C172 PLT AND A PA28 HAD LESS THAN LEGAL SEPARATION IN PAO CLASS D.

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MY FIRST APCH TO LNDG ON RWY 31 AT PAO ENDED IN A GAR BECAUSE I WAS FOLLOWING THE AIRPLANE AHEAD TOO CLOSELY AND IT HAD NOT CLRED THE RWY BY THE TIME I WAS ON SHORT FINAL. I MADE R XWIND AND DOWNWIND TURNS; THE RECOMMENDED PATTERN FOR RWY 31. ABEAM THE NUMBERS ON DOWNWIND; I RECEIVED THIS CLRNC: 'CLRED TO LAND RWY 31; #2 FOLLOWING THE CHEROKEE OVER THE AMPHITHEATER.' IN REALITY; MY PLANE WAS #3. THERE WAS A LOW-WING PLANE ON 1/2 TO 1 MI FINAL (#1) AND THE CHEROKEE ON ABOUT 2 MI FINAL (#2). I ASSUMED THAT ACFT #1 WAS THE CHEROKEE MENTIONED IN MY LNDG CLRNC; AND I DID NOT KNOW THAT THERE WAS ANOTHER PLANE OUT THERE (#2; THE CHEROKEE). ABOUT 20 SECONDS AFTER GETTING MY LNDG CLRNC; I PASSED ABEAM ACFT #1. I ASSUMED THAT IT WAS THE CHEROKEE; SO I TURNED R BASE AND WAS ABOUT TO TURN FINAL WHEN THE TWR CTLR BARKED AT ME 'ACFT X; YOU'RE CUTTING OFF YOUR LNDG TFC; GET BACK TO DOWNWIND!' THE CHEROKEE (#2) WAS NOW AT MY 11 O'CLOCK POS; ABOUT 100 FT ABOVE ME; 500 FT AWAY. HAD I CONTINUED MY PATTERN; WE MIGHT HAVE COLLIDED. I TURNED L TO ALLOW THE CHEROKEE TO PASS; THEN TURNED BACK TO FINAL WHEN AUTH BY TWR CTLR. THAT AUTH ACTUALLY CAME TOO SOON; AS I WAS STILL TOO CLOSE BEHIND THE CHEROKEE. TWR AUTH A 360 DEG; WHICH I EXECUTED AT FINAL APCH SPD WITH FULL FLAPS AT 600 FT AGL. THEN I LANDED WITHOUT FURTHER EXCITEMENT. THE TWR CTLR COMMITTED THESE ERRORS: 1) CLRING ME AS #2 TO LAND; RATHER THAN #3. OTHER PLTS HAVE SINCE TOLD ME THAT THE CTLRS AT THIS ARPT FREQUENTLY OMIT ACFT ON SHORT FINAL IN COUNTING UP THE LNDG ACFT FOR LNDG CLRNCS. I SEE NO REASON WHY THEY SHOULD DO THIS. TWR CTLRS SHOULD COUNT UP ALL ACFT THAT HAVE NOT YET LANDED. 2) USE OF THE 'AMPHITHEATER' AS A LANDMARK; NOT SHOWN ON THE VFR TERMINAL AREA CHART. THIS ARPT IS NOT MY HOME BASE; SO WHEN I SPOTTED ACFT #1 ON SHORT FINAL; I DIDN'T FIGURE THAT I NEEDED TO ASK FOR A CLARIFICATION 'UNFAMILIAR AMPHITHEATER.' IT SEEMED LIKE A MOOT POINT. IF I HAD KNOWN THAT THE AMPHITHEATER IS ACTUALLY 2 MI OUT; I WOULD HAVE BEEN MORE DILIGENT IN SEARCHING FOR THE CHEROKEE. I HAD STUDIED THE TAC PRIOR TO THIS FLT (THERE IS VERY COMPLICATED AIRSPACE HERE; WITH 3 CLASS D'S UNDERNEATH A SHELF OF CLASS B ALL WITHIN ABOUT 8 MI). IT DOES NOT SHOW THE 'AMPHITHEATER.' I COMMITTED THESE ERRORS: 1) FAILURE TO 'SEE AND AVOID' THE CHEROKEE UNTIL DANGEROUSLY CLOSE; A CONSEQUENCE OF FAILING TO VISUALLY 'CLR' THE FINAL APCH PATH WHILE ON BASE. I WILL BE MORE DILIGENT ABOUT THIS IN FUTURE. 2) TURNING BASE AND FINAL TOO SOON AFTER THE CHEROKEE HAD PASSED. EVEN THOUGH CLRED BY TWR; IT IS ULTIMATELY THE PLT'S RESPONSIBILITY TO DECIDE WHEN ADEQUATE SEPARATION HAS BEEN ACHIEVED. 3) EXECUTING THE 360 DEGS IN SLOW FLT WITH MARGINAL ALT. HAD I MADE AN ERROR; THERE WOULD HAVE BEEN INADEQUATE ALT TO RECOVER FROM A STALL/SPIN. RECOMMENDATIONS FOR PREVENTING SIMILAR INCIDENTS IN THE FUTURE: 1) TWR CTLRS SHOULD COUNT ALL ACFT IN THE AIR WHEN ISSUING #2; #3; ETC; LNDG CLRNCS. 2) TWR CTLRS SHOULD NOT USE THE 'AMPHITHEATER' AS A VFR LANDMARK UNTIL IT IS PUT ON THE CHART. 3) PUT THE 'AMPHITHEATER' ON THE SAN FRANCISCO VFR TERMINAL AREA CHART. IT IS AN EASILY-RECOGNIZED BUILDING ABOUT 2 MI FROM THE APCH END OF RWY 31 AT PAO. IF THE TWR IS GOING TO USE IT AS A LANDMARK; THEN LET PLTS KNOW WHAT IT LOOKS LIKE BY PUTTING IT ON THE CHART. IT WOULD FIT VERY NICELY ON THE INSET ON THE BACK OF THE TAC SHOWING ALL THE OTHER LANDMARKS AROUND PAO.