A C172 WITH A CFI AND CFI APPLICANT HAVE AN NMAC WITH A HELI WHILE IN MRI TWR'S AIRSPACE.

Narrative

WHILE IN THE MRI RWY 24 PATTERN; A HELI PASSED FROM R TO L DIRECTLY UNDER US. THE ESTIMATED ALT OF THE HELI WAS 600 FT MSL; OUR ESTIMATED ALT WAS 750 FT MSL AT THE TIME OF XING. I WAS IN THE L SEAT OBSERVING A CFI APPLICANT/STUDENT WHO WAS FLYING IN THE R SEAT. NO TFC CALLS WERE ISSUED BY TWR. THE HELI WAS LIKELY ARRIVING ON A GOLF COURSE ARR THAT CALLS FOR THAT FLT PATH AND ALT; WHICH CONFLICTS WITH THE STANDARD L TFC PATTERN FOR RWY 24. THE STUDENT DIDN'T SEE THE HELI AT ALL; AND I SAW IT ONLY AFTER IT HAD PASSED UNDER US AND WAS CLR TO THE W OF US. THE HELI IS PAINTED WITH A COLOR THAT MAKES IT DIFFICULT TO SEE IN THE CITY GND CLUTTER. I BELIEVE INCREASED HELI OPS AND THE DESIRE TO APPLY NOISE ABATEMENT POLICY AND PROCS HAVE CREATED AN UNSAFE OPERATING PRACTICE. SUPPLEMENTAL INFO FROM ACN 628456: WE WERE BOTH LOOKING IN ALL DIRECTIONS FOR OTHER TFC. HAD WE BEEN INFORMED THAT A HELI WAS GOING TO BE COMING FROM BEHIND US AND 100 FT BELOW US; WE COULD/WOULD HAVE EXTENDED OUR DOWNWIND AND LEVELED OFF.