Narrative:

Air carrier X, widebody transport, inbound to ewr. Atx Y an small transport, inbound to teb. Atx Y was level at 3000' when he spotted the widebody transport at his 10 O'clock position. He said the widebody transport appeared to be at his altitude so he executed a climbing left turn of a 180 degree. In reality, air carrier X was in a discretionary descent out of 4800' for 4000' assigned. Atx Y initiated this action on his own without informing me. I first became aware of the situation and observed his mode C showing 3200' and climbing as the widebody transport passed over. The cloud cover in the area was multi-layered and uneven which would lead me to believe that the atx Y actually saw the widebody transport at his altitude when in reality he was 1600' above. Sounds similar to the 1961 midair over new york. Supplemental information from acn 128706: while approaching ewr at assigned altitude of 4000, a discussion occurred between another aircraft and approach control regarding the second aircraft's altitude. After the second aircraft reported that he just performed an evasive maneuver to avoid a large aircraft and he pulled up to 3700' we became concerned. Ny approach asked 2ND aircraft for verification of altitude. He responded 3700'. He asked us for verification (we responded 4000') and approach then told other aircraft that the widebody transport (us) was assigned 4000'. In no case did approach indicate that we had made an altitude bust. The only possible thought in our mind was the fact the altimeter setting was quite low, 29.43 or so, the 2ND aircraft's pilot had to have misset his kollsman window.

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Original NASA ASRS Text

Title: ATX CLIMBED ABOVE ASSIGNED ALT AND CAME IN CLOSE PROX TO AN ACR WDB.

Narrative: ACR X, WDB, INBND TO EWR. ATX Y AN SMT, INBND TO TEB. ATX Y WAS LEVEL AT 3000' WHEN HE SPOTTED THE WDB AT HIS 10 O'CLOCK POSITION. HE SAID THE WDB APPEARED TO BE AT HIS ALT SO HE EXECUTED A CLIMBING LEFT TURN OF A 180 DEG. IN REALITY, ACR X WAS IN A DISCRETIONARY DSCNT OUT OF 4800' FOR 4000' ASSIGNED. ATX Y INITIATED THIS ACTION ON HIS OWN WITHOUT INFORMING ME. I FIRST BECAME AWARE OF THE SITUATION AND OBSERVED HIS MODE C SHOWING 3200' AND CLIMBING AS THE WDB PASSED OVER. THE CLOUD COVER IN THE AREA WAS MULTI-LAYERED AND UNEVEN WHICH WOULD LEAD ME TO BELIEVE THAT THE ATX Y ACTUALLY SAW THE WDB AT HIS ALT WHEN IN REALITY HE WAS 1600' ABOVE. SOUNDS SIMILAR TO THE 1961 MIDAIR OVER NEW YORK. SUPPLEMENTAL INFORMATION FROM ACN 128706: WHILE APCHING EWR AT ASSIGNED ALT OF 4000, A DISCUSSION OCCURRED BETWEEN ANOTHER ACFT AND APCH CTL REGARDING THE SECOND ACFT'S ALT. AFTER THE SECOND ACFT REPORTED THAT HE JUST PERFORMED AN EVASIVE MANEUVER TO AVOID A LARGE ACFT AND HE PULLED UP TO 3700' WE BECAME CONCERNED. NY APCH ASKED 2ND ACFT FOR VERIFICATION OF ALT. HE RESPONDED 3700'. HE ASKED US FOR VERIFICATION (WE RESPONDED 4000') AND APCH THEN TOLD OTHER ACFT THAT THE WDB (US) WAS ASSIGNED 4000'. IN NO CASE DID APCH INDICATE THAT WE HAD MADE AN ALT BUST. THE ONLY POSSIBLE THOUGHT IN OUR MIND WAS THE FACT THE ALTIMETER SETTING WAS QUITE LOW, 29.43 OR SO, THE 2ND ACFT'S PLT HAD TO HAVE MISSET HIS KOLLSMAN WINDOW.

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.