Narrative:

On the return leg of an overwater flight, our aircraft descended through the assigned altitude. We were talking to ZNY and had received a descent clearance to cross a point, 35 NM from robbinsville VOR at 7000' MSL. After starting our descent from 10,000', traffic was called out in our 1 O'clock 12 NM at 6000'. This traffic was called again and a frequency change to maguire approach issued. My initial call for check-in with approach was 'passing 6.3 for 6'. The captain and I immediately realized our error, power was added and a climb initiated back to 7000'. On descent, the altitude alert did sound and I did make a 1000' above assigned altitude call. However, preoccupation with the small commuter aircraft and the inability to get a visual on this traffic, kept our attention outside the cockpit. Later cockpit discussion led to the following reasons for our error: 1) the captain had subconsciously changed the assigned altitude in his mind to 6000', having heard that number several times in the space of a few minutes. 2) both myself and the F/east had been 'heads up' trying to find our traffic, and had failed to monitor our descent progress properly. 3) controller urgency to begin our descent and a busy frequency, was also a contributing factor in this altitude deviation. The other aircraft was finally spotted after we leveled at 7000' and our aircraft passed safely.

Related reports:

Google
 

Original NASA ASRS Text

Title: DESCENDED BELOW ASSIGNED ALT. CAPT HEARD ALT OF TRAFFIC CALLED AND GOT A MIND SET.

Narrative: ON THE RETURN LEG OF AN OVERWATER FLT, OUR ACFT DESCENDED THROUGH THE ASSIGNED ALT. WE WERE TALKING TO ZNY AND HAD RECEIVED A DSCNT CLRNC TO CROSS A POINT, 35 NM FROM ROBBINSVILLE VOR AT 7000' MSL. AFTER STARTING OUR DSCNT FROM 10,000', TFC WAS CALLED OUT IN OUR 1 O'CLOCK 12 NM AT 6000'. THIS TFC WAS CALLED AGAIN AND A FREQ CHANGE TO MAGUIRE APCH ISSUED. MY INITIAL CALL FOR CHECK-IN WITH APCH WAS 'PASSING 6.3 FOR 6'. THE CAPT AND I IMMEDIATELY REALIZED OUR ERROR, POWER WAS ADDED AND A CLIMB INITIATED BACK TO 7000'. ON DSCNT, THE ALT ALERT DID SOUND AND I DID MAKE A 1000' ABOVE ASSIGNED ALT CALL. HOWEVER, PREOCCUPATION WITH THE SMALL COMMUTER ACFT AND THE INABILITY TO GET A VISUAL ON THIS TFC, KEPT OUR ATTN OUTSIDE THE COCKPIT. LATER COCKPIT DISCUSSION LED TO THE FOLLOWING REASONS FOR OUR ERROR: 1) THE CAPT HAD SUBCONSCIOUSLY CHANGED THE ASSIGNED ALT IN HIS MIND TO 6000', HAVING HEARD THAT NUMBER SEVERAL TIMES IN THE SPACE OF A FEW MINUTES. 2) BOTH MYSELF AND THE F/E HAD BEEN 'HEADS UP' TRYING TO FIND OUR TFC, AND HAD FAILED TO MONITOR OUR DSCNT PROGRESS PROPERLY. 3) CTLR URGENCY TO BEGIN OUR DSCNT AND A BUSY FREQ, WAS ALSO A CONTRIBUTING FACTOR IN THIS ALT DEVIATION. THE OTHER ACFT WAS FINALLY SPOTTED AFTER WE LEVELED AT 7000' AND OUR ACFT PASSED SAFELY.

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.