Date: 201808
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 10
Light: Daylight
Aircraft: Small Aircraft
Operator: Personal
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: Small Aircraft; Low Wing; 1 Eng; Retractable Gear
Reporter organization: Personal
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Confusion; Situational Awareness
Anomaly: Conflict NMAC; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: I was on the last leg of a VFR flight in my Ercoupe 415-C; and was flying at 3;500 feet MSL direct to ZZZ; my destination. I had been monitoring ZZZ Approach on VHF frequency; but switched to ZZZ1 ATIS to check the local weather. Since ZZZ1's winds were gusting to 16 knots (stronger than forecast); I switched to ZZZ AWOS to check my arrival weather. I then also set ZZZ CTAF as my preset (secondary) frequency.About this same time; when I was just a few miles past [departure airport]; I watched a low-wing; single-engine; retractable gear airplane come from the east and then turn north/northeast about two miles ahead of and two thousand feet or so above me. I assumed that it would outrun me; as my panel-mounted Garmin NAV system showed my ground speed to only be 115 MPH. I kept an eye on this traffic ahead of me; though; and was surprised - after just a few minutes - to watch it make an abrupt; 180 degree descending left turn; passing just west of my position. The plane's pilot obviously also saw me as he went by; as that's when the 'fun' began.Spotting me; apparently (from that pilot's perspective); meant 'game on;' because that's when he began to erratically and unpredictably maneuver all around me! I had absolutely no idea; of course; of what he was up; what he was trying to do; or what he was going to do next. I momentarily descended to 2;500 feet MSL; thinking that would at least keep him from flying below me where I wouldn't be able to see him. In looking at the Sectional Chart later; I realized that I may have very momentarily (10 seconds or less) descended very slightly below 500 feet AGL as I maneuvered to avoid this other airplane. It is also more than probable that I did not.I also made a left; 360 degree turn at one point when he was behind me just to be able to watch for him as he turned behind me. However; since I had no idea of his intentions; experience or skill level; I decided it would be better to return to my original course and just maintain a stable heading so as to not further complicate his closures or maneuvers. I was concerned that he might misjudge a turn or closure rate and fly right into me; which is a risk not unfamiliar to me since my formation hours number in the hundreds. This pilot's maneuvering around me continued for several minutes. At one point; he even passed just around 100 feet above and maybe 200 yards or so right in front of me. And then; for his finale; I looked over and watched him pull up and 'park' just off of my left wing! It was then; since my left Ercoupe window was already down; that I looked him in the face and used my right arm to 'swat' at him; as if to say; 'Get away from me.' He apparently got the message; because he then peeled off to the left and that was the last I saw of him. This bizarre incident was very disturbing. I could not tell if this pilot was just undisciplined or if he was intoxicated; suicidal...or just what. In any case; I certainly felt threatened; harassed; bullied; targeted and continually endangered for the 10 to 13 minutes or so that this event lasted (it seemed much longer at the time!). My safety was certainly compromised by this pilot's antics. I even remember thinking that; even though I was constantly scanning for him to avoid the threat of a possible mid-air collision; I also needed to be careful that I didn't fixate on him so much that I ended up inadvertently flying my airplane into the ground!In hindsight; I wish that I had still been on ZZZ Approach's frequency so that I could have reported all of this; but I wasn't; and I was absolutely too busy watching to potentially evade this other airplane to even think about looking down at my radio control head to reinsert their frequency. The same was true of the camera I had in my cockpit: I just couldn't risk the distraction that trying to use it might have created. I attempted to phone ZZZ Approach as soon as I landed in ZZZ; but I did not get an answer. However; I was able to connect with and have a good conversation with a ZZZ1 Tower Controller who took the situation seriously and said that he would make certain that the right people looked into this event. I also called the FSDO and reported the details to them the next morning.I am documenting this event because I know that 'rogue' pilots who don't follow the rules DO exist throughout our nation's aviation system and that they are a danger to many. I don't know just how serious the threat of a mid-air collision with my Ercoupe really was on this flight; but not knowing what this other pilot's intentions were or what he was trying to do; certainly made the collision/accident threat real.




Synopsis: Ercoupe pilot reported that during flight; another aircraft was intentionally coming close and harassing; causing a collision threat.