Narrative:

I experienced a near midair collision in the vicinity of byron airport (C83) in byron; california. I was operating a C172 in the right closed traffic pattern for runway 30 at byron. While in the right downwind abeam the 30 numbers; I initiated my descent for the approach to landing; where I discovered that another aircraft was actually directly underneath my aircraft (the other aircraft was also on downwind; but at an altitude approximately 200 feet below me and slightly ahead of me). I was operating at 1100 feet MSL. I promptly stopped my descent; while keeping the aircraft in sight; maneuvered away from the other aircraft to allow adequate separation before coming in for my landing. During the entire approach; I was able to keep the aircraft in sight. After the landing; I made a full stop and taxi back where I tried to get the full registration of the aircraft. My student told me the registration; but I personally could not confirm this. The aircraft was using call sign 'aircraft Y' on the CTAF; which I heard myself. The airplane appeared to be a low-wing; single engine propeller plane. The airplane was last seen taxiing to the hangar area at byron airport; where I elected not to follow them. I was at byron airport with a student practicing landings in the traffic pattern. We were making regular position reports on the CTAF of 123.05. At the time; there were a significant amount of air traffic in the vicinity of byron; including glider towing and parachute operations. The CTAF was noticeably congested; particularly with traffic calls from other airports. While I was on right downwind to runway 30; I was aware there was 1 other airplane in the traffic pattern ahead of me; because I had heard their radio calls and could visually see them. At some time; aircraft Y reported they were on a right base to runway 30. Almost immediately; another aircraft (whose tail number I cannot remember) responded on the CTAF by saying that they were also on right base to runway 30 and asked for aircraft Y's specific location. I cannot remember the response on the CTAF. At the same time; I did not see a second aircraft on the base leg. A few moments later; I made a radio call announcing that I was on right downwind for runway 30; and almost immediately aircraft Y responded by announcing that they were also on right downwind for runway 30. I was confused because I was sure I heard that aircraft call on base just a few moments ago. I pitched the nose down to initiate my descent; and visually saw aircraft Y barely in front of me; about 200 feet below me. I immediately took evasive action. I stopped the descent and navigated my airplane outside of aircraft Y's flight path to the runway. At the time of the incident; weather was VFR with generally good visibility and ceilings. I had obtained the minute weather at byron prior to entering the traffic pattern. Winds were calm. I believe the cause of the incident was aircraft Y's decision to operate at an altitude below the tpa of 1;000 ft. Above airport elevation. Additionally; I believe that aircraft Y's radio call that they were on base was not accurate and led to confusion about the aircraft's actual position.

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Original NASA ASRS Text

Title: Flight instructor and student reported a near NMAC while in the traffic pattern at Byron; non-towered Airport.

Narrative: I experienced a near midair collision in the vicinity of Byron Airport (C83) in Byron; California. I was operating a C172 in the right closed traffic pattern for runway 30 at Byron. While in the right downwind abeam the 30 numbers; I initiated my descent for the approach to landing; where I discovered that another aircraft was actually directly underneath my aircraft (the other aircraft was also on downwind; but at an altitude approximately 200 feet below me and slightly ahead of me). I was operating at 1100 feet MSL. I promptly stopped my descent; while keeping the aircraft in sight; maneuvered away from the other aircraft to allow adequate separation before coming in for my landing. During the entire approach; I was able to keep the aircraft in sight. After the landing; I made a full stop and taxi back where I tried to get the full registration of the aircraft. My student told me the registration; but I personally could not confirm this. The aircraft was using call sign 'Aircraft Y' on the CTAF; which I heard myself. The airplane appeared to be a low-wing; single engine propeller plane. The airplane was last seen taxiing to the hangar area at Byron airport; where I elected not to follow them. I was at Byron airport with a student practicing landings in the traffic pattern. We were making regular position reports on the CTAF of 123.05. At the time; there were a significant amount of air traffic in the vicinity of Byron; including glider towing and parachute operations. The CTAF was noticeably congested; particularly with traffic calls from other airports. While I was on right downwind to runway 30; I was aware there was 1 other airplane in the traffic pattern ahead of me; because I had heard their radio calls and could visually see them. At some time; Aircraft Y reported they were on a right base to runway 30. Almost immediately; another aircraft (whose tail number I cannot remember) responded on the CTAF by saying that they were also on right base to runway 30 and asked for Aircraft Y's specific location. I cannot remember the response on the CTAF. At the same time; I did not see a second aircraft on the base leg. A few moments later; I made a radio call announcing that I was on right downwind for runway 30; and almost immediately Aircraft Y responded by announcing that they were also on right downwind for runway 30. I was confused because I was sure I heard that aircraft call on base just a few moments ago. I pitched the nose down to initiate my descent; and visually saw Aircraft Y barely in front of me; about 200 feet below me. I immediately took evasive action. I stopped the descent and navigated my airplane outside of Aircraft Y's flight path to the runway. At the time of the incident; weather was VFR with generally good visibility and ceilings. I had obtained the minute weather at Byron prior to entering the traffic pattern. Winds were calm. I believe the cause of the incident was Aircraft Y's decision to operate at an altitude below the TPA of 1;000 ft. above airport elevation. Additionally; I believe that Aircraft Y's radio call that they were on base was not accurate and led to confusion about the aircraft's actual position.

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.