Narrative:

Near miss on base/final runway 31 pao airport. Visibility was better than 6 mi, but there was low brown haze that was affecting visibility. Pattern was obviously busy, so I proceeded inbound slowly after receiving instructions 'right traffic runway 31, follow cessna.' I reported no traffic in sight, reply: 'over the bird house' (downwind 500 ft past runway 31 numbers). Still not in sight, slowed to minimum speed to avoid overtaking. Called again, reply: 'wide downwind over yacht harbor.' I did not spot my traffic over the abandoned harbor, but did spot a tan plane that appeared to be turning base over the currently operating marina less than 1 mi further downwind. 'Is my traffic turning base now?' 'affirmative, in sight.' somewhere after this call, there was a shift change at the tower. I began my base turn as the tan aircraft passed abeam on final. I noted that it was a low wing twin, but did not call tower to ask if they meant 'twin cessna.' as I turned onto final, there was a red and white high wing C172 maneuvering directly under me, perhaps less than 100 ft. I executed an emergency climb to avoid collision. The C172 had been on a wide downwind and I never spotted him. He must have turned back and flown under me as I was on downwind. A) I am a local pilot, however, even I got confused when I saw no traffic over te no-longer-present palo alto boat harbor, but did see traffic near the shoreline marina further downwind. This terminology might be amended to 'abandoned harbor' or 'old harbor' or replaced with a reference to something else (same thing, to a lesser extent with 'bird house'). B) my concentration on slow flight in the SR22 was absolutely a contributing distraction. 3) this could have been avoided if I had asked 'is it the tan aircraft turning right base over shoreline park harbor?' instead of 'is my traffic turning base now?' D) in an informal discussion with the controller, he said 'when you say you have the traffic in sight, we assume you've got it in sight.' true, but what traffic? If he had said red cessna it would have been clear. East) the shift change or general tower workload may have been a contributing factor, however I understand the responsibility for separation relies with me.

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Original NASA ASRS Text

Title: NMAC IN THE PAO TFC PATTERN ON A HAZY DAY WHEN TFC SIGHTED IS NOT THE TFC CALLED OUT.

Narrative: NEAR MISS ON BASE/FINAL RWY 31 PAO ARPT. VISIBILITY WAS BETTER THAN 6 MI, BUT THERE WAS LOW BROWN HAZE THAT WAS AFFECTING VISIBILITY. PATTERN WAS OBVIOUSLY BUSY, SO I PROCEEDED INBOUND SLOWLY AFTER RECEIVING INSTRUCTIONS 'R TFC RWY 31, FOLLOW CESSNA.' I RPTED NO TFC IN SIGHT, REPLY: 'OVER THE BIRD HOUSE' (DOWNWIND 500 FT PAST RWY 31 NUMBERS). STILL NOT IN SIGHT, SLOWED TO MINIMUM SPD TO AVOID OVERTAKING. CALLED AGAIN, REPLY: 'WIDE DOWNWIND OVER YACHT HARBOR.' I DID NOT SPOT MY TFC OVER THE ABANDONED HARBOR, BUT DID SPOT A TAN PLANE THAT APPEARED TO BE TURNING BASE OVER THE CURRENTLY OPERATING MARINA LESS THAN 1 MI FURTHER DOWNWIND. 'IS MY TFC TURNING BASE NOW?' 'AFFIRMATIVE, IN SIGHT.' SOMEWHERE AFTER THIS CALL, THERE WAS A SHIFT CHANGE AT THE TWR. I BEGAN MY BASE TURN AS THE TAN ACFT PASSED ABEAM ON FINAL. I NOTED THAT IT WAS A LOW WING TWIN, BUT DID NOT CALL TWR TO ASK IF THEY MEANT 'TWIN CESSNA.' AS I TURNED ONTO FINAL, THERE WAS A RED AND WHITE HIGH WING C172 MANEUVERING DIRECTLY UNDER ME, PERHAPS LESS THAN 100 FT. I EXECUTED AN EMER CLB TO AVOID COLLISION. THE C172 HAD BEEN ON A WIDE DOWNWIND AND I NEVER SPOTTED HIM. HE MUST HAVE TURNED BACK AND FLOWN UNDER ME AS I WAS ON DOWNWIND. A) I AM A LCL PLT, HOWEVER, EVEN I GOT CONFUSED WHEN I SAW NO TFC OVER TE NO-LONGER-PRESENT PALO ALTO BOAT HARBOR, BUT DID SEE TFC NEAR THE SHORELINE MARINA FURTHER DOWNWIND. THIS TERMINOLOGY MIGHT BE AMENDED TO 'ABANDONED HARBOR' OR 'OLD HARBOR' OR REPLACED WITH A REF TO SOMETHING ELSE (SAME THING, TO A LESSER EXTENT WITH 'BIRD HOUSE'). B) MY CONCENTRATION ON SLOW FLT IN THE SR22 WAS ABSOLUTELY A CONTRIBUTING DISTR. 3) THIS COULD HAVE BEEN AVOIDED IF I HAD ASKED 'IS IT THE TAN ACFT TURNING R BASE OVER SHORELINE PARK HARBOR?' INSTEAD OF 'IS MY TFC TURNING BASE NOW?' D) IN AN INFORMAL DISCUSSION WITH THE CTLR, HE SAID 'WHEN YOU SAY YOU HAVE THE TFC IN SIGHT, WE ASSUME YOU'VE GOT IT IN SIGHT.' TRUE, BUT WHAT TFC? IF HE HAD SAID RED CESSNA IT WOULD HAVE BEEN CLR. E) THE SHIFT CHANGE OR GENERAL TWR WORKLOAD MAY HAVE BEEN A CONTRIBUTING FACTOR, HOWEVER I UNDERSTAND THE RESPONSIBILITY FOR SEPARATION RELIES WITH ME.

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