IMMEDIATE COMMENT FROM ATC. SO WE STARTED A DSCNT TO FL180 AS WE APEXED OUR CLB AT FL185. SHORTLY THEREAFTER ATC CLRED US TO OUR CRUISE ALT OF FL200. I NOW THINK I SHOULD HAVE ASKED HIM WHY THE C130 AND US WERE EVENTUALLY GOING TO BE AT FL190 WITHIN 10 MI AND/OR WAS THERE GOING TO BE A CONFLICT. THAT QUESTION WOULD HAVE CLRED UP THE OBVIOUS FLT LVL CLRNC DISCREPANCY EARLY ON.

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.