The National Transportation Safety Board issued an urgent safety recommendation on April 14, asking the Federal Aviation Administration to prohibit further flight of a Zodiac Special Light Sport Aircraft which has been involved in six in-flight structural breakups since 2006.
The recommendation applies to the Zodiac CH-601XL, a low-wing, fixed-gear, single- engine, two-seat general aviation airplane designed by Zenair, Inc. In its urgent safety recommendation, the board cited four accidents in the United States and two in Europe in which a CH-601XL broke up in-flight, killing a total of ten people. Aerodynamic flutter, a phenomenon in which control surfaces can vibrate and, if unmitigated, can lead to catastrophic structural failure, is suspected in all of the accidents, the NTSB stated.
The FAA responded that it already is looking into concerns about all versions of the Zodiac aircraft, which were raised at an industry meeting in February, but the agency has no immediate plans to call for the airplanes to be grounded, according to FAA spokeswoman Laura J. Brown. She said that the FAA has told the ASTM that it should conduct a review of its LSA standards regarding aerodynamic flutter.
The CH-601XL was certified as a Special Light Sport Aircraft (S-LSA) by the FAA in 2005, a type of certification which does not require FAA approval of the airplane’s design. Instead, the airplane model is issued an airworthiness certificate if the manufacturer asserts that the plane meets industry accepted design standards and has passed a series ground and flight tests.
The Safety Board’s urgent recommendation to the FAA is to prohibit further flight of the Zodiac CH-601XL until it can determine that the airplane no longer is susceptible to aerodynamic flutter. The Safety Board’s investigations of the accidents that occurred in the U.S. point to a problem with the design of the flight control system, which makes the airplane susceptible to flutter, it said.
“The NTSB does not often recommend that all airplanes of a particular type be prohibited from further flight,” said NTSB Acting Chairman Mark V. 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 Board also found that the stick force gradient – a measure of the force applied to the control stick and the increase in lift that results – was not uniform throughout the range of motion, particularly at high vertical accelerations or Gs. Lessening of the gradient at high Gs could make the airplane susceptible to being over-controlled by the pilot inadvertently, which could create a condition in which the airplane is stressed beyond its design limits, leading to an in-flight structural failure.
Problems with the airspeed indication system also were identified, the Safety Board said. Errors with the correlation between the actual airspeed of the airplane and that shown on the instruments in the cockpit could result in the airplane being piloted at airspeeds exceeding design limits, which could compromise the plane’s structural integrity. While the airspeed indication issue has not been linked to any accidents, the Safety Board believes that this is a safety-of-flight issue that should be corrected.
The six accidents in which CH-601XLs suffered in-flight structural failures are one on February 8, 2006, near Oakdale, California, when a CH-601XL crashed after its wings collapsed, with two fatalities; on November 4, 2006, when a CH-601XL broke up in flight while cruising near Yuba City, California, with two fatalities; on February 5, 2008, when a CH-601XL crashed near Barcelona, Spain, after its wings folded up during a descent, shortly before landing (two fatalities); on April 7, 2008, when a CH-601XL broke up in flight near Polk City, Florida with one fatality; on September 14, 2008, when a CH 601XL crashed in the Netherlands with two fatalities; and on March 3, 2009, when a CH-601XL broke up in flight while cruising near Antelope Island, Utah, with one fatality.
In addition, the Safety Board asked the FAA to make a comprehensive evaluation of the wing and aileron system on the CH 601XL to identify design and/or operational changes that will reduce the potential for flutter; notify owners of CH-601XLs of any design and/or operational changes to the airplane and require them to implement the changes; work with ASTM International to incorporate standards for light sport airplanes that would reduce the likelihood of encountering in-flight flutter; evaluate the stick force gradient at the aft center of gravity, and especially at the higher Gs, and notify pilots of such effects; develop standards on stick force characteristics for light sport airplanes that minimize the possibility of pilot’s inadvertently over-controlling the airplane; ensure that the pilot’s airspeed indicator accurately reflects the airlane’s velocity and update pilot operating handbooks accordingly; and work with ASTM International to ensure that standards for light sport airplanes result in accurate airspeed indications and appropriate documentation in new airplane pilot operating handbooks.
The board’s investigations have identified several areas in which it believes the design standards for light sport airplanes are deficient. Because ASTM International provides the standards that are developed by industry working groups, the NTSB has asked ATSM to add requirements to ensure the standards for light sport airplanes reduce the potential for aerodynamic flutter to develop; develop standards on stick force characteristics for light sport airplanes that minimize the possibility of pilot’s inadvertently over-controlling the airplane; and ensure standards for light sport airplanes result in accurate airspeed indications and appropriate documentation in new airplane pilot operating handbooks.
To read the letter to the Federal Aviation Administration: www.ntsb.gov/recs/letters/2009/A09_30_37.pdf
To read the letter to ATSM International: www.ntsb.gov/recs/letters/2009/A09_38_40.pdf