Narrative:

Flying into lvk for a full stop landing from the southwest; I was assigned a runway to land on and given traffic to follow. Finding what I believed to be the correct traffic to follow I proceeded to enter a left downwind for 25L; then discovered additional traffic on final approach to follow. I extended my downwind slightly then turned left base behind the traffic to follow. On base; I noticed an aircraft below and immediately to my left. This aircraft had evidently just passed beneath me; right to left; on its final approach for 25R. I had erroneously extended my base leg past the final approach for 25L; into the approach path for 25R. I slowed down and tried to maintain altitude; thinking perhaps another aircraft had entered the final for the same runway I was supposed to land on; or I had again failed to correctly spot the traffic to follow. Tower requested my intentions. Not wanting to again end up on top of (and unable to keep an eye on) the traffic that had just passed underneath me; I requested a right 360; away from that aircraft. Tower called back but gave no instruction. Concerned about the consequences of extending my already overextended base leg further (seeing hills ahead) I announced I would be executing a right 360 to return to final. Tower permitted me my right 360 into a final for 25L and then assigned me number 1 for 25L. I landed 25L.a few false perceptions on my part contributed to this traffic conflict. 1) my first instruction from tower was to enter a left base for 25R; which within the same radio call was immediately corrected to 25L (the runway closer to me). It's possible; however; that this initial instruction; though promptly corrected; set a (false) subconscious expectation on my end that I would be going for the farther runway. 2) I misjudged my base turn; likely because the right runway is so much more prominent when approaching the airport from the east as one does on final for the 25 runways (25L is significantly shorter). 3) the aircraft I initially misidentified as my traffic to follow was likely on final for the parallel; which contributed to my confusion and an unstable approach.it would likely have been more prudent; upon discovering I had overflown the final approach for the parallel runway; to ask tower for permission to enter a right downwind for 25R instead; or to simply continue the extended base and wait for instruction. I executed the maneuver I deemed necessary for safety; however; in hindsight; I based this decision on incomplete information. The perceived danger of the hills ahead was likely inflated as there is ample room to the north of the runway for the downwind leg; but any action in this situation could have set me up for another traffic conflict depending on where in the pattern other landing aircraft were.

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Original NASA ASRS Text

Title: PA-28 pilot reported an NMAC in the pattern at LVK airport.

Narrative: Flying into LVK for a full stop landing from the southwest; I was assigned a runway to land on and given traffic to follow. Finding what I believed to be the correct traffic to follow I proceeded to enter a left downwind for 25L; then discovered additional traffic on final approach to follow. I extended my downwind slightly then turned left base behind the traffic to follow. On base; I noticed an aircraft below and immediately to my left. This aircraft had evidently just passed beneath me; right to left; on its final approach for 25R. I had erroneously extended my base leg past the final approach for 25L; into the approach path for 25R. I slowed down and tried to maintain altitude; thinking perhaps another aircraft had entered the final for the same runway I was supposed to land on; or I had again failed to correctly spot the traffic to follow. Tower requested my intentions. Not wanting to again end up on top of (and unable to keep an eye on) the traffic that had just passed underneath me; I requested a right 360; away from that aircraft. Tower called back but gave no instruction. Concerned about the consequences of extending my already overextended base leg further (seeing hills ahead) I announced I would be executing a right 360 to return to final. Tower permitted me my right 360 into a final for 25L and then assigned me Number 1 for 25L. I landed 25L.A few false perceptions on my part contributed to this traffic conflict. 1) My first instruction from Tower was to enter a left base for 25R; which within the same radio call was immediately corrected to 25L (the runway closer to me). It's possible; however; that this initial instruction; though promptly corrected; set a (false) subconscious expectation on my end that I would be going for the farther runway. 2) I misjudged my base turn; likely because the right runway is so much more prominent when approaching the airport from the east as one does on final for the 25 runways (25L is significantly shorter). 3) The aircraft I initially misidentified as my traffic to follow was likely on final for the parallel; which contributed to my confusion and an unstable approach.It would likely have been more prudent; upon discovering I had overflown the final approach for the parallel runway; to ask Tower for permission to enter a right downwind for 25R instead; or to simply continue the extended base and wait for instruction. I executed the maneuver I deemed necessary for safety; however; in hindsight; I based this decision on incomplete information. The perceived danger of the hills ahead was likely inflated as there is ample room to the north of the runway for the downwind leg; but any action in this situation could have set me up for another traffic conflict depending on where in the pattern other landing aircraft were.

Data retrieved from NASA's ASRS site 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.