SMA ON FINAL AT NON TWR ARPT HAS NMAC.

Narrative

DEPARTING PAO IN A C-152 ON A VFR FLT IN CLR AND UNLIMITED CEILING CONDITIONS TO HAF. MADE POS CALLS ON HAF CTAF AT 10 NM AND 5 NM S FROM HAF. ACFT ON CTAF RPTED RWY 30 IN USE. A NUMBER OF ACFT WERE IN THE PATTERN. I CALLED TURNING THE 45 FOR R TFC RWY 30 AND 1000 FT AND WAS FOLLOWING A CITABRIA (I WAS #3 BEHIND THE CITABRIA WHEN I ENTERED DOWNWIND. THE #1 ACFT; THE CITABRIA; THE CHEROKEE WELL BEHIND ME (HE WAS TURNING XWIND WHEN I WAS TURNING DOWNWIND FROM THE 45); AND I WERE ALL BROADCASTING DOWNWIND; BASE; FINAL; AND SHORT FINAL ON CTAF PER ROUTINE. SHORTLY AFTER I CALLED SHORT FINAL RWY 30 AND WHILE AT ABOUT 200 FT; THE CHEROKEE BEHIND ME BROADCAST '2 ACFT ON SHORT FINAL HALF MOON BAY. SOMEBODY DO SOMETHING!' I LOOKED L AND SAW A SECOND CHEROKEE JUST BEHIND MY L WINGTIP ABOUT 10 FT BELOW ME; AND LESS THAN 100 FT HORIZ DISTANCE FROM ME. I IMMEDIATELY INITIATED A GAR WITH A R TURN AND BROADCAST MY INTENTIONS TO TURN XWIND BACK INTO A R DOWNWIND FOR RWY 30. WHILE ON FINAL; MY ACFT AND THE CHEROKEE SHARING SHORT FINAL MUST HAVE BEEN ON A CONVERGING COURSE FOR THE RWY THRESHOLD. ON MY SECOND FINAL APCH; THE CHEROKEE THAT HAD CALLED THE CONFLICT FOR ME OFFERED TO HELP ME STRANGLE THE OTHER CHEROKEE PLT WHO MUST HAVE MADE EITHER A STRAIGHT-IN OR L APCH FOR RWY 30 AND NEVER BROADCAST HIS POS OR INTENTIONS AND MUST NOT HAVE BEEN MONITORING CTAF. HE NEVER INITIATED EVASIVE ACTION AND I SUSPECT NEVER SAW MY ACFT AND PROBABLY EVEN TODAY DOES NOT REALIZE THAT THERE WAS EVER A PROB ON THIS 'ROUTINE' APCH TO HAF. OBVIOUSLY NEITHER THE PLT IN THE CONFLICTING CHEROKEE NOR I SAW AND AVOIDED EACH OTHER UNTIL THE CHEROKEE BEHIND ME CALLED THE CONFLICT. I BELIEVE THAT I AM HERE TODAY BECAUSE THE PLT IN THE CHEROKEE BEHIND ME SAW THE SIT AND BROADCAST THE CONFLICT ON CTAF. THE JUDGEMENTS AND DECISIONS THAT ARE QUESTIONABLE IN THIS INCIDENT INCLUDE: THE PLT OF THE CONFLICTING CHEROKEE MAKING A NON-STANDARD APCH; HIS FAILURE TO MONITOR CTAF OR BROADCAST HIS POS OR INTENTIONS ON CTAF; AND HIS AND MY FAILURE TO SEE AND AVOID.