Narrative:

Descended to MDA of 1900 MSL prematurely. I was being vectored to a localizer 16 approach into bgm on completion of an IFR flight from franklin. WX reported to be marginal VFR, ceiling 500 broken, visibility 6. Instruments for the approach set as follows: VOR #1 ith radial 135. VOR #2 i-aaj, localizer 16. Cleared to descend to 3600, heading 130. Pick up localizer, cleared for the approach 16. Told to contact tower, did so as localizer centered, commenced descent to 3100 as published. Tower cleared me to land. VOR #1 centered up, I commenced a rapid descent to published MDA of 1900. Established ground contact but remained in and out of cloud bases. Tower reported loss of my transponder, then urgently demanded my altitude. The urgency scared me, I replied 'MDA' then '1900' alarmed but not knowing what was amiss, controller stated something about 'aurey' (int/OM) but with all that was going through my mind then, I don't remember what. After landing controller asked why I had descended to 1900 before aurey. I replied I thought I was inside aurey, scared me, glad to be here. (Aircraft is IFR current. VOR's check within limits.) lessons: I should have defined aurey with bgm R401. The more oblique angle relative to the final approach to 16 of ith R135, plus greater distance, makes it inherently less accurate. In the future I will avoid defining points with navaids that are distant and subtend a small angle (less than 45 degree) if at all possible, or request another approach. I should have missed the approach once the error was discovered. Controller might have directed me to miss, as I was confused, knew something was wrong, but not quite sure what. In any case I should have executed a miss on my own accord once I thought something wrong immediately, sort out the problem later. In the future I will do exactly that.

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Original NASA ASRS Text

Title: GA SMA DESCENDED TO MDA BEFORE REACHING THE IAF.

Narrative: DESCENDED TO MDA OF 1900 MSL PREMATURELY. I WAS BEING VECTORED TO A LOC 16 APCH INTO BGM ON COMPLETION OF AN IFR FLT FROM FRANKLIN. WX REPORTED TO BE MARGINAL VFR, CEILING 500 BROKEN, VISIBILITY 6. INSTRUMENTS FOR THE APCH SET AS FOLLOWS: VOR #1 ITH RADIAL 135. VOR #2 I-AAJ, LOC 16. CLRED TO DSND TO 3600, HDG 130. PICK UP LOC, CLRED FOR THE APCH 16. TOLD TO CONTACT TWR, DID SO AS LOC CENTERED, COMMENCED DSCNT TO 3100 AS PUBLISHED. TWR CLRED ME TO LAND. VOR #1 CENTERED UP, I COMMENCED A RAPID DSCNT TO PUBLISHED MDA OF 1900. ESTABLISHED GND CONTACT BUT REMAINED IN AND OUT OF CLOUD BASES. TWR REPORTED LOSS OF MY XPONDER, THEN URGENTLY DEMANDED MY ALT. THE URGENCY SCARED ME, I REPLIED 'MDA' THEN '1900' ALARMED BUT NOT KNOWING WHAT WAS AMISS, CTLR STATED SOMETHING ABOUT 'AUREY' (INT/OM) BUT WITH ALL THAT WAS GOING THROUGH MY MIND THEN, I DON'T REMEMBER WHAT. AFTER LNDG CTLR ASKED WHY I HAD DESCENDED TO 1900 BEFORE AUREY. I REPLIED I THOUGHT I WAS INSIDE AUREY, SCARED ME, GLAD TO BE HERE. (ACFT IS IFR CURRENT. VOR'S CHECK WITHIN LIMITS.) LESSONS: I SHOULD HAVE DEFINED AUREY WITH BGM R401. THE MORE OBLIQUE ANGLE RELATIVE TO THE FINAL APCH TO 16 OF ITH R135, PLUS GREATER DISTANCE, MAKES IT INHERENTLY LESS ACCURATE. IN THE FUTURE I WILL AVOID DEFINING POINTS WITH NAVAIDS THAT ARE DISTANT AND SUBTEND A SMALL ANGLE (LESS THAN 45 DEG) IF AT ALL POSSIBLE, OR REQUEST ANOTHER APCH. I SHOULD HAVE MISSED THE APCH ONCE THE ERROR WAS DISCOVERED. CTLR MIGHT HAVE DIRECTED ME TO MISS, AS I WAS CONFUSED, KNEW SOMETHING WAS WRONG, BUT NOT QUITE SURE WHAT. IN ANY CASE I SHOULD HAVE EXECUTED A MISS ON MY OWN ACCORD ONCE I THOUGHT SOMETHING WRONG IMMEDIATELY, SORT OUT THE PROBLEM LATER. IN THE FUTURE I WILL DO EXACTLY THAT.

Data retrieved from NASA's ASRS site as of August 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.