Extra 200/300 Series
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 14; Flight Crew Total 1269; Flight Crew Type 850
Communication Breakdown; Situational Awareness
Conflict NMAC
Flight Crew Took Evasive Action
Environment - Non Weather Related; Human Factors
I was beginning an aerobatic practice (solo) in an area just offshore of the beach near [Location].  This is an area which meets all requirements for aerobatics; is generally safe; and generates no noise complaints.  A flight school at ZZZ sometimes trains in this area. First; I cleared the area visually; the cleared again by checking for any aircraft identified on my GPS display (connected to MODE S transponder); and finally by performing a 270 degree clearing turn.  Satisfied that the area was clear; I positioned over my box center and began an approximately 40 degree dive from 3;500 feet to 2;500 feet to start my aerobatic sequence. At this time; just as I was pulling level; I saw [an aircraft] headed almost straight at me; several hundred feet away. I pulled a hard right climbing turn; and easily avoided any collision risk.  The other aircraft also began a turn to their right; and left the area.To reduce the risk of a future event; I did some homework. I determined it was from a ZZZ flight school.  I called the flight school to see if they use a coordination frequency (they do; XXX.YY) so that I can better clear the area in the future; and announce intentions.  I also made the instructor aware that aerobatic activity is common in a very small area that they may wish to avoid (or use heightened vigilance).In reviewing the trajectory data I note that I was headed approximately southbound along the coast.  The other aircraft had been over land heading east; then turned to the north along the coast.  The other aircraft had been flying at around 1;000 ft over land when I cleared (I was at 3;500); and would have been impossible to see in the ground clutter.Factors:(1) Difficulty of seeing other aircraft in ground clutter.(2) Changes of altitude and heading by both aircraft.(3) High closing speed.(4) Because the other aircraft had been low while approaching; it was probably below ZZZ radar coverage and would not have appeared on my display as part of my SA.Plan for risk reduction:I hope that using the flight school's coordination frequency will significantly reduce surprises. (I typically remained on ZZZ1's frequency.)
GA pilot reported an NMAC in aerobatic practice area with an aircraft from a flight school. Pilot stated they would now use the school's frequency to maintain situational awareness.