Narrative:

While level at FL250 our flight was called by ATC and requested to expedite descent to FL240. Complied. Later I learned from center supervisor of ZOA that tape readouts indicated our flight should have been at FL260. When I talked to the first officer later about the incident he indicated he had been off frequency getting ATIS when ATC gave the altitude change. When he returned to ATC frequency he said I was pointing at 25000 ft in the altitude alert window and he pointed to it, then began descent to FL250. It is possible as captain and PNF misset the altitude in the altitude alert window. I believe I must have misread 25000 for 26000. The digital readouts make it a little difficult to distinguish a '5' from a '6.' since the first officer was off frequency he did not hear the ATC clearance. The next breakdown may have been when I communicated to the first officer about the altitude he may not have heard me say 260 but thought I said 250. I did not know he had been off frequency until later. Had I so known, I might have been more careful of informing him of what had occurred while he was not listening. The contributing factors were instrumentation difficult to read, 2-MAN crew having to depart frequency for information and failure to impose rigid cockpit discipline by me, captain, to see that SOP was completely complied with.

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Original NASA ASRS Text

Title: MLG FLC DSNDS TO WRONG ALT.

Narrative: WHILE LEVEL AT FL250 OUR FLT WAS CALLED BY ATC AND REQUESTED TO EXPEDITE DSCNT TO FL240. COMPLIED. LATER I LEARNED FROM CTR SUPVR OF ZOA THAT TAPE READOUTS INDICATED OUR FLT SHOULD HAVE BEEN AT FL260. WHEN I TALKED TO THE FO LATER ABOUT THE INCIDENT HE INDICATED HE HAD BEEN OFF FREQ GETTING ATIS WHEN ATC GAVE THE ALT CHANGE. WHEN HE RETURNED TO ATC FREQ HE SAID I WAS POINTING AT 25000 FT IN THE ALT ALERT WINDOW AND HE POINTED TO IT, THEN BEGAN DSCNT TO FL250. IT IS POSSIBLE AS CAPT AND PNF MISSET THE ALT IN THE ALT ALERT WINDOW. I BELIEVE I MUST HAVE MISREAD 25000 FOR 26000. THE DIGITAL READOUTS MAKE IT A LITTLE DIFFICULT TO DISTINGUISH A '5' FROM A '6.' SINCE THE FO WAS OFF FREQ HE DID NOT HEAR THE ATC CLRNC. THE NEXT BREAKDOWN MAY HAVE BEEN WHEN I COMMUNICATED TO THE FO ABOUT THE ALT HE MAY NOT HAVE HEARD ME SAY 260 BUT THOUGHT I SAID 250. I DID NOT KNOW HE HAD BEEN OFF FREQ UNTIL LATER. HAD I SO KNOWN, I MIGHT HAVE BEEN MORE CAREFUL OF INFORMING HIM OF WHAT HAD OCCURRED WHILE HE WAS NOT LISTENING. THE CONTRIBUTING FACTORS WERE INSTRUMENTATION DIFFICULT TO READ, 2-MAN CREW HAVING TO DEPART FREQ FOR INFO AND FAILURE TO IMPOSE RIGID COCKPIT DISCIPLINE BY ME, CAPT, TO SEE THAT SOP WAS COMPLETELY COMPLIED WITH.

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.