In an unusual move on Tuesday, the NTSB issued an "urgent safety recommendation" asking the FAA to prohibit further flight of the Zodiac CH-601XL, a light sport aircraft that has been involved in six in-flight structural breakups since 2006. The airplane is designed by Zenair. The board cited four accidents in the U.S. and two in Europe in which a CH-601XL broke up in-flight, killing a total of 10 people. According to the NTSB, there is a problem with the airplane design that makes it susceptible to aerodynamic flutter -- a phenomenon in which the control surfaces of the airplane can suddenly vibrate, and if unmitigated, can lead to catastrophic structural failure. The CH-601XL was certified as an LSA in 2005. The NTSB wants the fleet grounded until the FAA can determine that the problem has been solved. "The NTSB does not often recommend that all airplanes of a particular type be prohibited from further flight," said NTSB Acting Chairman Mark Rosenker. "In this case, we believe such action will save lives. Unless the safety issues with this particular Zodiac model are addressed, we are likely to see more accidents in which pilots and passengers are killed in airplanes that they believed were safe to fly."
The NTSB also found the stick force gradient was not uniform, and was lesser at high Gs, which could make the airplane susceptible to over-control by the pilot, which could lead to over-stressing the design limits and lead to in-flight structural failure. The board also made several requests of ASTM International, the entity that provides the design standards for light sport aircraft. NTSB says ASTM should add requirements to ensure the standards for LSAs reduce the potential for aerodynamic flutter to occur, develop standards on stick-force characteristics that minimize the possibility inadvertent over-controlling by the pilot, and ensure standards for LSAs result in accurate airspeed indications and appropriate documentation in new airplane pilot operating handbooks.