Narrative:

During descent on the olympia 2 arrival to seattle, experienced copilot was hand flying the aircraft, which is common to maintain proficiency. We had received clearance to 'descend via the olympia 2 arrival landing south' which meant we were good to descend to 16000 ft over olympia VOR and then down to 12000 ft. As we approached olm VOR it was clear we would have to deviation around a cloud buildup. At this time, I believe we were talking to sea approach control. The controller told us to deviation to the west of the buildup and to maintain 13000 ft. We continued to descend below 16000 ft as we deviated around the buildup. As we cleared the buildup, I told the controller that we could turn back on course. The controller cleared us back on course and about this time asked us our altitude which was about 12500 ft. When I reported '12500 ft correcting,' the controller cleared us to continue descent to 12000 ft. The remainder of the approach and landing was uneventful. My only suggestion for precluding a similar mistake is to encourage use of the autoplt during high workload or abnormal events during flight, such as our deviation around WX during descent. The autoplt and MCP were properly set up including the altitude alerter, but because the airplane was being hand flown, and workload was relatively high with approach control radio xmissions and deviation around WX, neither me nor the copilot heard the altitude alerter chimes telling us we had flown below our assigned altitude of 13000 ft.

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Original NASA ASRS Text

Title: MLG ACFT ON ARR ROUTING AND ALT RESTR DEVIATED AROUND WX AND DSNDED 500 FT BELOW ASSIGNED ALT.

Narrative: DURING DSCNT ON THE OLYMPIA 2 ARR TO SEATTLE, EXPERIENCED COPLT WAS HAND FLYING THE ACFT, WHICH IS COMMON TO MAINTAIN PROFICIENCY. WE HAD RECEIVED CLRNC TO 'DSND VIA THE OLYMPIA 2 ARR LNDG S' WHICH MEANT WE WERE GOOD TO DSND TO 16000 FT OVER OLYMPIA VOR AND THEN DOWN TO 12000 FT. AS WE APCHED OLM VOR IT WAS CLR WE WOULD HAVE TO DEV AROUND A CLOUD BUILDUP. AT THIS TIME, I BELIEVE WE WERE TALKING TO SEA APCH CTL. THE CTLR TOLD US TO DEV TO THE W OF THE BUILDUP AND TO MAINTAIN 13000 FT. WE CONTINUED TO DSND BELOW 16000 FT AS WE DEVIATED AROUND THE BUILDUP. AS WE CLRED THE BUILDUP, I TOLD THE CTLR THAT WE COULD TURN BACK ON COURSE. THE CTLR CLRED US BACK ON COURSE AND ABOUT THIS TIME ASKED US OUR ALT WHICH WAS ABOUT 12500 FT. WHEN I RPTED '12500 FT CORRECTING,' THE CTLR CLRED US TO CONTINUE DSCNT TO 12000 FT. THE REMAINDER OF THE APCH AND LNDG WAS UNEVENTFUL. MY ONLY SUGGESTION FOR PRECLUDING A SIMILAR MISTAKE IS TO ENCOURAGE USE OF THE AUTOPLT DURING HIGH WORKLOAD OR ABNORMAL EVENTS DURING FLT, SUCH AS OUR DEV AROUND WX DURING DSCNT. THE AUTOPLT AND MCP WERE PROPERLY SET UP INCLUDING THE ALT ALERTER, BUT BECAUSE THE AIRPLANE WAS BEING HAND FLOWN, AND WORKLOAD WAS RELATIVELY HIGH WITH APCH CTL RADIO XMISSIONS AND DEV AROUND WX, NEITHER ME NOR THE COPLT HEARD THE ALT ALERTER CHIMES TELLING US WE HAD FLOWN BELOW OUR ASSIGNED ALT OF 13000 FT.

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.