Narrative:

On left downwind to runway 17L. Adjacent to runway end (normal gear down point); observed an aircraft on the runway about to depart; therefore; extended the downwind slightly. With the departing aircraft in motion; I told my student to start his turn to base leg. Immediately; the wing dropped; an aircraft was observed converging at an angle I would estimate of 30 degrees both the student (flying) and I pitched up hard. The converging aircraft passing directly beneath our aircraft. On recovery; ATC was informed by me of a near miss. Note: it is my belief that had we extended undercarriage at the normal position and commenced a standard 500 FPM descent a collision would have been unavoidable. Supplemental information from acn 829731: I received clearance as a student pilot to fly VFR at or below 8;000 ft MSL to the east practice area. After preparing for takeoff in C172; I called the tower and was cleared to cross runway 17R and position and hold runway 12. Immediately after takeoff on runway 12; I was told to turn to a 080 degree heading; which I complied. The problem arose when I headed on the 080 degree heading into traffic pattern for runway 17L. Shortly after turning on the 080 degree heading; tower contacted me to maintain 6;500 ft MSL. I did not hear that first contact and did not reply; continuing to climb. I then received a second contact to maintain 6;500 ft MSL due to traffic on the downwind; but the tower did not state the runway number; so it was unclear to where the traffic was. I replied 'roger.' I really questioned if that could be possible because of the terrain in front of me but I made it my intent to descend. What contributed to the incident was that I did not maintain 6;500 ft MSL or contact tower to state 'unable.' the other aircraft (aircraft Y) had to take corrective action and pulled up in a climb to avoid an accident. Once I had the aircraft in sight; I descended to avoid a collision as well. To mitigate these events; I will make efforts to be more aware of the situation -- especially where training for both pilots and controllers is taking place.

Related reports:

Google
 

Original NASA ASRS Text

Title: Light twin on downwind for Runway 17L at COS with instructor and student have NMAC with student pilot departing Runway 12 heading 080 degrees as instructed by Tower. Call from Tower for student pilot to level at 6500 FT is late and goes unheard. Controller training was apparently also taking place.

Narrative: On left downwind to Runway 17L. Adjacent to runway end (normal gear down point); observed an aircraft on the runway about to depart; therefore; extended the downwind slightly. With the departing aircraft in motion; I told my student to start his turn to base leg. Immediately; the wing dropped; an aircraft was observed converging at an angle I would estimate of 30 degrees Both the student (flying) and I pitched up hard. The converging aircraft passing directly beneath our aircraft. On recovery; ATC was informed by me of a near miss. Note: It is my belief that had we extended undercarriage at the normal position and commenced a standard 500 FPM descent a collision would have been unavoidable. Supplemental information from ACN 829731: I received clearance as a student pilot to fly VFR at or below 8;000 FT MSL to the east practice area. After preparing for takeoff in C172; I called the Tower and was cleared to cross Runway 17R and position and hold Runway 12. Immediately after takeoff on Runway 12; I was told to turn to a 080 degree heading; which I complied. The problem arose when I headed on the 080 degree heading into traffic pattern for Runway 17L. Shortly after turning on the 080 degree heading; Tower contacted me to maintain 6;500 FT MSL. I did not hear that first contact and did not reply; continuing to climb. I then received a second contact to maintain 6;500 FT MSL due to traffic on the downwind; but the Tower did not state the runway number; so it was unclear to where the traffic was. I replied 'Roger.' I really questioned if that could be possible because of the terrain in front of me but I made it my intent to descend. What contributed to the incident was that I did not maintain 6;500 FT MSL or contact Tower to state 'Unable.' The other aircraft (Aircraft Y) had to take corrective action and pulled up in a climb to avoid an accident. Once I had the aircraft in sight; I descended to avoid a collision as well. To mitigate these events; I will make efforts to be more aware of the situation -- especially where training for both pilots and Controllers is taking place.

Data retrieved from NASA's ASRS site as of April 2012 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.