Narrative:

We were on an IFR flight from monterey to san francisco, ca. Southeast of the woodside VOR, bay approach had cleared us for a visual approach to runway 28L at sfo with the restriction to cross abeam the san carlos airport above 3000'. Approximately over the osi VOR a red and white small aircraft came into my field of view from under the small transport nose and moving from right to left. It appeared the small aircraft was practicing stalls as it was in a nose high attitude and in a right turn, but it did not appear to be climbing. I did not take evasive action because, a) the high rate of overtake would take us past the small aircraft before any correction would take effect, and B) even though we came to within 200' vertical and horizontal, it was apparent by the relative motion of both aircraft that we would not collide. When I mentioned to approach that we had the traffic in sight, he said he didn't see anyone in our vicinity. Moments later he questioned me about which traffic I was referring to. Contributing factors: my descending below the latitude limits of the TCA once being cleared for the visual approach. I did not realize this until I took a closer look at the sfo TCA chart a few mins ago, but I will certainly pay closer attention to this on future visual approachs. The failure of either the ATC radar to detect the small aircraft, or the failure of the controller to see the traffic in his scope. The fact that the small aircraft was practicing its airwork so close to the TCA and over such a widely used NAVAID.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR SMT HAD NAMC WITH SMA OPERATING BELOW THE FLOOR OF THE TCA WHEN ACR DESCENDED BELOW THE TCA.

Narrative: WE WERE ON AN IFR FLT FROM MONTEREY TO SAN FRANCISCO, CA. SE OF THE WOODSIDE VOR, BAY APCH HAD CLRED US FOR A VISUAL APCH TO RWY 28L AT SFO WITH THE RESTRICTION TO CROSS ABEAM THE SAN CARLOS ARPT ABOVE 3000'. APPROX OVER THE OSI VOR A RED AND WHITE SMA CAME INTO MY FIELD OF VIEW FROM UNDER THE SMT NOSE AND MOVING FROM RIGHT TO LEFT. IT APPEARED THE SMA WAS PRACTICING STALLS AS IT WAS IN A NOSE HIGH ATTITUDE AND IN A RIGHT TURN, BUT IT DID NOT APPEAR TO BE CLBING. I DID NOT TAKE EVASIVE ACTION BECAUSE, A) THE HIGH RATE OF OVERTAKE WOULD TAKE US PAST THE SMA BEFORE ANY CORRECTION WOULD TAKE EFFECT, AND B) EVEN THOUGH WE CAME TO WITHIN 200' VERT AND HORIZ, IT WAS APPARENT BY THE RELATIVE MOTION OF BOTH ACFT THAT WE WOULD NOT COLLIDE. WHEN I MENTIONED TO APCH THAT WE HAD THE TFC IN SIGHT, HE SAID HE DIDN'T SEE ANYONE IN OUR VICINITY. MOMENTS LATER HE QUESTIONED ME ABOUT WHICH TFC I WAS REFERRING TO. CONTRIBUTING FACTORS: MY DSNDING BELOW THE LAT LIMITS OF THE TCA ONCE BEING CLRED FOR THE VISUAL APCH. I DID NOT REALIZE THIS UNTIL I TOOK A CLOSER LOOK AT THE SFO TCA CHART A FEW MINS AGO, BUT I WILL CERTAINLY PAY CLOSER ATTN TO THIS ON FUTURE VISUAL APCHS. THE FAILURE OF EITHER THE ATC RADAR TO DETECT THE SMA, OR THE FAILURE OF THE CTLR TO SEE THE TFC IN HIS SCOPE. THE FACT THAT THE SMA WAS PRACTICING ITS AIRWORK SO CLOSE TO THE TCA AND OVER SUCH A WIDELY USED NAVAID.

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