Narrative:

We were on vectors for the ILS runway 1R at iad, 4000 ft MSL assigned. We were then cleared to intercept the localizer for runway 1R. Shortly thereafter we were cleared (or thought we were cleared) to 'cross mosby at 2000 ft, cleared ILS runway 1R approach.' we then commenced our descent to 2000 ft. At 2300 ft approach control requested our altitude. We responded '2300 ft.' approach responded that we should climb to 3000 ft and that our clearance was to cross mosby at 3000 ft. We started our climb back to 3000 ft. At 2500 ft, approach control requested our current altitude, to which we responded 2500 ft. Approach responded that we could maintain our current altitude and were then cleared again for the ILS runway 1R approach, which was completed without further incident. The captain and I both believe that the clearance was to cross mosby at 2000 ft. I know I responded with 2000 ft and set it in the altitude window, which the captain confirmed. It is frustrating that if our response to the clearance was incorrect, that approach control (which was not very busy at the time) did not catch an incorrect readback of a clearance.

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Original NASA ASRS Text

Title: B757 FLC WITH DSCNT CLRNC MISUNDERSTOOD ALT ASSIGNMENT AND READ BACK WHAT THEY HEARD WHICH WAS NOT CAUGHT BY THE CTLR UNTIL CTLR SAW THEM DSNDING THROUGH THE ASSIGNED.

Narrative: WE WERE ON VECTORS FOR THE ILS RWY 1R AT IAD, 4000 FT MSL ASSIGNED. WE WERE THEN CLRED TO INTERCEPT THE LOC FOR RWY 1R. SHORTLY THEREAFTER WE WERE CLRED (OR THOUGHT WE WERE CLRED) TO 'CROSS MOSBY AT 2000 FT, CLRED ILS RWY 1R APCH.' WE THEN COMMENCED OUR DSCNT TO 2000 FT. AT 2300 FT APCH CTL REQUESTED OUR ALT. WE RESPONDED '2300 FT.' APCH RESPONDED THAT WE SHOULD CLB TO 3000 FT AND THAT OUR CLRNC WAS TO CROSS MOSBY AT 3000 FT. WE STARTED OUR CLB BACK TO 3000 FT. AT 2500 FT, APCH CTL REQUESTED OUR CURRENT ALT, TO WHICH WE RESPONDED 2500 FT. APCH RESPONDED THAT WE COULD MAINTAIN OUR CURRENT ALT AND WERE THEN CLRED AGAIN FOR THE ILS RWY 1R APCH, WHICH WAS COMPLETED WITHOUT FURTHER INCIDENT. THE CAPT AND I BOTH BELIEVE THAT THE CLRNC WAS TO CROSS MOSBY AT 2000 FT. I KNOW I RESPONDED WITH 2000 FT AND SET IT IN THE ALT WINDOW, WHICH THE CAPT CONFIRMED. IT IS FRUSTRATING THAT IF OUR RESPONSE TO THE CLRNC WAS INCORRECT, THAT APCH CTL (WHICH WAS NOT VERY BUSY AT THE TIME) DID NOT CATCH AN INCORRECT READBACK OF A CLRNC.

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.