AN INSTRUCTOR PLT ALLOWS THE SPI STUDENT PLT TO DSND BELOW THEIR IFR ASSIGNED ALT WHILE ATTEMPTING A VISUAL ON PHN; MI.

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DEPARTED ST CLAIR COUNTY ARPT ON STUDENT TRAINING FLT. UPON REACHING 2300 FT MSL; PICKED UP AN IFR CLRNC FROM SELFRIDGE APCH (MTC). MY INTENT WAS TO CONDUCT AIRWORK ABOVE THE OVCST AND; AS SUCH; I HAD FILED AN IFR FLT PLAN TO VFR ON TOP. INITIAL ALT RESTR WAS TO CLB AND MAINTAIN 6000 FT MSL AND THEN RPT IF NOT ON TOP BY THAT ALT. AT 6000 FT MSL THE ACFT HAD NOT REACHED THE TOP OF THE OVCST AND I BEGAN TO NOTICE THE FORMATION OF RIME ICE. I IMMEDIATELY ADVISED SELFRIDGE APCH CTL THAT I WANTED TO DSND TO VFR CONDITIONS AND THEN ENTER THE PATTERN AT PHN VFR. ALL THE WHILE I WAS PROVIDING INSTRUCTION TO A PRIMARY STUDENT. ATC ASSIGNED ME AN ALT OF 2300 FT. UPON REACHING 2300 FT MSL I NOTICED THE ARPT AND BEGAN TO DSND FOR THE PATTERN ALT. WHEN DSNDING THROUGH 1800 FT MSL; I WAS QUERIED BY MTC APCH ABOUT MY ALT. BECAUSE OF A HIGH WORKLOAD SIT -- STUDENT TRAINING; DUSK CONDITIONS; AND THE ACCUMULATION OF A SIGNIFICANT AMOUNT OF ICE -- I FAILED TO COMPLY WITH MY ALT RESTR OF 2300 FT MSL. UPON SEEING THE ARPT; I CONTINUED WITH OTHER DUTIES IN ANTICIPATION OF A LNDG RATHER THAN REMEMBERING I WAS ON AN IFR FLT PLAN. I IMMEDIATELY CANCELED IFR AND LANDED UNEVENTFULLY. IN THE FUTURE; AN INCREASED VIGILANCE SHOULD BE MAINTAINED IN ORDER TO ENSURE COMPLIANCE WITH ASSIGNED ALTS. IF NECESSARY; THE INSTRUCTOR SHOULD FLY THE AIRPLANE; RATHER THAN CONTINUING ON AN INSTRUCTIONAL FLT.