A PA34 PLT ATTEMPTS AN ILS APCH WITH A GYRO THAT IS MALFUNCTIONING AND LETS HIMSELF GET 130 FT BELOW MINIMUMS BEFORE GOING MISSED.

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SUX RWY 21 RPTING 1800 FT RVR. I HAD A DIRECTIONAL GYRO WHICH WAS PRECESSING OFF HDG EXCESSIVELY; BUT I OPTED TO ATTEMPT THE APCH ANYWAY. I KEPT MOVING THE HDG BUG R; TRYING TO CORRECT FOR INST ERROR. IT TOOK ALL MY CONCENTRATION TO STAY ON LOC AND GLIDE. I GOT TO 1630 FT WHICH IS MSL MINIMUMS AND LOOKED OUT. ALL I SAW WAS FOG. I SUDDENLY SAW GND CONTACT TO MY L AND TURNED MY HEAD TO LOOK. I SAW AN ACR B757 WAITING TO DEPART. I LOOKED AHEAD AND THE ALTIMETER SAID 1500 FT; 130 FT UNDER MINIMUMS WITH NOTHING VISIBLE AHEAD. I PULLED UP HARD; APPLIED FULL PWR; AND CLBED OUT. I MISSED THE GND BY PERHAPS 70 FT. I ATTRIBUTE THIS NEAR ACCIDENT TO 3 FACTORS: 1) I WAS PREOCCUPIED WITH AN INST PROB; 2) I DID NOT IMMEDIATELY EXECUTE A MISSED; AND 3) HAVING DONE 4 NEAR MINIMUMS ILS'S IN THE LAST MONTH; I HAD EVERY EXPECTATION THAT I WOULD BE SUCCESSFUL. I FORGOT TO RAISE THE GEAR UNTIL 2000 FT AGL AND DIDN'T GET RID OF APCH FLAPS UNTIL ENRTE TO MY VFR ALTERNATE. I RPTED TO APCH CTL THAT MY DIRECTIONAL GYRO WAS SHOT AND THEY LET ME DIVERT TO PIR WHICH WAS VFR AT THE TIME. I WILL NEVER AGAIN BE SO COMPLACENT ABOUT FOG AND I WILL NOT ATTEMPT IT WITH DEFECTIVE EQUIP. AND I WILL EXPECT TO GO MISSED.