Narrative:

We were being vectored for a visual approach to runway 21 and were quite high. We were being stepped down from 9000 ft to 7000 ft, being assigned headings and looking for the airport. After setting and confirming that 7000 ft had been set, both pilots turned our attention back outside the aircraft. Spoilers and slats extended we were descending rapidly. I noticed the aircraft was in vertical speed mode and airspeed was slowing. I reached up and switched the aircraft to IAS since I thought the vertical speed mode was left over from the intermediary descent to 9000 ft and then when we were reclred to 7000 ft the aircraft was already capturing 9000 ft. What I believe was actually happening was we were descending so fast that the aircraft was already capturing 7000 ft, so when I hit IAS I inadvertently canceled the altitude capture. Since our attention was diverted outside, the aircraft descended through 7000 ft and we did not catch it until we were already descending through 6700 ft. When we realized it I immediately clicked off the autoplt, stopped the descent and began a climb back to 7000 ft. About that time approach queried us about what altitude we were descending to and I responded we were leveling at 7000 ft. He then reclred us for 6000 ft. There did not appear to be any traffic conflict and approach did not make any further mention of our descent.

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Original NASA ASRS Text

Title: AN ACR MD80 FLC DSNDED BELOW THEIR ASSIGNED ALT WHEN THE FLC INADVERTENTLY SWITCHED THE MODE OF THE AUTOFLT SYS AFTER IT HAD GONE TO THE CAPTURE MODE AT A HIGH RATE OF DSCNT. THE CAPT FAILED TO MONITOR THE EFFECTS OF THE CHANGE HE INITIATED IN THE SYS AND THE FO DID NOT MONITOR THE ACFT DURING A CRITICAL PHASE OF FLT.

Narrative: WE WERE BEING VECTORED FOR A VISUAL APCH TO RWY 21 AND WERE QUITE HIGH. WE WERE BEING STEPPED DOWN FROM 9000 FT TO 7000 FT, BEING ASSIGNED HEADINGS AND LOOKING FOR THE ARPT. AFTER SETTING AND CONFIRMING THAT 7000 FT HAD BEEN SET, BOTH PLTS TURNED OUR ATTN BACK OUTSIDE THE ACFT. SPOILERS AND SLATS EXTENDED WE WERE DSNDING RAPIDLY. I NOTICED THE ACFT WAS IN VERT SPD MODE AND AIRSPD WAS SLOWING. I REACHED UP AND SWITCHED THE ACFT TO IAS SINCE I THOUGHT THE VERT SPD MODE WAS LEFT OVER FROM THE INTERMEDIARY DSCNT TO 9000 FT AND THEN WHEN WE WERE RECLRED TO 7000 FT THE ACFT WAS ALREADY CAPTURING 9000 FT. WHAT I BELIEVE WAS ACTUALLY HAPPENING WAS WE WERE DSNDING SO FAST THAT THE ACFT WAS ALREADY CAPTURING 7000 FT, SO WHEN I HIT IAS I INADVERTENTLY CANCELED THE ALT CAPTURE. SINCE OUR ATTN WAS DIVERTED OUTSIDE, THE ACFT DSNDED THROUGH 7000 FT AND WE DID NOT CATCH IT UNTIL WE WERE ALREADY DSNDING THROUGH 6700 FT. WHEN WE REALIZED IT I IMMEDIATELY CLICKED OFF THE AUTOPLT, STOPPED THE DSCNT AND BEGAN A CLB BACK TO 7000 FT. ABOUT THAT TIME APCH QUERIED US ABOUT WHAT ALT WE WERE DSNDING TO AND I RESPONDED WE WERE LEVELING AT 7000 FT. HE THEN RECLRED US FOR 6000 FT. THERE DID NOT APPEAR TO BE ANY TFC CONFLICT AND APCH DID NOT MAKE ANY FURTHER MENTION OF OUR DSCNT.

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.