RV-6
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 10; Flight Crew Total 1000; Flight Crew Type 500
Situational Awareness
Conflict NMAC
General None Reported / Taken
Airspace Structure; Human Factors
[I] departed approximately 1 minute before other aircraft; both going to DCU. Climbed to 4;500 MSL to get out of low altitude turbulence but had head wind. From talk with other pilot after event he stayed low to avoid head wind. Turned on autopilot direct DCU. At approximately 15 miles out from DCU I started powered descent (still with autopilot on) to get under HSV Class C shelf. [I] tuned to DCU Unicom [and] called location as 15 miles northeast DCU. Heard call from other aircraft; one being subject aircraft. He reported 14 miles out from the northeast. Started looking for the aircraft but did not see him. From previous flights similar to this one; an aircraft can be many miles apart while being the same distance to the northeast. Next call was from subject airplane 'You just passed over the top of me'. Both aircraft landed. He reported that I passed over 100-200 FT directly overhead.Problem:1. Both aircraft on GPS track autopilot from the same airport to the same airport. 2. Did not realize that I could have an aircraft under the nose that would stay hidden due to #1. above. 3. Failure to communicate altitude information between aircraft to provide better situational awareness. 4. Class C shelf over airport contributed as it funnels aircraft low at a distance.To prevent a recurrence: 1. Offset GPS track when other aircraft are going the same direction. 2. Communicate altitude in position report. 3. Communicate with ATC as they would have seen the other aircraft as an intersecting target. ([I] did not as ATC is not always the most cooperative with experimental aircraft. 'Experimental XXX remain clear of Class C airspace').
Experimental pilot reports a NMAC during descent; with another Vans aircraft that had departed one minute earlier with the same destination.