On June 26, 1996, at 1900 eastern daylight time, a Navion, NA1, N4096K, sustained substantial damage during landing when the left main landing gear collapsed during roll out. The airplane skidded off the runway and the left wing impacted a tree. The commercial pilot and one passenger reported no injuries. The 14 CFR Part 91 flight was returning to Skylane Airport, Evansville, Indiana, where it had departed on a local flight. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he was making a full stop at the grass airstrip. He reported that he touched down about 400 feet from the approach end of runway 36, indicating about 70 mph. He reported that he continued the rollout for 500 feet beyond the touchdown point when he heard a loud "thump" on the left side of the airplane, just after he had retracted the flaps. He reported that the left landing gear collapsed and the left wing contacted the grass strip. The airplane started veering to the left even though the pilot applied full right rudder and aileron. The pilot indicated that the airplane continued down the runway and turning to the left for about another 400 feet. The plane's left wing struck a tree which was 10 feet west of the 150 foot wide runway. The pilot reported that after the airplane impacted the tree, the nose gear and right gear collapsed, and the propeller struck the ground. The pilot reported that he turned off the fuel, electrical system, radios, and master switch, and evacuated the airplane. The aircraft and accident site were examined by Airworthiness and Operations Inspectors of the Federal Aviation Administration (FAA). The inspection revealed that the aircraft first touched down slightly right of centerline at about 1000 feet from the approach end of runway 36. The inspector reported that the track of the airplane indicated that the aircraft veered left heading straight to the point where it impacted the tree. About 400 to 450 feet after touchdown, the left flap made contact with the grass airstrip. A 250 to 300 foot gouge in the turf was made by the inboard section of the left flap. The inspector reported that the left wing struck the tree at the mid section of the wing. The nose gear sheared off. The flaps were in the up position. The left and right main gear were up, not extended. The gear handle was in the down position. The inspectors reported that when the aircraft was picked up the landing gear came down and locked. The inspectors examined the landing gear and no mechanical discrepancies were noted on the left or right main landing gear.