C152 CFI PERFORMS A GAR AT DWH TO AVOID ACFT ON FINAL AHEAD OF HIS ACFT. ACFT Y WAS APCHING THE WRONG RWY.

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ON A ROUTINE TRAINING FLT I APCHED DWH FROM THE W AND WAS TOLD TO ENTER A R BASE FOR RWY 17R. ANOTHER CESSNA WAS IN THE PATTERN FOR RWY 17R AND WHEN WE WERE APPROX 2 1/2 MI OUT THEY WERE INSTRUCTED TO DO TOUCH AND GO ON RWY 17L. I LOST VISUAL CONTACT WITH THE OTHER ACFT AGAINST GND CLUTTER AS THEY WERE LOWER WHEN WE WERE ENTERING R BASE. AS WE TURNED FINAL I SAW THE OTHER ACFT LOWER AND IMMEDIATELY AHEAD SLIGHTLY R OF COURSE FOR RWY 17R. I IMMEDIATELY QUERIED THE TWR 'VERIFY (US) CLRED TOUCH AND GO RWY 17R AND ACFT AHEAD CLRED RWY 17L.' TO THAT POINT I HAD NOT BEEN ISSUED AN ADVISORY OR A CLRNC TO LAND EVEN THOUGH I WAS RADAR IDENTED. TWR AGAIN INSTRUCTED THE OTHER ACFT TO MAKE TFC FOR RWY 17L BUT I PERCEIVED THEM TO BECOME STATIONARY AND LARGER AS THEY ADDED FLAPS; SO I IMMEDIATELY CLBED TO THE R AND CALLED TWR AS A GAR. THEY CLRED US FOR 360 DEG TURN TO FINAL TOUCH AND GO RWY 17R. I BELIEVE THE PROB WAS CAUSED BY SEVERAL FACTORS: 1) CASUAL MONITORING BY TWR (NO ADVISORY OF TFC TO THE PARALLEL RWY). 2) INSTRUCTION TO OTHER ACFT TO SWITCH RWY WAS NOT ACKNOWLEDGED CORRECTLY THE FIRST TIME; AND AN ACKNOWLEDGMENT WAS NOT RESOLICITED BY THE TWR. 3) POOR RWY ALIGNMENT OF OTHER ACFT. 4) INADEQUATE VISUAL SCANNING TECHNIQUES OF MYSELF AND OTHER PLT. 5) I WAS INSTRUCTED TO 'ENTER R BASE' BUT NOT TO 'RPT ENTERING' WHICH LED ME TO EXPECT THEY WERE MORE CLOSELY MONITORING OUR POS BY RADAR OR VISUAL CONTACT. TO CORRECT THE PROB: 1) PLTS SHOULD BE INSTRUCTED TO 'RPT ENTERING' TO AID ATC AS WELL AS OTHER PLTS IN THE PATTERN WITH ORIENTATION. 2) PARALLEL APCHS SHOULD BE CLOSELY MONITORED AND TIMELY ADVISORIES ISSUED. 3) AT LEAST A STRONG WORD SHOULD BE SHARED WITH PLTS WHO WANDER INTO ADJACENT FINAL APCH COURSES (I HAVE NO WAY OF KNOWING IF THE OTHER PLT WAS LATER TALKED WITH ON GND FREQ OR TELEPHONE).