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Flutter issues according to NTSB report on CH-601XL 6 crashes with 10 fatalities

posted Apr 17, 2009, 1:56 AM by Naresh Sharma

NTSB Asks FAA To Immediately Ground Zodiac CH-601XL Aircraft

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, which has been involved in six in-flight structural breakups since 2006. 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 NTSB wants the U.S. 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 result in in-flight structural failure. The board also made several requests of ASTM International, the entity that provides the design standards for light sport aircraft. The 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. For more details, click here for the full text of the NTSB's safety recommendation letter (PDF) and click here for the NTSB letter to the ASTM (PDF).

FAA And Industry Respond To NTSB Zodiac Concerns

The FAA is already looking into concerns about all versions of the Zodiac CH-601XL aircraft, which were raised at an industry meeting back in February, FAA spokeswoman Laura J. Brown told AVweb on Tuesday, but she added that the agency has no immediate plans to call for the airplanes to be grounded. "The manufacturer already has told owners to check the aileron control cable tensions," she said. The FAA has formed a special review team with members from the FAA and the industry to investigate the problem. Brown added that the FAA has told the ASTM that it should conduct a review of its LSA standards regarding aerodynamic flutter. The CH-601XL airplane is sold in a kit version by Zenith Aircraft, which is run by Sebastian Heintz, and is also sold as an S-LSA by AMD (Aircraft Manufacturing & Design), which is run by Matthew Heintz. The CH-601XL was certified as an S-LSA in 2005. In the six accidents cited by the NTSB, two of the aircraft were experimental amateur-built (one in California and one in Utah), one in California was an S-LSA manufactured by AMD, and one in Florida was an S-LSA built by the Czech Aircraft Works. The other two crashes were in the Netherlands and in Spain, and it is not clear what version of the aircraft was involved.

On Wednesday, Zenith Aircraft posted a notice online stating that the company first learned of the NTSB's safety recommendation on Tuesday, when the press release was issued. "We continue to believe wing flutter will not occur if the control cables are adjusted properly," the notice reads. "Nonetheless, we are carefully considering the points raised in the memo, including whether the Zodiac CH-601XL is susceptible to wing flutter. Each accident discussed in the NTSB memo occurred under different circumstances. Some of the accidents are still being investigated and what caused those accidents has not been determined. Zenith Aircraft will communicate with the FAA about the issues raised in the NTSB memo. We will provide more information after we thoroughly consider the issues raised in the NTSB memo and we have spoken with the FAA about those issues." Matthew Heintz told EAA the company is in the process of evaluating the NTSB report, but expressed confidence in the aircraft's design. "We absolutely do have confidence in the aircraft ... If there is something wrong with the design, we want to fix it," he said. He added the NTSB's report on the California accident involving an AMD S-LSA did not cite aerodynamic flutter as a cause of the accident. Company-issued service bulletins have instructed owners to inspect all control cables and adjust as necessary so that they are within the prescribed parameters, EAA said.

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