Narrative:

I was instructing a student on takeoffs and lndgs in a cessna 170B (high wing) on runway 8 at boulder municipal airport. We had just departed runway 8 on the upwind leg when we heard a bonanza (low wing) report 'entering the pattern on the crosswind leg at 7700 ft MSL and descending for landing on runway 8.' traffic pattern altitude is 6100 ft MSL at this airport. As an aviation safety counselor, I radioed that it is not advisable to descend in the traffic pattern. The bonanza pilot said he would leave the pattern for descent and return to downwind. Feeling the problem was solved I turned my attention to my student who continued flying a standard l-hand traffic pattern. When my student and I turned downwind we reported our position 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 report 'entering the pattern at midfield downwind.' this got my attention and I looked around and spotted the bonanza turning at 300 ft or less directly above our cessna 170. I reported our close proximity, but the bonanza pilot said he had 'a visual on us.' I again restated my concern about his desire to descend in the traffic pattern and he stated he would perform a '360' for spacing. Here we had the classic low wing aircraft descending on a high wing aircraft in the traffic pattern which could have resulted in a tragic mid-air collision had we not seen each other or had we not been paying attention to the radio. The readers should note that traffic pattern altitude should be attained well before entering the airspace in the vicinity of any airport. Also, the outcome of this event was uneventful because both pilots were monitoring the CTAF radio frequency and communicating with each other.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 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.