AN SMA PVT PLT LANDS AT THE WRONG ARPT AFTER HIS ACFT SUFFERS A PROB WITH ITS DIRECTIONAL GYROSCOPE NOT AGREEING WITH THE ACFT'S COMPASS.

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I WAS FLYING FROM PROVINCETOWN (PVC) TO NORTH CENTRAL (SFZ); HDG 265 DEGS; TO OVERFLY PYM AS CHKPOINT. AT THAT POINT I CHANGED HDG TO 285 DEGS FOR SFZ. I SET THE ADF FREQ IN THE ACFT TO 241 KH FOR THE SFZ NDB AND MY VOR TO PUTNAM VOR; 117.4. BOTH NEEDLES CTRED AS I FLEW 285 DEGS (IT SHOULD BE NOTED THAT I PERIODICALLY RESET DIRECTIONAL GYROSCOPE DURING THIS FLT). I MADE VISUAL CONTACT WITH TAN ON MY L AND SOON AFTERWARDS 1B9 ON MY R AS I HELD 285 DEGS. ALL WAS AS IT SHOULD BE. AS AN ADDITIONAL NAVAID I HAD A 2ND VOR TUNED TO 112.7; BOS; AND SET TO 240 DEGS. WHEN I INTERCEPTED THE BOS 240 DEG RADIAL I WOULD MAKE A L TURN TO 240 DEGS AND BE LINED UP FOR RWY 23 SFZ. AFTER PASSING 1B9; MY DIRECTIONAL GYROSCOPE AND COMPASS HAD DIFFERENT NUMBERS AND RESETTING DIRECTIONAL GYROSCOPE DID NOT HELP. WITH ALL THE VERT HAZE AND MY APPARENT NOW CLOSE PROX TO SFZ I TURNED L; CTRED THE CDI ON 240 DEGS AND FLEW TO WHAT I BELIEVED WAS SFZ. I SAW A 'CROSSED RWY' FIELD DIALED 122.7 (SFZ UNICOM) AND BROADCAST DOWNWIND; BASE; AND FINAL. MY DEFECTIVE DIRECTIONAL GYROSCOPE INDICATED MY RWY ALIGNMENT FOR RWY 23 SO ALL SEEMED FINE. I DID NOT HEAR MUCH ON 122.7 EXCEPT THAT RWY 23 WAS ACTIVE. LACK OF COM AT UNCTLED ARPTS IS NOT UNUSUAL. I LANDED; TAXIED BACK; AT THIS POINT I SAW A BLINKING GREEN LIGHT 'CLRED TO TAXI' -- OK. (THIS WAS FIRST HINT THAT THIS MIGHT NOT BE SFZ.) AS I TOOK OFF; WAITING FOR XWIND TFC; I NOTICED THE OTHER END OF RWY WAS RWY 17 NOT RWY 5 -- ONLY THEN WHEN IN THE AIR DID I REALIZE I HAD LANDED INADVERTENTLY AT OWD; APPARENTLY WHILE TRACKING 240 DEGS OUTBOUND DIRECT SFZ. FOLLOWED INBOUND TO SFZ. THIS WAS MY PRIMARY DIRECTION INDICATOR WITH DIRECTIONAL GYROSCOPE OUT AND NOT SURE OF COMPASS.