Narrative:

We were descending into ZZZ from YYY; repositioning an aircraft for a part 135 revenue flight. Out of approximately 17000 ft MSL; we encountered IMC with turbulence (moderate) and rain (moderate). I activated our D328 jet's anti-ice system; which includes AC electric elevator horn (tips) heat; and the left side failed. I cycled the switch approximately 3 times in 1-2 mins; however; it remained failed. About this time my first officer and I noticed a strong smell of burning electrical equipment. I looked back to the flight attendant sitting in the last row near the lavatory; who indicated she too smelled smoke. We were already descending; and I advised ATC we had smoke in the aft and needed lower. There appeared to be a haze in the cockpit as well. ATC gave immediate vectors to ZZZ and still lower altitudes. As the first officer and I prepared to don oxygen masks and goggles; the smell improved. We ran checklists; declared an emergency; set up for an ILS at ZZZ. Upon landing without incident; crash fire rescue equipment met us and checked aircraft with a heat camera; finding no hot spots. It was determined by maintenance to be an electrical relay beneath my seat which had failed/overheated when I had cycled the failed elevator horn heat; causing smoke and burning smell.

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Original NASA ASRS Text

Title: A D328 FLT CREW NOTICED A BURNING ELECTRICAL SMELL AFTER USING ELECTRICAL ANTI-ICE. THEY WERE DSNDING TO DEST. CREW DECLARED AN EMER AND LANDED.

Narrative: WE WERE DSNDING INTO ZZZ FROM YYY; REPOSITIONING AN ACFT FOR A PART 135 REVENUE FLT. OUT OF APPROX 17000 FT MSL; WE ENCOUNTERED IMC WITH TURB (MODERATE) AND RAIN (MODERATE). I ACTIVATED OUR D328 JET'S ANTI-ICE SYS; WHICH INCLUDES AC ELECTRIC ELEVATOR HORN (TIPS) HEAT; AND THE L SIDE FAILED. I CYCLED THE SWITCH APPROX 3 TIMES IN 1-2 MINS; HOWEVER; IT REMAINED FAILED. ABOUT THIS TIME MY FO AND I NOTICED A STRONG SMELL OF BURNING ELECTRICAL EQUIP. I LOOKED BACK TO THE FLT ATTENDANT SITTING IN THE LAST ROW NEAR THE LAVATORY; WHO INDICATED SHE TOO SMELLED SMOKE. WE WERE ALREADY DSNDING; AND I ADVISED ATC WE HAD SMOKE IN THE AFT AND NEEDED LOWER. THERE APPEARED TO BE A HAZE IN THE COCKPIT AS WELL. ATC GAVE IMMEDIATE VECTORS TO ZZZ AND STILL LOWER ALTS. AS THE FO AND I PREPARED TO DON OXYGEN MASKS AND GOGGLES; THE SMELL IMPROVED. WE RAN CHKLISTS; DECLARED AN EMER; SET UP FOR AN ILS AT ZZZ. UPON LNDG WITHOUT INCIDENT; CFR MET US AND CHKED ACFT WITH A HEAT CAMERA; FINDING NO HOT SPOTS. IT WAS DETERMINED BY MAINT TO BE AN ELECTRICAL RELAY BENEATH MY SEAT WHICH HAD FAILED/OVERHEATED WHEN I HAD CYCLED THE FAILED ELEVATOR HORN HEAT; CAUSING SMOKE AND BURNING SMELL.

Data retrieved from NASA's ASRS site as of January 2009 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.