Risk Control in Tiltrotor UAV Development

High risk exists in development of the unusual and unconventional aircraft, tiltrotor. Although extensive risk control efforts were exerted in development of the tiltrotor UAV, accidents and incidents were inevitable. Three accidents incurred crashes. The crashes occurred with low-cost small-scale vehicles but lessons learned were invaluable. Each crashes and incidents gave important lessons in development of the flight control system. Many lessons from previous flights were reflected in the flight control software development and enhanced safety in the full-scale flight. In the development of the small-scale tiltrotor UAV, special ground test methods were also developed to reduce the risks. The special test methods were applied to the full-scale tiltrotor UAV for risk reduction. The ground tests include ironbird test, 4-degree-of-freedom rig test and tethered test. Many faults were found during the special ground and tethered tests in advance, which prevented serious accidents otherwise they might have happened during flight test. This type of risk control contributed to rapid progress in conversion flight with full-scale tiltrotor UAV. The conversion flight from helicopter to airplane mode was accomplished within seven months after maiden flight.