M20P PLT IS VECTORED TO 79J ARPT IN IMC; MAKES 2 INCORRECT APCHS BEFORE DIVERTING TO ANOTHER ARPT AND THEN DETERMINES AN ALTIMETER SETTING ERROR.

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WHAT WAS TO BE A SIMPLE; SHORT VFR FLT TURNED OUT TO BE A PERILOUS APCH TO LOW CEILINGS; DURING WHICH I ENDANGERED MYSELF AND MY PAX. I HAD SPENT A COUPLE OF DAYS ON VACATION NEAR MONTGOMERY; AL. THE WX HAD BEEN GOOD THROUGHOUT THE WK EXCEPT FOR THE TYPICAL SPORADIC AFTERNOON AND EVENING TSTMS. I LOOKED AT THE WX ON THE WX CHANNEL; BUT DIDN'T GET A PREFLT WX BRIEFING PRIOR TO DEPARTING WILLOW POINT (AL71) APPROX XA15 IN BEAUTIFUL CLR WX. I HAD INTENDED TO DROP OFF A PAX AT MY HOME FIELD OF ANDALUSIA-OPP (79J) AND CONTINUE TO DESTIN; FL (DTS). EVERYTHING WAS FINE APPROX 30 MI N OF 79J WHEN I SAW A LAYER OF CLOUDS IN THE ANDALUSIA DIRECTION BELOW OUR LEVEL OF 2500 FT. I HAD RECEIVED FLT FOLLOWING FROM MGM AND HE HAD JUST CANCELED RADAR SVC. I CONTACTED CAIRNES APCH (THE FACILITY COVERING ANDALUSIA) AND REQUESTED AN IFR DSCNT EXPECTING A THIN LAYER OF CLOUDS. HE ISSUED A SQUAWK CODE AND TOLD ME TO EXPECT RADAR VECTORS FOR THE NDB APCH. SINCE I HAD IFR CERTIFIED GPS ON BOARD; I ASKED FOR THE GPS APCH; BUT HE WAS APPARENTLY BUSY WITH OTHER TFC AND DIDN'T HEAR ME. I DECIDED TO TAKE THE NDB APCH USING THE GPS SINCE I WAS FAMILIAR WITH THE APCH AT MY HOME FIELD (OR AT LEAST I THOUGHT I WAS). I CHKED THE ASOS AND WAS SURPRISED TO HEAR CEILING OF 300 FT AND 1 MI VISIBILITY IN FOG. AT THIS POINT; I BEGAN A SERIES OF ERRORS AND POOR DECISIONS WHICH COULD HAVE LED TO A TRAGIC OUTCOME. WHEN THE CTLR TURNED ME ONTO FINAL AND CLRED ME FOR THE APCH; I DON'T REMEMBER SETTING THE ALTIMETER OR CHKING IT. FOR SOME REASON; I HAD IN MY MIND THAT THE MINIMUMS FOR THE APCH WERE 800 FT (ACTUALLY; IT IS 800 FT AGL; OR 1150 FT). I DSNDED ON THE APCH TO 800 FT; BUT NEVER SAW THE RWY. AS I ALSO WAS SURPRISED THE TWR AT 79J WASN'T OPEN; SINCE IT USUALLY IS ON WKDAYS. ON THE MISSED APCH; I CALLED CAIRNES AND REQUESTED THE GPS APCH. IN MY EXPERIENCE; THE ASOS CAN BE QUITE UNRELIABLE CONCERNING CEILINGS AND I THOUGHT IT WAS WORTH ANOTHER TRY. THE CTLR ASSIGNED AN ALT AND CLRED ME TO THE FAF FOR THE GPS TO RWY 29. I TURNED ONTO THE FINAL APCH COURSE OUTSIDE THE FIX JUST AFTER THE CTLR CLRED ME FOR THE APCH. I REMEMBER AT THAT TIME CHKING THE ASOS AGAIN AND ADJUSTING MY ALTIMETER SETTING. I THEN DSNDED TO THE MINIMUM WHICH WAS 860 FT MSL. I FLEW A DISTANCE AT THAT ALT THEN THE FOG THINNED AND I SAW THE TREE TOPS. I REALIZED I WAS TOO LOW AND SHOULD HAVE GONE MISSED APCH AT THAT TIME; BUT EXPECTED TO SEE THE RWY ANY SECOND AND CONTINUED. THE FOG THICKENED AGAIN AND I LOST SIGHT OF THE GND AND EXECUTED THE MISSED APCH PROC. I CONTACTED APCH AND REQUESTED AN IFR CLRNC DIRECTLY TO DESTIN. I CHKED THE ASOS AGAIN AND DISCOVERED I HAD SET THE ALTIMETER INCORRECTLY; ENOUGH TO MAKE A 200-300 FT ALTIMETER ERROR. I KNEW I WAS FORTUNATE TO ESCAPE THAT APCH. IN SUMMARY; I FEEL THAT BY NOT GETTING A PREFLT WX BRIEFING; I DIDN'T PREPARE ADEQUATELY FOR THE FLT I UNDERTOOK. I HAD MADE THE SAME FLT SCORES OF TIME AND HAD NEVER ENCOUNTERED THE SIT PREVIOUSLY. I LET MY GUARD DOWN CONCERNING WX AND IT LED TO A TENSE SIT. WHEN I ENCOUNTERED THE UNEXPECTED LOW IFR CONDITIONS AT MY DEST; I JUST DIDN'T HANDLE IT WELL. I WAS WELL RESTED AND HEALTHY; BUT FOR SOME REASON MADE TECHNICAL ERRORS; PROCEDURAL ERRORS AND POOR JUDGEMENTS. I RELIED ON MY MEMORY INSTEAD OF REVIEWING THE APCH PLATE ON THE FIRST APCH; I SET THE ALTIMETER INCORRECTLY; I FAILED TO EXECUTE A MISSED APCH WHEN I DISCOVERED I WAS TOO LOW AND DIDN'T HAVE THE RWY IN SIGHT; AND OVERALL JUST BLEW THE WHOLE THING. LIKE SO MANY ACCIDENTS I READ ABOUT; THIS NEAR DISASTER WOULD HAVE BEEN AVERTED IF ANY OF SEVERAL THINGS HAD BEEN DIFFERENT. IF I HAD GOTTEN WX PRIOR TO THE FLT; I WOULD HAVE MADE DIFFERENT PLANS. IF I HAD LOOKED AT THE PLATE ON THE FIRST APCH OR SET THE ALTIMETER PROPERLY ON THE SECOND; I WOULD HAVE BEEN SAFELY ABOVE DANGER. IF THE TWR HAD BEEN OPEN; THEY COULD HAVE TOLD ME WHAT THE ACTUAL CONDITIONS WERE AND I WOULDN'T HAVE TRIED THE APCH. I DO KNOW THAT I HAVE LEARNED A LESSON FROM THIS EPISODE; SEVERAL IN FACT. IDOUBT I WILL EVER TAKE THE WX FOR GRANTED. I KNOW I WILL NEVER FAIL TO BE SURE THE ALTIMETER IS PROPERLY SET. I KNOW I WILL IMMEDIATELY EXECUTE A MISSED APCH IF ANYTHING IS AMISS AND WILL NEVER RELY ON MEMORY ONLY FOR DETAILS OF AN APCH; EVEN ONE AT MY HOME FIELD. I KNOW I WILL DILIGENTLY ATTEMPT TO PRACTICE INST PROCS MORE REGULARLY; WITH MY INSTRUCTOR OR IFR CERTIFIED CHK PLT.