In a Safety Recommendation released Apr. 14, the National Transportation Safety Board (NTSB) "has investigated a series of in-flight structural breakups of Zodiac CH-601XL airplanes designed by Zenair, Inc...in the United States in the last 3 years. The Safety Board is also aware of several in-flight structural breakups of CH-601XLs that have occurred abroad. It appears that aerodynamic flutter is the likely source of four of the U.S. accidents and of at least two foreign accidents. The Safety Board believes urgent action is needed by the Federal Aviation Administration (FAA) to prevent additional in-flight breakups. Two of the accident airplanes were classified as Special Light Sport Aircraft (S-LSA)."
In six of the breakups, NTSB noted 10 people were killed.
Aerodynamic flutter occurs when aerodynamic and structural forces interact and lead to unsafe structural vibration in the airplane. Left undamped, the vibrations can quickly lead to structural failure.
NTSB's urgent recommendation to the FAA is to prohibit further flight of the CH-601XL until it can determine the airplane is no longer susceptible to aerodynamic flutter. The NTSB asserts its investigations of the US accidents point to a problem with the design of the flight control system, which makes the airplane susceptible to flutter.
Said NTSB Acting Chairman Mark V. Rosenker. "...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 report went on to specify stick force and airspeed indicator anomalies.
Also notable in the report was NTSB's expression of concern for the US light-sport aircraft industry.
"The Board also identified several areas in which the design standards for light sport airplanes were deficient."
Instead of the FAA-mandated certification typical for general aviation aircraft, FAA accepts ASTM International standards, developed by industry working groups as certification that LSA designs are safe to fly.
NTSB has asked the ASTM to take the following actions:
1. Add requirements to reduce the potential for aerodynamic flutter to develop
2. develop standards on stick force characteristics for light sport airplanes that minimize the possibility of pilot’s inadvertently over-controlling the airplane
3. ensure standards for LSA for accurate airspeed indications and appropriate documentation in new airplane pilot operating handbooks.
Expect a buzz saw of comments and activity around this surprising development in the weeks ahead.
-photo courtesy Zenith Aircraft Co.
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