NMAC BTWN A C170 ON THE DOWNWIND LEG AND A BEECH BONANZA ENTERING THE TFC PATTERN AT AN UNCTLED; NON TWR ARPT.

Narrative

I WAS INSTRUCTING A STUDENT ON TKOFS AND LNDGS IN A CESSNA 170B (HIGH WING) ON RWY 8 AT BOULDER MUNICIPAL ARPT. WE HAD JUST DEPARTED RWY 8 ON THE UPWIND LEG WHEN WE HEARD A BONANZA (LOW WING) RPT 'ENTERING THE PATTERN ON THE XWIND LEG AT 7700 FT MSL AND DSNDING FOR LNDG ON RWY 8.' TFC PATTERN ALT IS 6100 FT MSL AT THIS ARPT. AS AN AVIATION SAFETY COUNSELOR; I RADIOED THAT IT IS NOT ADVISABLE TO DSND IN THE TFC PATTERN. THE BONANZA PLT SAID HE WOULD LEAVE THE PATTERN FOR DSCNT AND RETURN TO DOWNWIND. FEELING THE PROB WAS SOLVED I TURNED MY ATTN TO MY STUDENT WHO CONTINUED FLYING A STANDARD L-HAND TFC PATTERN. WHEN MY STUDENT AND I TURNED DOWNWIND WE RPTED OUR POS VIA THE RADIO AND LEVELED OFF AT THE TPA OF 6100 FT MSL. AS WE CONTINUED OUR DOWNWIND; WE ARRIVED AT MIDFIELD DOWNWIND AND HEARD THE BONANZA RPT 'ENTERING THE PATTERN AT MIDFIELD DOWNWIND.' THIS GOT MY ATTN AND I LOOKED AROUND AND SPOTTED THE BONANZA TURNING AT 300 FT OR LESS DIRECTLY ABOVE OUR CESSNA 170. I RPTED OUR CLOSE PROX; BUT THE BONANZA PLT SAID HE HAD 'A VISUAL ON US.' I AGAIN RESTATED MY CONCERN ABOUT HIS DESIRE TO DSND IN THE TFC PATTERN AND HE STATED HE WOULD PERFORM A '360' FOR SPACING. HERE WE HAD THE CLASSIC LOW WING ACFT DSNDING ON A HIGH WING ACFT IN THE TFC PATTERN WHICH COULD HAVE RESULTED IN A TRAGIC MID-AIR COLLISION HAD WE NOT SEEN EACH OTHER OR HAD WE NOT BEEN PAYING ATTN TO THE RADIO. THE READERS SHOULD NOTE THAT TFC PATTERN ALT SHOULD BE ATTAINED WELL BEFORE ENTERING THE AIRSPACE IN THE VICINITY OF ANY ARPT. ALSO; THE OUTCOME OF THIS EVENT WAS UNEVENTFUL BECAUSE BOTH PLTS WERE MONITORING THE CTAF RADIO FREQ AND COMMUNICATING WITH EACH OTHER.