Narrative:

While descending into phl airport we were issued a clearance to 9000 ft at vcn with our discretion out of 13000 ft (we were at 15000 ft). I descended to 13000 ft as we were still 30 NM from vcn. I was flying the aircraft on autoplt. ATC said something but I didn't catch it. I had 9000 ft set in the altitude window according to our previous clearance and had hit altitude hold at 13000 ft. As I saw the distance to vcn appropriate for the descent to 9000 ft I started down at 1000 FPM. At 12500 ft the captain suddenly realized the situation and informed me the controller had told us to maintain 13000 ft for traffic. I quickly disconnected the autoplt and returned to 13000 ft. I noticed another aircraft on the TCASII which remained outside of 10 NM from our position. ATC never called us concerning the deviation. In retrospect I was concerned with the captain's lack of exactness in checklist response throughout the trip and had talked to him that morning about it. In altitude procedures he did not strictly follow the standard of pointing to a new altitude until the other pilot acknowledged it. I had set our lowest altitude of the clearance in and, upon receiving 13000 ft until advised, he never pointed to the window for me to change it as the PF on autoplt. My hearing problem and his procedural deviation caused this altitude excursion. I think we have come a long, good way in attacking these procedural problems but this tells me we have a long way to go.

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Original NASA ASRS Text

Title: ACR DSNDS BELOW ASSIGNED ALT.

Narrative: WHILE DSNDING INTO PHL ARPT WE WERE ISSUED A CLRNC TO 9000 FT AT VCN WITH OUR DISCRETION OUT OF 13000 FT (WE WERE AT 15000 FT). I DSNDED TO 13000 FT AS WE WERE STILL 30 NM FROM VCN. I WAS FLYING THE ACFT ON AUTOPLT. ATC SAID SOMETHING BUT I DIDN'T CATCH IT. I HAD 9000 FT SET IN THE ALT WINDOW ACCORDING TO OUR PREVIOUS CLRNC AND HAD HIT ALT HOLD AT 13000 FT. AS I SAW THE DISTANCE TO VCN APPROPRIATE FOR THE DSCNT TO 9000 FT I STARTED DOWN AT 1000 FPM. AT 12500 FT THE CAPT SUDDENLY REALIZED THE SIT AND INFORMED ME THE CTLR HAD TOLD US TO MAINTAIN 13000 FT FOR TFC. I QUICKLY DISCONNECTED THE AUTOPLT AND RETURNED TO 13000 FT. I NOTICED ANOTHER ACFT ON THE TCASII WHICH REMAINED OUTSIDE OF 10 NM FROM OUR POS. ATC NEVER CALLED US CONCERNING THE DEV. IN RETROSPECT I WAS CONCERNED WITH THE CAPT'S LACK OF EXACTNESS IN CHKLIST RESPONSE THROUGHOUT THE TRIP AND HAD TALKED TO HIM THAT MORNING ABOUT IT. IN ALT PROCS HE DID NOT STRICTLY FOLLOW THE STANDARD OF POINTING TO A NEW ALT UNTIL THE OTHER PLT ACKNOWLEDGED IT. I HAD SET OUR LOWEST ALT OF THE CLRNC IN AND, UPON RECEIVING 13000 FT UNTIL ADVISED, HE NEVER POINTED TO THE WINDOW FOR ME TO CHANGE IT AS THE PF ON AUTOPLT. MY HEARING PROB AND HIS PROCEDURAL DEV CAUSED THIS ALT EXCURSION. I THINK WE HAVE COME A LONG, GOOD WAY IN ATTACKING THESE PROCEDURAL PROBS BUT THIS TELLS ME WE HAVE A LONG WAY TO GO.

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.