The pilot/owner had completed the assembly of his home built airplane and had confirmed that the airplane was airworthy, before taxiing for takeoff. A witness at the sod airstrip observed the pilot's takeoff and climb to about 500 feet. The pilot made shallow turns and returned back over the airstrip, and was last seen flying in a wings level attitude west of the airstrip. The witness reported hearing a cracking sound and then saw the left wing and left door separate from the airframe. The airplane crashed in an adjacent field about 300 yards from the sod airstrip. Examination of the airframe and fractures failed to disclose a material failure. However, according to the building plans, the spar tongue which attaches the wing to the airframe, was not properly constructed with a plywood plate on the aft side of the tongue. The examination further confirmed that the wing fittings and bolts were secure. A review of the airframe maintenance log failed to disclose a current periodic inspection. The Federal Aviation Administration's