NMAC BTWN A PIPER PA28 TURNING FINAL FOR SJC RWY 29; AND A SUPER MD80 ON FINAL TO RWY 30L. THE PA28 HAD OVERSHOT RWY 29; WHICH WAS ONLY 500 FT FROM RWY 30L.

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I WAS MIDWAY THROUGH A TURN FROM L BASE TO FINAL FOR RWY 29 AT SAN JOSE INTL. I WAS CLRED TO LAND. I WAS AT APPROX 800 FT AND AN MD80 FLEW DIRECTLY UNDER MY ACFT; WITH ABOUT 60-70 FT OF VERT SEPARATION. I DID NOT SEE THE MD80 UNTIL IT WAS BELOW AND TO MY L AS I MADE MY TURN TO FINAL. IT FIRST APPEARED ON A LINE WHICH BISECTED THE ANGLE CREATED BTWN THE LEADING EDGE OF MY L WING AND THE 'POST' AT THE FORWARD EDGE OF MY L FRONT WINDOW. BACKGROUND: I WAS THE SOLE OCCUPANT OF MY ACFT. I WAS ON A VFR FLT. DEPARTED PALO ALTO WITH HALF MOON BAY AS AN INITIAL DEST; THEN TO SAN JOSE INTL FOR SOME NIGHT LNDGS. UPON CLBING TO 4500 FT AND ON THE W SIDE OF THE PACIFIC COAST RANGE; I COULD SEE HALF MOON BAY WAS NOT VFR; SO I TURNED SE; CLBED TO 5500 FT UNTIL ABOUT 15 MI SW OF SAN JOSE. WHILE LEVEL AT 5500 FT I GOT SAN JOSE'S ATIS; AND CONTACTED BAY APCH WITH A REQUEST INBOUND FULL STOP AT SAN JOSE INTL. I ADVISED APCH THAT I WAS UNFAMILIAR WITH THE ARPT. HE HAD ME DSND TO 3000 FT. WHEN I WAS AT ABOUT 3300 FT HE HAD ME DSND TO 2000 FT. SAN JOSE TWR DESCRIBED AN AIMING POINT OF 'THE PRUNEYARD' AND I WAS CLRED TO LAND RWY 29 AND ADVISED ME OF AN MD80 ON AN 8 MI FINAL. I DIDN'T CLRLY HEAR WHICH RWY AND ASKED 'SAY AGAIN' I HAD CLRLY HEARD THE TA; AND RESPONDED 'NO JOY.' SAN JOSE TWR CONFIRMED RWY 29. I ALSO INDICATED I WASN'T FAMILIAR WITH THE 'PRUNEYARD.' TWR THEN ADVISED ME TO LOOK FOR A SINGLE LARGE BUILDING TO THE L OF THE DOWNTOWN GROUP OF LARGE BUILDINGS. TWR ADVISED THE MD80 OF MY ACFT AT 10 O'CLOCK POS; 1 1/2 MI; TO WHICH HE RESPONDED; 'WE'RE LOOKING.' TWR FURTHER ADVISED ME OF A DEPARTING B757 CAUSING WAKE TURB; WIND 300 DEGS AT 6 KTS. TWR FURTHER ADVISED THE MD80 'YOUR TFC IS POS AND HOLD ON RWY 30.' TO THE BEST OF MY KNOWLEDGE; THAT WAS THE LAST XMISSION PRIOR TO THE MD80 PASSING UNDERNEATH MY ACFT.