Zenith Aircraft stands by its Zodiac CH-601XL light sport aircraft design, despite calls from the NTSB to ground it. The recommendation marks the first time the safety agency has addressed an issue with an aircraft classed in the light sport category. The basic Zodiac design in question has been in production as a kit since 1984, but was certified as a light sport aircraft in 2005. Since then, there have been six inflight breakups involving the design resulting in 10 fatalities. The accidents have been tentatively attributed to aerodynamic flutter. NTSB Acting Chairman Mark Rosenker has called for mandatory grounding of all existing Zodiac 601XLs until the issue is resolved. The FAA has yet to issue such a grounding order. Sebastian Heintz, president of Zenith Aircraft, said the company takes the issue seriously and has commissioned two third-party companies to perform flutter analysis. "There are no indications of flutter," Heintz said. One Zodiac builder-pilot places blame for flutter accidents on the owners, saying they should be particularly careful to ensure the control cable tension is adjusted properly. Asked if they would fly in a CH-601XL, several officials at Sun 'n Fun adopted a wait-and-see attitude, pending further investigation by the FAA and NTSB.