Narrative:

My student and I were on an extended downwind to follow traffic over cherry creek reservoir on a straight in approach to 17L. I entered the base to follow traffic and aircraft Y was instructed to follow us. As we reached the 1 mile point on final approach; I saw aircraft Y fly right under us and intercept the final approach course. I immediately increased power and performed a go-around while contacting centennial tower to find out what happened. I was instructed to offset to the right of the runway (west of 17R) on my go-around. I complied and immediately requested a full-stop terminate on 28 in order to get on the ground as soon as possible. I asked tower for clarification on what happened and they informed me they were discussing it and would get back to me. After landing and shutdown I called the tower supervisor to get clarification on the chain of events and was informed that aircraft Y failed to follow assigned traffic; which was me; and cut me off on final approach. Aircraft Y pilot lost sight of me and only saw the airplane over the numbers which was the traffic I was sequenced behind. I believe the reason aircraft Y didn't see me was because we were on a collision course and there was no relative movement in his windscreen to alert him to my presence. Upon losing sight of me; aircraft Y should have immediately requested a traffic update from tower which would have prevented the near miss. Another factor that could have attributed to the near miss is the design of both aircraft. I was flying a high-wing [aircraft] and he was in a low-wing [aircraft]. That could have contributed to the visibility issues as I only saw him through my side window as he flew directly beneath me.

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Original NASA ASRS Text

Title: GA flight instructor reported a NMAC on final approach to APA when another aircraft turned base prematurely.

Narrative: My student and I were on an extended downwind to follow traffic over Cherry Creek Reservoir on a straight in approach to 17L. I entered the base to follow traffic and Aircraft Y was instructed to follow us. As we reached the 1 mile point on final approach; I saw Aircraft Y fly right under us and intercept the final approach course. I immediately increased power and performed a go-around while contacting Centennial Tower to find out what happened. I was instructed to offset to the right of the runway (west of 17R) on my go-around. I complied and immediately requested a full-stop terminate on 28 in order to get on the ground as soon as possible. I asked tower for clarification on what happened and they informed me they were discussing it and would get back to me. After landing and shutdown I called the tower supervisor to get clarification on the chain of events and was informed that Aircraft Y failed to follow assigned traffic; which was me; and cut me off on final approach. Aircraft Y pilot lost sight of me and only saw the airplane over the numbers which was the traffic I was sequenced behind. I believe the reason Aircraft Y didn't see me was because we were on a collision course and there was no relative movement in his windscreen to alert him to my presence. Upon losing sight of me; Aircraft Y should have immediately requested a traffic update from tower which would have prevented the near miss. Another factor that could have attributed to the near miss is the design of both aircraft. I was flying a high-wing [aircraft] and he was in a low-wing [aircraft]. That could have contributed to the visibility issues as I only saw him through my side window as he flew directly beneath me.

Data retrieved from NASA's ASRS site 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.