Narrative:

Incident occurred at the end of a cross country flight under VFR. Destination 3b2. Went into unforecast and unrpted IMC the afternoon of the incident. I was notified of this by the bos approach controller when I checked in for VFR flight following. Requested and received a pop-up clearance for the NDB-6 approach to 3b2, after holding VFR for about 15 mins. There was no urgency to proceed, since I had arrived vicinity 3b2 on flight planned time of 1:45, having started with full fuel, conservatively estimated as 5:00 endurance. However, since all reporting stations surrounding 3b2 were still reporting VFR at the time, I developed the expectation of quick descent through a thin deck, spotting the field 3 mi out, and a routine landing. While orbiting VFR, I requested and received permission to contact unicom. The unicom was answered by another pilot, who gave me 3b2 altimeter of 29.93, winds favoring runway 6. Altimeter settings are the key to this incident. For the NDB-6 approach to 3b2, the MDA is 520 ft MSL with 3b2 altimeter, but 600 ft MSL with boston altimeter. When clearing and vectoring me for the approach, the controller 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 approach. Inside the FAF, when descent to 600 ft MSL left me in solid IMC, without thinking much about it, I reset to 29.93 and descended to 520 ft MSL. Almost immediately the controller gave me a low altitude alert, and I reset to 30.18 and climbed back to 600 ft MSL. About 1 hour later, after having landed and refueled at owd, I filed IFR to 3b2 and flew the same approach. (At owd, I had called 3b2 and been informed that the ceiling had raised to 600 ft, other aircraft had gotten in, and AFSS WX update had all stations still reporting 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 approach path, but I could have encountered one, if I had stayed down there, near the field or starting the miss. Lesson learned: always pay attention to conflicting altimeter settings. Always use the higher one if the lower one is from a less reliable source.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.