NMAC IN TFC PATTERN.

Narrative

*3 ACFT WERE RPTING TO TWR FOR ENTRY TO RWY 17L. 2 WERE TO CALL ABEAM TWR; L DOWNWIND. THE THIRD WAS ON A PRACTICE INST APCH TO 35R AND TOLD TO RPT A MISSED APCH AND TO THEN JOIN IN DOWNWIND AND RPT ABEAM. RPTR; ACFT Y; ASSUMED THIRD ACFT WAS WELL TO REAR AND CONCENTRATED ON SPOTTING #1 ACFT X AHEAD. #3 ACFT Z WAS SEEN WHEN HE PULLED ACROSS AND ABOVE BY 100 FT JOINING THE DOWNWIND. ACFT Z WAS A HIGH WING SMA. ACFT Y (RPTR'S) WAS A LOW WING. RPTR FEELS THAT HIS LOW WING OBSTRUCTED VISION ON ACFT Z. RPTR CRITICIZES TWR FOR NOT USING BETTER ATC PRACTICES. TWR DID NOT DO A GOOD JOB OF COORDINATING THE TFC ENTERING ITS ATA. SHE HAD 3 PLANES ALL CONVERGING TO THE SAME SPOT AND DID NOT RECOGNIZE THE POTENTIAL FOR CONFLICT; NOR VISUALLY IDENTING THE 3 PLANES; ALTHOUGH IT SHOULD HAVE BEEN EASY TO DO SO. I SUGGEST THAT TWR NOT DIRECT 2 PLANES TO THE SAME SPOT IN THE PATTERN WITHOUT VISUAL CONTACT. IT WOULD BE BEST TO REQUEST RPTS FROM ALL PLTS PRIOR TO ENTERING THE PATTERN; SO EACH WILL KNOW THE OTHER'S POS. TWR NEEDS TO BE ALERT AND HAVE VISUAL CONTACT WITH ARRIVING ACFT TO PREVENT COLLISION.