On December 15, 1993, approximately 1600 mountain standard time, N360HP, an Ebershoff Steen Skybolt, sustained substantial damage in flight near Erie, Colorado. The pilot was not injured. Visual meteorological conditions prevailed for the test flight. The following is based on the pilot/operator report and a subsequent interview. Maintenance had recently been performed on the engine and the pilot was test flying the highly modified aerobatic airplane prior to it being returned to service. He lowered the nose 45 degrees below the horizon and accelerated to 190 MPH. Immediately, the flight controls began to flutter. He reduced power, raised the nose, and slowed the airplane. The pilot flew back to Jeffco Airport in Broomfield, Colorado, and landed uneventfully. Post-accident examination revealed a fractured lower right aft wing spar, torn fabric on the lower left aileron, and a broken actuator. According to a University of Denver metallurgist, the actuator "had buckled in compression overload."