SAFETY ALERT GIVEN TO GA SMA PLT WHEN APCH CTLR NOTES ACFT IN PROX TO GND DURING IAP NDB APCH. ALTDEV ALT EXCURSION.

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INCIDENT OCCURRED AT THE END OF A XCOUNTRY FLT UNDER VFR. DEST 3B2. WENT INTO UNFORECAST AND UNRPTED IMC THE AFTERNOON OF THE INCIDENT. I WAS NOTIFIED OF THIS BY THE BOS APCH CTLR WHEN I CHKED IN FOR VFR FLT FOLLOWING. REQUESTED AND RECEIVED A POP-UP CLRNC FOR THE NDB-6 APCH TO 3B2; AFTER HOLDING VFR FOR ABOUT 15 MINS. THERE WAS NO URGENCY TO PROCEED; SINCE I HAD ARRIVED VICINITY 3B2 ON FLT PLANNED TIME OF 1:45; HAVING STARTED WITH FULL FUEL; CONSERVATIVELY ESTIMATED AS 5:00 ENDURANCE. HOWEVER; SINCE ALL RPTING STATIONS SURROUNDING 3B2 WERE STILL RPTING VFR AT THE TIME; I DEVELOPED THE EXPECTATION OF QUICK DSCNT THROUGH A THIN DECK; SPOTTING THE FIELD 3 MI OUT; AND A ROUTINE LNDG. WHILE ORBITING VFR; I REQUESTED AND RECEIVED PERMISSION TO CONTACT UNICOM. THE UNICOM WAS ANSWERED BY ANOTHER PLT; WHO GAVE ME 3B2 ALTIMETER OF 29.93; WINDS FAVORING RWY 6. ALTIMETER SETTINGS ARE THE KEY TO THIS INCIDENT. FOR THE NDB-6 APCH TO 3B2; THE MDA IS 520 FT MSL WITH 3B2 ALTIMETER; BUT 600 FT MSL WITH BOSTON ALTIMETER. WHEN CLRING AND VECTORING ME FOR THE APCH; THE CTLR GAVE ME 30.18 BOSTON ALTIMETER. I WROTE DOWN BOTH SETTINGS BUT REMAINED INITIALLY ON 30.18; NOT REALLY EXPECTING THE DIFFERENCE TO MATTER. AS IT TURNED OUT; 3B2 WAS WELL BELOW MINIMUMS WHEN I FLEW THE APCH. INSIDE THE FAF; WHEN DSCNT TO 600 FT MSL LEFT ME IN SOLID IMC; WITHOUT THINKING MUCH ABOUT IT; I RESET TO 29.93 AND DSNDED TO 520 FT MSL. ALMOST IMMEDIATELY THE CTLR GAVE ME A LOW ALT ALERT; AND I RESET TO 30.18 AND CLBED BACK TO 600 FT MSL. ABOUT 1 HR LATER; AFTER HAVING LANDED AND REFUELED AT OWD; I FILED IFR TO 3B2 AND FLEW THE SAME APCH. (AT OWD; I HAD CALLED 3B2 AND BEEN INFORMED THAT THE CEILING HAD RAISED TO 600 FT; OTHER ACFT HAD GOTTEN IN; AND AFSS WX UPDATE HAD ALL STATIONS STILL RPTING VFR.) THIS TIME; HOWEVER; THE LICENSED WX OBSERVER AT 3B2 ANSWERED UNICOM AND TOLD ME THAT THEIR ALTIMETER WAS OTS. INCIDENT ANALYSIS -- CONTRIBUTING FACTORS: GET HOMEITIS AND OPTIMISTIC EXPECTATION. I SHOULD HAVE (BUT DID NOT) BECOME SUSPICIOUS ABOUT THE DIFFERENCE IN THE ALTIMETER SETTINGS. ALTHOUGH THERE WAS NOT TIME TO QUERY; I SHOULD HAVE REMINDED MYSELF 'FROM HIGH TO LOW; LOOK OUT BELOW' AND STAYED AT 600 FT MSL ON BOSTON ALTIMETER. IN HINDSIGHT; THE FOLLOWING CALCULATION IS CHILLING: DIFFERENCE IN MDAS: 600-520 = 80 FT. DIFFERENCE IN SETTINGS: 30.18 - 29.93 = 0.25 X 100 FT/0.1 INCHES = 250 OVER 330 FT. THUS; I WAS PROBABLY DOWN TO 270 FT AGL FOR A BRIEF TIME. FORTUNATELY; THERE ARE NO OBSTACLES ON THE NDB-6 APCH PATH; BUT I COULD HAVE ENCOUNTERED ONE; IF I HAD STAYED DOWN THERE; NEAR THE FIELD OR STARTING THE MISS. LESSON LEARNED: ALWAYS PAY ATTN TO CONFLICTING ALTIMETER SETTINGS. ALWAYS USE THE HIGHER ONE IF THE LOWER ONE IS FROM A LESS RELIABLE SOURCE.