Narrative:

A corporate light transport and a commuter mdt. Both aircraft were southbound on L489 north of huo VORTAC with air carrier Y ahead by 15 mi to start and below by 2000'. I descended air carrier Y to 6000 and was stepping light transport X down above. Air carrier Y air carrier Y had to be at 6000 just south of huo and the LR35 had to be at 7000 at the same point. I gave light transport X a clearance to 11,000 and he read it back correctly. I subsequently cleared light transport X to 10,000 and light transport X responded that he was already descending to 10,000 and asked how I heard him. I told light transport X that the previous clearance was to 11,000 and again cleared him to 10,000. As air carrier Y leveled at 6000 just north of huo, I issued a restriction to cross 5 mi north of huo at or below 8000, descend and maintain 7000 and indicated 7000 as the assigned altitude in the NAS data block. Light transport X read back cross 5 mi north at or below 8000 descend and maintain 5000. I didn't catch the incorrect readback. The radar associate did not hear the readback as he was on a landline. As both aircraft passed over huo I observed the mode C of light transport X at 6400 and the conflict alert initiated. I validated the mode C off light transport X and light transport X said first I assigned him 7000, then I assigned him 5000. I issued immediate turns to both aircraft and climbed light transport X back to 7000 to reestablish separation. I feel the light transport X pilot contributed to this operational error thru lax attention to ATC instructions and indicated by his incorrect execution of 2 clrncs within 3 mins. One at 10,000 instead of 11,000 and the incorrect restriction at huo. I feel that if the radar associate had not been on the landline, that the extra ears may have helped note the bad readback in time to correct it before it became a problem. Supplemental information from acn 123691: captain of light transport en route from boston logan to teterboro we got a descent clearance to 'cross 10 mi west of huguenot (huo) VOR at 7000'.' I read it back that way. Later boston center said we were missing our crossing restriction. We were about 7 mi northeast of huo still descending to 7000'. We were tracking in on a northeastern radial and were going to track out of huo on the 207 degree radial to coate intersection (17 mi fix). The person flying assumed 10 mi west was on the 207 degree radial. However, boston said that they meant for us to be at 7000' 10 mi before huo.

Related reports:

Google
 

Original NASA ASRS Text

Title: LESS THAN STANDARD SEPARATION BETWEEN LTT X AND ACR Y. SYSTEM ERROR.

Narrative: A CORPORATE LTT AND A COMMUTER MDT. BOTH ACFT WERE SBND ON L489 N OF HUO VORTAC WITH ACR Y AHEAD BY 15 MI TO START AND BELOW BY 2000'. I DESCENDED ACR Y TO 6000 AND WAS STEPPING LTT X DOWN ABOVE. ACR Y ACR Y HAD TO BE AT 6000 JUST S OF HUO AND THE LR35 HAD TO BE AT 7000 AT THE SAME POINT. I GAVE LTT X A CLRNC TO 11,000 AND HE READ IT BACK CORRECTLY. I SUBSEQUENTLY CLRED LTT X TO 10,000 AND LTT X RESPONDED THAT HE WAS ALREADY DESCENDING TO 10,000 AND ASKED HOW I HEARD HIM. I TOLD LTT X THAT THE PREVIOUS CLRNC WAS TO 11,000 AND AGAIN CLRED HIM TO 10,000. AS ACR Y LEVELED AT 6000 JUST N OF HUO, I ISSUED A RESTRICTION TO CROSS 5 MI N OF HUO AT OR BELOW 8000, DSND AND MAINTAIN 7000 AND INDICATED 7000 AS THE ASSIGNED ALT IN THE NAS DATA BLOCK. LTT X READ BACK CROSS 5 MI N AT OR BELOW 8000 DSND AND MAINTAIN 5000. I DIDN'T CATCH THE INCORRECT READBACK. THE RADAR ASSOCIATE DID NOT HEAR THE READBACK AS HE WAS ON A LANDLINE. AS BOTH ACFT PASSED OVER HUO I OBSERVED THE MODE C OF LTT X AT 6400 AND THE CONFLICT ALERT INITIATED. I VALIDATED THE MODE C OFF LTT X AND LTT X SAID FIRST I ASSIGNED HIM 7000, THEN I ASSIGNED HIM 5000. I ISSUED IMMEDIATE TURNS TO BOTH ACFT AND CLIMBED LTT X BACK TO 7000 TO REESTABLISH SEPARATION. I FEEL THE LTT X PLT CONTRIBUTED TO THIS OPERATIONAL ERROR THRU LAX ATTN TO ATC INSTRUCTIONS AND INDICATED BY HIS INCORRECT EXECUTION OF 2 CLRNCS WITHIN 3 MINS. ONE AT 10,000 INSTEAD OF 11,000 AND THE INCORRECT RESTRICTION AT HUO. I FEEL THAT IF THE RADAR ASSOCIATE HAD NOT BEEN ON THE LANDLINE, THAT THE EXTRA EARS MAY HAVE HELPED NOTE THE BAD READBACK IN TIME TO CORRECT IT BEFORE IT BECAME A PROBLEM. SUPPLEMENTAL INFORMATION FROM ACN 123691: CAPT OF LTT ENRTE FROM BOSTON LOGAN TO TETERBORO WE GOT A DSCNT CLRNC TO 'CROSS 10 MI W OF HUGUENOT (HUO) VOR AT 7000'.' I READ IT BACK THAT WAY. LATER BOSTON CENTER SAID WE WERE MISSING OUR XING RESTRICTION. WE WERE ABOUT 7 MI NE OF HUO STILL DESCENDING TO 7000'. WE WERE TRACKING IN ON A NORTHEASTERN RADIAL AND WERE GOING TO TRACK OUT OF HUO ON THE 207 DEG RADIAL TO COATE INTXN (17 MI FIX). THE PERSON FLYING ASSUMED 10 MI W WAS ON THE 207 DEG RADIAL. HOWEVER, BOSTON SAID THAT THEY MEANT FOR US TO BE AT 7000' 10 MI BEFORE HUO.

Data retrieved from NASA's ASRS site as of August 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.