Date: 201901
Local time of day: 1201-1800
Location: C83.Airport, CA
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Skyhawk 172/Cutlass 172
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Initial Approach
Airspace: Class E NCT
ATC / advisory: CTAF C83
Second aircraft: Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Reporter organization: Personal
Reporter function: Instructor
Reporter qualification: Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Commercial
Human factors: Communication Breakdown
Second reporter function: Single Pilot; Pilot Flying
Anomaly: Conflict NMAC
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Airport
Primary problem: Human Factors

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.


Second narrative: I was not aware there was a problem until contacted 5 days later I was landing at Byron airport CA (C83). As usual I announced 45 degree to 30 followed by downwind call;base call;final call and clear of runway 30 to taxi. I did hear and monitor other aircraft in the vicinity however no one announced they were in a downwind for 30. I was not informed on the radio before or after said incident nor did anyone inform me at anytime on the radio. I have not been told were he was relative to me so I don't know what the right of way criteria was. Basically the problem I see was a lack of communication on the radio in the traffic pattern on that day. I know I gave often and clear position calls all the way in. Also the lack of information afforded me is another issue as I can't understand the circumstances that I was unaware of.


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