Narrative:

Aircraft X reported humpty inbound on ILS/DME 21 approach and was instructed to report 4.3 DME. Small aircraft Y reported 2 mi left base and was instructed to turn to the left downwind to follow aircraft X approaching a 3 mi final. Small aircraft Y acknowledged the instruction. I noticed small aircraft Y continuing on the base leg, I again issued the instructions to turn right and enter the left downwind. Small aircraft Y still did not follow instructions. It was no longer feasible for small aircraft Y to turn right because of this position in relation to the final approach course. I then instructed small aircraft Y to turn towards the runway. Small aircraft Y began to turn right and aircraft X descended rapidly to avoid small aircraft Y. Due to the volume of traffic and student pilots at prescott it is paramount that instructors insure the student complies with instructions when they are first issued.

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Original NASA ASRS Text

Title: SMA Y FAILED TO FOLLOW ATC INSTRUCTIONS. NMAC. PLT DEVIATION.

Narrative: ACFT X REPORTED HUMPTY INBND ON ILS/DME 21 APCH AND WAS INSTRUCTED TO REPORT 4.3 DME. SMA Y REPORTED 2 MI LEFT BASE AND WAS INSTRUCTED TO TURN TO THE LEFT DOWNWIND TO FOLLOW ACFT X APCHING A 3 MI FINAL. SMA Y ACKNOWLEDGED THE INSTRUCTION. I NOTICED SMA Y CONTINUING ON THE BASE LEG, I AGAIN ISSUED THE INSTRUCTIONS TO TURN RIGHT AND ENTER THE LEFT DOWNWIND. SMA Y STILL DID NOT FOLLOW INSTRUCTIONS. IT WAS NO LONGER FEASIBLE FOR SMA Y TO TURN RIGHT BECAUSE OF THIS POSITION IN RELATION TO THE FINAL APCH COURSE. I THEN INSTRUCTED SMA Y TO TURN TOWARDS THE RWY. SMA Y BEGAN TO TURN RIGHT AND ACFT X DESCENDED RAPIDLY TO AVOID SMA Y. DUE TO THE VOLUME OF TFC AND STUDENT PLTS AT PRESCOTT IT IS PARAMOUNT THAT INSTRUCTORS INSURE THE STUDENT COMPLIES WITH INSTRUCTIONS WHEN THEY ARE FIRST ISSUED.

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.