Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Last 90 Days 50; Flight Crew Total 3500; Flight Crew Type 500
Situational Awareness
Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action
Human Factors
Private pilot instrument student was flying. I was instructing from right seat. Always scanning visually for traffic as well as the ADS-B display; while we were northbound I noticed a target on ADS-B (Automatic Dependent Surveillance - Broadcast)  less than a mile behind us at -300 feet (we were at 3;100 MSL). Target was gaining on us and climbing toward our altitude. When target showed -100 feet and nearly merged with us on the ADS-B display (approaching from our 7:00 position); student initiated a climbing right turn to the east since it appeared the target aircraft had no idea we existed. Thankfully; that maneuver started to put some distance as well as altitude between us. However; the target started turning right as well. We continued climbing and turned further right (net 180 by then); while it appeared target was still at their previous altitude (now -700 thanks to our climb) and maintaining a roughly easterly heading but still in a right turn. After we continued our right turn (now heading west); I for the first time caught a visual on the other aircraft at roughly our 4:00 position and 700 feet below (where ADS-B said they were at that moment). They appeared to be still turning right; perhaps practicing 360s -- I don't know. In any case; we were continuing to put distance and altitude between us such that we resumed our northerly heading back to home airport without further incident other than perhaps a minute during which our altitude was 3;800 MSL -- not a legal VFR cruising altitude -- while we made sure the other aircraft was not trying to chase us. We then descended to 3;000 MSL and contacted Portland Approach for flight following for the assurance of an extra set of eyes.
Flight Instructor reported a NMAC in cruise near SLE airport.