Narrative:

Was cleared to descend from FL230 to FL210 by ZAB. At the time, I was deviating around small storm cells and dealing with light icing conditions. My altitude preselect/capture had been malfunctioning and thus did not have the capture function engaged. I was using the instrument as a reminder only. Distraction by the WX, I missed my descent altitude and did not notice the discrepancy until I reached FL197, 1300 ft below my assigned altitude. I called ZAB to confirm that I had only been cleared to FL210. I leveled off at FL197 and began climb back to FL210. He stated he had noticed my excursion from my assigned altitude, but then cleared me down to FL190. The rest of the trip proceeded without event. Factors: WX causing distrs/tunnel vision fixation caused me to lose focus on altitude. Dependence on altitude alert/capture function of malfunctioning instrument. Over reliance caused comfort level of altitude capture that should not have been there.

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Original NASA ASRS Text

Title: DURING A SINGLE PLT IFR OP, THE PLT OF A PIPER CHEYENNE II, PA31 II T (PAYE), DSNDED BELOW ASSIGNED ALT DURING DSCNT DUE TO A WX DISTR AND THE FACT THAT THE AUTOPLT PRESELECT FEATURE WAS NOT WORKING.

Narrative: WAS CLRED TO DSND FROM FL230 TO FL210 BY ZAB. AT THE TIME, I WAS DEVIATING AROUND SMALL STORM CELLS AND DEALING WITH LIGHT ICING CONDITIONS. MY ALT PRESELECT/CAPTURE HAD BEEN MALFUNCTIONING AND THUS DID NOT HAVE THE CAPTURE FUNCTION ENGAGED. I WAS USING THE INST AS A REMINDER ONLY. DISTR BY THE WX, I MISSED MY DSCNT ALT AND DID NOT NOTICE THE DISCREPANCY UNTIL I REACHED FL197, 1300 FT BELOW MY ASSIGNED ALT. I CALLED ZAB TO CONFIRM THAT I HAD ONLY BEEN CLRED TO FL210. I LEVELED OFF AT FL197 AND BEGAN CLB BACK TO FL210. HE STATED HE HAD NOTICED MY EXCURSION FROM MY ASSIGNED ALT, BUT THEN CLRED ME DOWN TO FL190. THE REST OF THE TRIP PROCEEDED WITHOUT EVENT. FACTORS: WX CAUSING DISTRS/TUNNEL VISION FIXATION CAUSED ME TO LOSE FOCUS ON ALT. DEPENDENCE ON ALT ALERT/CAPTURE FUNCTION OF MALFUNCTIONING INST. OVER RELIANCE CAUSED COMFORT LEVEL OF ALT CAPTURE THAT SHOULD NOT HAVE BEEN THERE.

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.