Narrative:

As we approached dulles, we encountered moderate turbulence, so I slowed from normal approach speed to 180 KTS. As the turbulence decreased, ATC requested we resume 'best speed', so I increased airspeed to 220 KTS. All this was during descent to 5000'. As ATC reported the traffic we were following, they gave us a turn to a heading toward the airport and a descent. As I began the turn and started the descent, I looked to the altitude alerter, where we set the altitude we're cleared to. It was still set to 5000'. I asked the captain (PNF, working radios) what altitude we had been cleared to. He said 3000', asked me if I disagreed, and set it in the alerter. I was not sure, but thought that 3000' was the correct altitude, so I continued to descend to 3000'. As we passed through 3600' ATC called telling us to maintain 4000'. Both the captain and I thought we were cleared to 3000', and do not know if we were in error. We both agree that if either thought 3000' was wrong we would have called to confirm. In retrospect, I probably should have told him that I did not know for sure what altitude we had been cleared to rather than 'not disagreeing' with 3000'. This might have made him more suspicious of his understanding, and might have prompted us to call for verification. Our increased activity due to the turbulence, request for increased speed, and search for traffic made us more susceptible to error, and should have made us more skeptical of our assumptions.

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Original NASA ASRS Text

Title: FLT CREW OF LTT DESCENDING ON APCH TO IAD AT 3600' MSL WAS CLEARED TO MAINTAIN 4000' BY IAD APCH CTLR.

Narrative: AS WE APCHED DULLES, WE ENCOUNTERED MODERATE TURB, SO I SLOWED FROM NORMAL APCH SPD TO 180 KTS. AS THE TURB DECREASED, ATC REQUESTED WE RESUME 'BEST SPD', SO I INCREASED AIRSPD TO 220 KTS. ALL THIS WAS DURING DSNT TO 5000'. AS ATC RPTED THE TFC WE WERE FOLLOWING, THEY GAVE US A TURN TO A HDG TOWARD THE ARPT AND A DSNT. AS I BEGAN THE TURN AND STARTED THE DSNT, I LOOKED TO THE ALT ALERTER, WHERE WE SET THE ALT WE'RE CLRED TO. IT WAS STILL SET TO 5000'. I ASKED THE CAPT (PNF, WORKING RADIOS) WHAT ALT WE HAD BEEN CLRED TO. HE SAID 3000', ASKED ME IF I DISAGREED, AND SET IT IN THE ALERTER. I WAS NOT SURE, BUT THOUGHT THAT 3000' WAS THE CORRECT ALT, SO I CONTINUED TO DSND TO 3000'. AS WE PASSED THROUGH 3600' ATC CALLED TELLING US TO MAINTAIN 4000'. BOTH THE CAPT AND I THOUGHT WE WERE CLRED TO 3000', AND DO NOT KNOW IF WE WERE IN ERROR. WE BOTH AGREE THAT IF EITHER THOUGHT 3000' WAS WRONG WE WOULD HAVE CALLED TO CONFIRM. IN RETROSPECT, I PROBABLY SHOULD HAVE TOLD HIM THAT I DID NOT KNOW FOR SURE WHAT ALT WE HAD BEEN CLRED TO RATHER THAN 'NOT DISAGREEING' WITH 3000'. THIS MIGHT HAVE MADE HIM MORE SUSPICIOUS OF HIS UNDERSTANDING, AND MIGHT HAVE PROMPTED US TO CALL FOR VERIFICATION. OUR INCREASED ACTIVITY DUE TO THE TURB, REQUEST FOR INCREASED SPD, AND SEARCH FOR TFC MADE US MORE SUSCEPTIBLE TO ERROR, AND SHOULD HAVE MADE US MORE SKEPTICAL OF OUR ASSUMPTIONS.

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.