Narrative:

I was working the stewart sector at boston ARTCC with light to moderate traffic. Air carrier X, small transport was inbound to newark via huo V489 coate at 7000 and had reported level. Small transport Y was inbound to teterboro via the same route at 6000. The air carrier X had been issued the small transport Y traffic prior to reporting level. On my final transmission to air carrier X, I told him the 6000 traffic was just below him and less than 1 mi away, and to contact new york approach control. The pilot, who spoke with an accent, acknowledged he was looking and was changing frequencys. Later, reviewing the tape, air carrier X mumbled 6000 in his readback, and was looking for the traffic. My guess is because he speaks broken english and usually enters my TRACON's airspace at 6000, when he heard me mention 6000 in issuing the traffic, he took that as a clearance to 6000 even though he knew there was traffic at 6000 and hadn't visually sighted him yet. When the targets merged on my display, I lost altitude data in my data block for air carrier X, for a few seconds. When I got it back, it indicated he was at 6000. Because this is a common computer false readout (altitude errors because the information gets scrambled due to the close proximity of the targets), I paid no attention to the information. I had no reason to believe that air carrier X was anything, but at 7000. Evidently, the ny controller didn't fully understand his initial check-in transmission of descending to 6000 either, otherwise he may have been able to stop his descent.

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Original NASA ASRS Text

Title: ACR X FLT CREW NON ADHERENCE TO ATC CLRNC. DESCENDING TO OCCUPIED ALT.

Narrative: I WAS WORKING THE STEWART SECTOR AT BOSTON ARTCC WITH LIGHT TO MODERATE TFC. ACR X, SMT WAS INBND TO NEWARK VIA HUO V489 COATE AT 7000 AND HAD REPORTED LEVEL. SMT Y WAS INBND TO TETERBORO VIA THE SAME ROUTE AT 6000. THE ACR X HAD BEEN ISSUED THE SMT Y TFC PRIOR TO REPORTING LEVEL. ON MY FINAL XMISSION TO ACR X, I TOLD HIM THE 6000 TFC WAS JUST BELOW HIM AND LESS THAN 1 MI AWAY, AND TO CONTACT NEW YORK APCH CTL. THE PLT, WHO SPOKE WITH AN ACCENT, ACKNOWLEDGED HE WAS LOOKING AND WAS CHANGING FREQS. LATER, REVIEWING THE TAPE, ACR X MUMBLED 6000 IN HIS READBACK, AND WAS LOOKING FOR THE TFC. MY GUESS IS BECAUSE HE SPEAKS BROKEN ENGLISH AND USUALLY ENTERS MY TRACON'S AIRSPACE AT 6000, WHEN HE HEARD ME MENTION 6000 IN ISSUING THE TFC, HE TOOK THAT AS A CLRNC TO 6000 EVEN THOUGH HE KNEW THERE WAS TFC AT 6000 AND HADN'T VISUALLY SIGHTED HIM YET. WHEN THE TARGETS MERGED ON MY DISPLAY, I LOST ALT DATA IN MY DATA BLOCK FOR ACR X, FOR A FEW SECONDS. WHEN I GOT IT BACK, IT INDICATED HE WAS AT 6000. BECAUSE THIS IS A COMMON COMPUTER FALSE READOUT (ALT ERRORS BECAUSE THE INFO GETS SCRAMBLED DUE TO THE CLOSE PROX OF THE TARGETS), I PAID NO ATTN TO THE INFO. I HAD NO REASON TO BELIEVE THAT ACR X WAS ANYTHING, BUT AT 7000. EVIDENTLY, THE NY CTLR DIDN'T FULLY UNDERSTAND HIS INITIAL CHECK-IN XMISSION OF DESCENDING TO 6000 EITHER, OTHERWISE HE MAY HAVE BEEN ABLE TO STOP HIS DSCNT.

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.