The Curtiss was approaching Louisville Airport (SDF) runway 24 when it ballooned slightly during the flare out. Power was applied and the aircraft entered a steep climb. The angle of attack continued to increase until the aircraft stalled at an altitude of about 300 feet. The plane struck the ground with the nose and left wing, bursting into flames. PROBABLE CAUSE: “Structural failure of the left elevator in flight, causing loss of control. The structural failure was brought about by the left outboard hinge bolt backing out of the assembly. The underlying cause was improper maintenance which resulted in the installation of hinge bolts and bearings not meeting specifications, and inadequate inspection which failed to detect this condition.”