Date: 202009
Local time of day: 1801-2400
Location: ZZZ.TRACON, US
Flight conditions: VMC
Weather elements / visibility: Haze / Smoke
Light: Daylight
Aircraft: RV-12
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Transponder
Component problem: Improperly Operated
Reporter organization: Personal
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Commercial; Flight Crew Multiengine
Human factors: Troubleshooting; Human-Machine Interface
Second reporter function: Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural FAR; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: I was riding along with an owner in his experimental aircraft. The weather conditions were VFR; however; visibility was somewhat reduced due to likely smoke/haze and light scatter in the area. Departed normally from ZZZ and headed northwest bound. Listened to ATIS and contacted Approach control and indicated our intentions for continued northwest bound flight for their awareness. We were not planning to fly near ZZZ1 airport; but the airspace is very busy within approximately 30 nm of the field; so participating in traffic advisories is good practice. Typically; TRACON would provide a squawk code and RADAR identification; however; this evening approach control responded to our request and stated 'aircraft calling in; maintain VFR'. As the airspace was very busy with multiple VFR and IFR aircraft during this time; we took this response from ATC as 'we're not interested in another VFR aircraft' particularly since we were not directly arriving into or departing from ZZZ1 airport itself. Despite this response from ATC; we continued to monitor ZZZ1 approach frequency on 132.3.  We leveled-off at 4;500 ft. MSL continued flying northwest bound. Approximately 10-15 mins later; about 5-8 nm south of ZZZ2 airport when we noticed potential conflict traffic on the aircraft's MFD and through the Foreflight App with ADS-B in. Using technology; we noticed the aircraft was flying directly towards us and climbing into our altitude; with altitude difference noted as 500; 400; 300; etc. below and difference decreasing. We attempted to establish visual contact with the aircraft; cross-referencing the iPad display and MFD. The aircraft was noted crossing under our right wing; approximately 300-400 ft. before we noticed the aircraft. Although we were monitoring Approach; it was unclear what instructions the other aircraft was provided regarding our position. In either case; the other aircraft was closer than I typically prefer; without prior coordination. Later on that same flight; we contacted Center to request flight following for the return leg through the ZZZ1 airspace. Center provided us a squawk code; then a few minutes later stated 'Aircraft X; I seem to have lost your altitude squawk; state your altitude.' As we were now returning and southeast bound; I responded with 'Aircraft X; level; five-thousand five hundred.' After receiving this call from ATC; I queried the aircraft owner to see if the transponder 'ALT' reporting was selected on through his MFD. Upon checking; it appeared the transponder was 'ON' but had inadvertently been selected out of the 'ALT' squawk mode. It is possible this inadvertent selection could have contributed to the earlier potential traffic hazard witnessed earlier in the flight. I discussed the setting with the owner and indicated that the transponder should be left in the 'ALT' setting continuously (unless otherwise requested by ATC). We continued uneventfully for the remainder of the flight; despite very active ATC communications; active vectoring and traffic avoidance while returning in and through the airspace into ZZZ. The vectoring; traffic avoidance and unexpected headwinds upon the return slightly delayed our arrival into ZZZ; however; we landed uneventfully.


Second narrative: On way to Walhalla area from ZZZ to look at fall colors noticed an aircraft from UND Aerospace on a general track towards us at about 1000 ft below our altitude. I believe they were also changing course somewhat also. We monitored as the distance closed. We were on autopilot at 4500 ft MSL and on course of 320. As the other plane closed to 1-2 mi. they began to climb. They seemed to level off about 500 ft. below us but just before our paths crossed they climbed to an estimated 300 ft. below us. My plane is equipped with ADS-B in and out. I was accompanied by my son-in-law who is a CFI. Later in the flight he noticed that the transponder was set to ON not ALT. Because of this I am not sure if the other aircraft was receiving information on our altitude. In hind sight we should have altered our course to avoid crossing like this. I believe this was an instance where having two pilots was a factor in not taking action. In the past when flying with my wife; I have taken action to maneuver around another aircraft in a similar situation. On The other hand; based on the maneuvering the other aircraft was doing it was not clear which direction we would need to go to avoid crossing paths. A lesson learned for me is to check that my transponder is set to ALT on future flights.


Synopsis: Sport/Recreation Pilots reported their transponder was not in the ALT position during the flight and this probably contributed to the NMAC.