Date: 201906
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 15
Light: Daylight
Aircraft: RV-8
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Landing; Final Approach
Airspace: Class G ZZZ
ATC / advisory: CTAF ZZZ
Second aircraft: Mooney Aircraft Undifferentiated or Other Model
Reporter organization: Personal
Reporter function: Instructor
Reporter qualification: Flight Crew Sea; Flight Crew Rotorcraft; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Communication Breakdown
Anomaly: Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Taxi
Contributing factors: Human Factors; Aircraft
Primary problem: Human Factors

Narrative: While on a transition training flight; I was the instructor pilot; my student entered the traffic pattern at ZZZ and announced turning left downwind for Runway XX. Between that radio call and his next radio call; announcing that we were turning left base; the student inadvertently pressed (and held) the flip-flop button on the stick; which switched the radio to 121.5. This is an annoying feature built into this radio (Garmin). When my student announced on the radio that we were turning left base; someone notified us 'You're on GUARD'. Having briefed this possible situation; my student immediately flipped back to the CTAF freq. We then turned onto final for Runway XX. Neither me or my student; after the fact; can remember if we made a radio call announcing our turn to final. Student proceeded to fly a good approach to a good landing and I instructed him to exit at the taxiway to our left; about 1;000 feet ahead so we could reconfigure our radio frequency. As we slowed near the taxiway; I looked to our left and was startled to see a MOONEY; less than 15 feet off our left wing tip; rolling along with us; as if we had just landed in formation. No surprise; he wasn't happy with us; and we weren't happy with him. Apparently he never heard us on the radio; and we never heard him. I'm a relatively high time pilot and my student has even more experience than I do; and neither of us depend on the radio for traffic avoidance. Throughout 3 hours of training on this day; we constantly reminded each other of visual call outs; clear on final before we turned. Neither of us saw the Mooney; period. Neither of us feel good about that; as things turned out we both realized that we had screwed up; the Mooney had to have been either in front of us (on final while we were on base) or to our right (long final; or underneath us); were not sure. However we ended up in this situation; it seems that luck is the only reason we didn't collide. During a discussion in person; on the ground; the Mooney pilot was livid that we had flown right over him on final; nearly hitting him. Because neither me nor my student ever saw him; that may be true. Sometimes things just happen; even to good people doing everything right. We simply didn't see him. That was a mistake; and we own that. That said; I have never personally seen two airplanes land in formation by accident. I believe the gentleman in the Mooney intentionally landed next to us and I'm not very happy about that.




Synopsis: Pilot reported minor radio problem which was corrected; but upon landing; they landed in formation with another aircraft unknown to the reporter.