A STUDENT PLT AND INSTRUCTOR EXPERIENCED AN ATTITUDE INDICATOR FAILURE AT 200-300 FT ABOVE DECISION HT WHILE ON AN ILS APCH. THE INSTRUCTOR SUSPECTED A VACUUM SYS FAILURE; FOLLOWED PROCS FOR NO GYRO OP; AND DECLARED A MISSED APCH. THE FLT DIVERTED TO AN ARPT THAT WAS RPTING VMC.

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I WAS RECEIVING INSTRUCTION IN AN MO20 THAT I RECENTLY PURCHASED. MY INSURANCE COMPANY REQUIRED 15 HRS OF DUAL WITH A CFI AND THIS FLT WAS THE FINAL FLT. DURING THE LAST 2 FLTS WE HAD PRACTICED INST APCHS. AFTER FLYING FROM CLE TO CAK TO YNG AND CONDUCTING NUMEROUS SUCCESSFUL APCHS; WE STOPPED FOR LUNCH. I RECEIVED A BRIEF AND CIE WX WAS FORECAST AS CEILING 1500 FT AND 4 MI VISIBILITY. I FILED AND WE DEPARTED YNG. WE CLBED TO 4000 FT AND ENCOUNTERED SOLID IFR. I HAVE NOT LOGGED A LOT OF ACTUAL (4 HRS AT THIS POINT) AND FELT A LITTLE UNCOMFORTABLE; BUT WAS DOING OK. DURING THE APCH THE CTLR KEPT ASKING US TO MAINTAIN A HIGHER SPD THAN NORMAL AND ATIS INDICATED CEILINGS MUCH LOWER THAN FORECAST. AS WE WERE NEARING DECISION HT (200-300 FT ABOVE DECISION HT); I WAS HAVING TROUBLE STAYING ON LOC AND THE ATTITUDE INDICATOR STARTED TO TUMBLE. AT THIS POINT THE INSTRUCTOR TOOK THE PLANE AND TRIED TO GET ON THE LOC. HE BECAME CONFUSED BY THE MALFUNCTIONING ATTITUDE INDICATOR AND WE RAPIDLY WENT AT LEAST 90 DEGS OFF THE APCH COURSE. WE CALLED MISSED WITH LOST GYROS AND CLBED OUT (CAUSING CONSIDERABLE CONFUSION TO ATC). WE REQUESTED AND WERE VECTORED TO YNG WHERE WE WERE ABLE TO LAND VISUALLY. I HAD THE ATTITUDE INDICATOR REPLACED THERE. ATC REQUESTED WE CALL THEM CONCERNING A 'FLT DEV' AND TOOK INFO FROM BOTH MYSELF AND MY INSTRUCTOR. SUPPLEMENTAL INFO FROM ACN 332341: THE ACFT IS EQUIPPED WITH THE DIAL HDG INDICATOR AND TURN AND BANK INDICATOR. THE VACUUM SYS HAS A HIGH AND LOW VACUUM LIGHT; NO SUCTION GAUGE. WHEN THE ISNT WENT INOP I TOOK OVER AND EXECUTED A MISSED APCH. SINCE THE ATTITUDE INDICATOR DID NOT WORK I DID NOT TRUST THE DIRECTIONAL GYRO -- WITH ONLY THE TURN AND BANK AND A BOUNCING MAGNETIC COMPASS; I DEVIATED FROM RWY HDG. WHEN PRACTICING NO GYRO APCHS THE INSTS ARE COVERED AND LOST IMMEDIATELY; THERE'S NO WAY TO SIMULATE THE GRADUAL LOSS OF GYROSCOPIC INSTS. THUS; WHEN RECEIVING CONFLICTING INFO AT A CRITICAL PHASE OF FLT; DISORIENTATION CAN HAPPEN VERY QUICKLY.