INST STUDENT PLT LOST CTL OF THE ACFT DUE TO ENCOUNTERING ICING CONDITIONS DURING APCH.

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THE PROB AROSE ON A FLT FROM SAT. THE CRISIS AROSE IN THE LAST 10-15 MINS OF THE FLT. WE WERE ON TOP OF A 6000 FT OVCST LAYER APCHING LBB. OUTSIDE AIR TEMP WAS PLUS 5 DEGS C; SURFACE TEMP AT LBB WAS APPROX 24 DEGS F. MY FATHER- IN-LAW (WHO IS AN INST STUDENT OF MINE) WANTED TO SHOOT A COUPLED ILS TO RWY 17R. WHILE BEING VECTORED ON A L DOWNWIND; I INFORMED MY FATHER-IN-LAW THAT I WAS GOING TO SET UP THE OM ON THE GPS. WHILE ENTERING THE INFO INTO THE GPS; I HAD AN IMMEDIATE FEELING OF AN AIRPLANE TRANSITIONING FROM CRUISE TO A SLOW FLT CONDITION. I LOOKED OUT THE WINDOW AND SAW APPROX 1 1/2 INCHES OF ICE ON THE LEADING EDGES OF THE ACFT; AND LOOKED AT THE ATTITUDE INDICATOR AND SAW 8 DEGS NOSE UP AND AIRSPD THAT HAD DECREASED FROM OUR 17 INCHES AT 234 RPM; 120 KT AIRSPD; TO 80-85 KIAS. I TOLD MY FATHER-IN-LAW THAT I WANTED HIM TO DISENGAGE THE AUTOPLT AND FEEL THE HANDLING CHARACTERISTICS OF THE PLANE. HE TURNED OFF THE AUTOPLT AND ALMOST IMMEDIATELY WE HAD A STALL WARNING HORN; BUFFETED; AND THE ACFT BROKE TO THE R. I TOOK CTL OF THE ACFT AND RECOVERED WITH A 400 FT LOSS OF ALT. OUR LAST ASSIGNED ALT WAS 5000 FT AND WE WERE NOW AT 4600 FT; ABLE TO HOLD ALT WITH HIGH PWR SETTINGS. I INFORMED THE CTLR THAT WE HAD AN ICE PROB AND COULD ACCEPT NO DELAY. WE WERE IMMEDIATELY CLRED FOR THE APCH AND LANDED WITH NO FURTHER PROBS. THE FORECAST FOR OUR DEST CALLED FOR 2000- 3000 FT CEILINGS AND IMPROVING CONDITIONS WITH A CHANCE FOR 4000-5000 FT CEILINGS. OUR LNDG WX WAS 500 FT BROKEN AND 2.5 MI VISIBILITY; WITH SE WINDS. THERE WAS AN AIRMET OUT FOR THE AREA FOR OCCASIONAL LIGHT TO MODERATE RIME ICING. I THOUGHT THAT AS WE WOULD BE IN IMC CONDITIONS FOR ONLY 10-15 MINS WHILE ON APCH; AND WITH THE BRIEFINGS AND PIREPS I'D RECEIVED; THERE WOULD BE NO PROB. I WAS WRONG. IT TOOK ONLY 1-2 MINS FOR THE ICE TO CREATE A SERIOUS DRAG AND HANDLING PROB. WE HAD FLOWN THROUGH SUPER-COOLED WATER AND NOW KNOW JUST HOW DANGEROUS SIT CAN BE TO ACFT CERTIFIED FOR NON-ICE. I'VE LEARNED A VALUABLE LESSON. IT TAKES THE RIGHT TOOL TO DO A JOB; AND I DID NOT HAVE THE RIGHT TOOL. I BUSTED AN ALT RESTR BECAUSE I FLEW INTO ICING CONDITIONS. NEVER AGAIN.