A PVT PLT; WORKING ON AN INST RATING; FLEW THROUGH THE MDA ON A VOR APCH.

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WE DEPARTED MLI AROUND XY30 AM ON FEB/FRI/93. SKY PARTIALLY OBSCURED; 3/4 MI VISIBILITY. WE HAD CLBED ABOVE THE FOG AT 800-900 FT. IT WAS CLR; SUN SHINING; VISIBILITY 10-15 MI. MY STUDENT (A PRIVATE PLT WORKING ON HIS INST RATING) ASKED FOR AND RECEIVED A CLRNC FOR A LCL IFR TO A VOR RWY 3 APCH TO CWI (ON OUR OWN). WE RECEIVED THE CLRNC BEFORE DEPARTING MLI. WE FLEW DIRECT TO CVA VOR (IAF) AT 3000 FT; FLEW THE PROC TURN OUTBOUND. AS WE CROSSED OVER THE FAF WE WERE STILL AT 3000 FT. WE SHOULD HAVE DSNDED TO 2300 FT. I TOLD STUDENT TO START HIS DSCNT TWICE; HE WAS FIXATING ON THE CDI. THE STUDENT WAS HAVING A PROB MAINTAINING A HDG AND KEEPING THE CDI CTRED ON THE 040 DEG RADIAL OFF CVA VOR. ATC TOLD US WE WERE CLRED FOR VOR RWY 3 APCH AT CWI ABOUT 2 MI FROM HILLZ (USED WITH THE ILS 3 APCH); TO SWITCH TO ADVISORY FREQ. I TOLD STUDENT AGAIN TO GET DOWN TO MDA WHICH WAS 1400 FT MSL. I WAS TALKING ABOUT THE APCH; CHKING THE TFC FREQ (STUDENT HAD NOT TALKED TO CWI YET) WHEN I NOTICED HE HAD DSNDED THROUGH MDA. I TOLD HIM TO STOP HIS DSCNT IMMEDIATELY (HE WAS AT 1000 FT MSL); AND CLB BACK UP TO MDA AT 1400 FT; WHICH HE DID. HIND SIGHT AS TO HOW THIS HAPPENED: 1) INSTRUCTOR WAS DISTRACTED; I HAD FLOWN WITH THIS STUDENT ON NUMEROUS OCCASIONS; HE ALWAYS PERFORMED ABOVE AVERAGE; BUT THIS TIME HE WAS HAVING MORE TROUBLE THAN USUAL MAINTAINING HDGS AND ALT. HE WAS SLOW TO START DSCNT INITIALLY. 2) WE WERE ALWAYS IN VFR CONDITIONS. I SUBCONSCIOUSLY WAS PREPARED TO DSND INTO THE FOG; WHICH NEVER HAPPENED. WE WERE STILL ABOVE THE FOB AT 1000 FT. 3) WE HAD FLOWN THE ILS 3 APCH AT CWI SEVERAL TIMES AND THAT DECISION HT WAS 1000 FT; WE WERE ALSO FAMILIAR WITH THE AREA SUCH OBSTRUCTIONS; WHICH I BELIEVE GAVE US A FALSE SENSE OF SECURITY. 4) SOMETIMES IT IS HARD TO TEACH WHILE IN ACTUAL CONDITIONS OR ON ACTUAL IFR CLRNC. THERE IS NO ROOM FOR ERROR.