Narrative:

I was providing OJT on local control at the time of the event. Aircraft X entered the traffic pattern via left traffic and aircraft Y entered via mid-field left-downwind entry. The aircraft were on converging courses after a few minutes. To resolve the conflict; the trainee controller told aircraft Y to widen out to the left and pass behind aircraft X. While the developmental was providing control instructions; I looked and found both aircraft out the window. The instructions alleviated the conflict.several minutes later; aircraft X was opposite direction with an aircraft inbound on a GPS approach. The GPS approach brings aircraft into the pattern slightly offset south of final and often in the face of left-downwind traffic. The trainee controller told aircraft X to widen out their left-downwind and quoted traffic about the approaching aircraft. Aircraft Y who had been following aircraft X did not widen out their downwind and due to a speed reduction by aircraft X; ended up passing aircraft X.the two aircraft passing each other placed aircraft Y on the left side of aircraft X. As aircraft X attempted left base; they noticed the aircraft off their left wing and reported something about 'traffic blowing through the pattern.' I was distracted by the complexity of the situation and trying to assist the trainee controller to keep the operation flowing. I failed to notice that aircraft Y had passed or even encroached upon aircraft X because aircraft X's radar tag was covering up data tag for aircraft Y and the tag from the aircraft on the GPS approach was also interfering.the GPS approach causes unnecessary conflicts with left-downwind traffic due to the opposite direction nature of the two pattern legs. A thorough review of the GPS course and potentially re-writing the approach to bring aircraft 'straight-in' would provide great benefit and improve safety all around. Had the approaching aircraft on the GPS approach not been a factor for the two aircraft in left-downwind; the two aircraft would have continued to follow each other preventing a 'cut out' and near midair collision report.my recommendation is to revise the GPS approach to mimic a straight-in.

Related reports:

Google
 

Original NASA ASRS Text

Title: PAO Tower Controllers and a pilot reported an NMAC in the traffic pattern. The GPS Runway 31 approach was cited as a contributing factor.

Narrative: I was providing OJT on Local Control at the time of the event. Aircraft X entered the traffic pattern via left traffic and Aircraft Y entered via mid-field left-downwind entry. The aircraft were on converging courses after a few minutes. To resolve the conflict; the trainee controller told Aircraft Y to widen out to the left and pass behind Aircraft X. While the developmental was providing control instructions; I looked and found both aircraft out the window. The instructions alleviated the conflict.Several minutes later; Aircraft X was opposite direction with an aircraft inbound on a GPS Approach. The GPS Approach brings aircraft into the pattern slightly offset south of final and often in the face of left-downwind traffic. The trainee controller told Aircraft X to widen out their left-downwind and quoted traffic about the approaching aircraft. Aircraft Y who had been following Aircraft X did not widen out their downwind and due to a speed reduction by Aircraft X; ended up passing Aircraft X.The two aircraft passing each other placed Aircraft Y on the left side of Aircraft X. As Aircraft X attempted left base; they noticed the aircraft off their left wing and reported something about 'traffic blowing through the pattern.' I was distracted by the complexity of the situation and trying to assist the trainee controller to keep the operation flowing. I failed to notice that Aircraft Y had passed or even encroached upon Aircraft X because Aircraft X's RADAR tag was covering up data tag for Aircraft Y and the tag from the aircraft on the GPS approach was also interfering.The GPS approach causes unnecessary conflicts with left-downwind traffic due to the opposite direction nature of the two pattern legs. A thorough review of the GPS course and potentially re-writing the approach to bring aircraft 'straight-in' would provide great benefit and improve safety all around. Had the approaching aircraft on the GPS approach not been a factor for the two aircraft in left-downwind; the two aircraft would have continued to follow each other preventing a 'cut out' and NMAC report.My recommendation is to revise the GPS approach to mimic a straight-in.

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.