Narrative:

I was on the second leg of a VFR flight from dpa-jvl-osh and had received a WX briefing around xa a.M. Before departing dpa. The forecast was for good VFR conditions along the whole route for the rest of the day. I departed jvl around XC30 for osh in excellent VFR conditions. About 20 mi southwest of osh I could see a low cloud deck in the osh vicinity. I tuned in osh ATIS and discovered that osh was 800 ft overcast with 3 mi visibility. I circled in the clear, called chicago center, and requested a VOR 27 approach into osh. Center told me to climb and maintain 4000 ft for the osh VOR 27 approach via radar vectors. A few minutes later I was cleared down to 2800 ft, which put me in the clouds, and I began to pick up mixed ice (1/4-1/2 inches). About 8 mi east of osh as I was intercepting the final approach course the gear warning horn sounded and my gear extended automatically because of ice accumulation on the pitot mast which controls the automatic gear extension. At first I thought that the horn was a stall warning horn, but then realized it was the gear warning. The distraction of the unexpected gear extension combined with very light turbulence and the altered flight characteristics of the aircraft due to icing caused me to descend to approximately 2500 ft which was below my assigned altitude of 2800 ft. I was immediately cleared for the approach and descended to the MDA of 1200 ft and the approach and landing continued normally. I should have checked the WX again when I was at jvl even though it was a short hop, the WX at jvl was good, and had earlier been forecast to be good at osh all day. I should not have allowed myself to be distracted by the gear warning horn to the extent of allowing descent below my assigned altitude. I am seriously considering having the automatic gear extension mechanism disabled on the aircraft.

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Original NASA ASRS Text

Title: GA SMA ALTDEV OVERSHOT DURING DSCNT FOR IAP VOR APCH TO OSH IN ICING CONDITIONS.

Narrative: I WAS ON THE SECOND LEG OF A VFR FLT FROM DPA-JVL-OSH AND HAD RECEIVED A WX BRIEFING AROUND XA A.M. BEFORE DEPARTING DPA. THE FORECAST WAS FOR GOOD VFR CONDITIONS ALONG THE WHOLE RTE FOR THE REST OF THE DAY. I DEPARTED JVL AROUND XC30 FOR OSH IN EXCELLENT VFR CONDITIONS. ABOUT 20 MI SW OF OSH I COULD SEE A LOW CLOUD DECK IN THE OSH VICINITY. I TUNED IN OSH ATIS AND DISCOVERED THAT OSH WAS 800 FT OVCST WITH 3 MI VISIBILITY. I CIRCLED IN THE CLR, CALLED CHICAGO CENTER, AND REQUESTED A VOR 27 APCH INTO OSH. CENTER TOLD ME TO CLB AND MAINTAIN 4000 FT FOR THE OSH VOR 27 APCH VIA RADAR VECTORS. A FEW MINUTES LATER I WAS CLRED DOWN TO 2800 FT, WHICH PUT ME IN THE CLOUDS, AND I BEGAN TO PICK UP MIXED ICE (1/4-1/2 INCHES). ABOUT 8 MI E OF OSH AS I WAS INTERCEPTING THE FINAL APCH COURSE THE GEAR WARNING HORN SOUNDED AND MY GEAR EXTENDED AUTOMATICALLY BECAUSE OF ICE ACCUMULATION ON THE PITOT MAST WHICH CONTROLS THE AUTOMATIC GEAR EXTENSION. AT FIRST I THOUGHT THAT THE HORN WAS A STALL WARNING HORN, BUT THEN REALIZED IT WAS THE GEAR WARNING. THE DISTR OF THE UNEXPECTED GEAR EXTENSION COMBINED WITH VERY LIGHT TURB AND THE ALTERED FLT CHARACTERISTICS OF THE ACFT DUE TO ICING CAUSED ME TO DSND TO APPROX 2500 FT WHICH WAS BELOW MY ASSIGNED ALT OF 2800 FT. I WAS IMMEDIATELY CLRED FOR THE APCH AND DSNDED TO THE MDA OF 1200 FT AND THE APCH AND LNDG CONTINUED NORMALLY. I SHOULD HAVE CHKED THE WX AGAIN WHEN I WAS AT JVL EVEN THOUGH IT WAS A SHORT HOP, THE WX AT JVL WAS GOOD, AND HAD EARLIER BEEN FORECAST TO BE GOOD AT OSH ALL DAY. I SHOULD NOT HAVE ALLOWED MYSELF TO BE DISTRACTED BY THE GEAR WARNING HORN TO THE EXTENT OF ALLOWING DSCNT BELOW MY ASSIGNED ALT. I AM SERIOUSLY CONSIDERING HAVING THE AUTOMATIC GEAR EXTENSION MECHANISM DISABLED ON THE ACFT.

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.