What happened
During a certification flight intended to test flutter characteristics, a corporate jet was descending to reach a target speed of Mach 0.884. The aircraft, serving as a specialized test bed, was known to exhibit a right-wing roll tendency at high speeds due to deviations in wing and aileron twist. As the aircraft accelerated, the pilot utilized full left aileron to maintain stability. While completing a test point at a 30-degree right-wing-low attitude, the aircraft entered an uncontrollable right roll.
Over a period of 49 seconds, the aircraft descended from approximately 30,500 feet, completing roughly seven rolls before a near-vertical impact with the ground. Telemetry analysis indicated that just prior to the onset of the rolls, the elevator moved to a 3.5-degree trailing-edge-up position and the heading drifted right. Shortly thereafter, the rudder moved from 2 degrees trailing-edge-left to 6.5 degrees trailing-edge-left. The combination of these elevator and rudder inputs coincided with a significant increase in the rightward deviation. Although the pilot attempted to level the wings and raise the nose, the aircraft continued to diverge from stable flight and accelerated beyond its demonstrated diving speed. There is no definitive conclusion as to whether the pilot could have reduced speed quickly enough to prevent the divergence.
Findings
Post-accident high-speed wind tunnel testing revealed that lateral stability decreased as Mach number and angle of attack increased. Specifically, stability became negative above Mach 0.83. The investigation found that rudder inputs intended to assist with lateral trim could actually worsen the instability at certain speeds. Furthermore, trailing-edge-up elevator inputs increased the angle of attack, which further degraded lateral stability. At speeds exceeding Mach 0.86, roll authority was found to deteriorate, and by Mach 0.88, both the upper and lower surfaces of the ailerons were experiencing separated flow.