An experimental airplane of the same series that the National Transportation Safety Board urged the Federal Aviation Administration to ground seven months ago, until a flight control problem could be corrected, was involved in another fatal accident last week.
On November 6, 2009, a Zodiac CH-601XL, an experimental amateur-built airplane, was destroyed as a result of an in- flight breakup near Agnos, Arkansas, killing the pilot who was the sole occupant. The debris field was scattered over an area more than 600 feet long. Both wings had separated from the fuselage in-flight.
In April 2009, the NTSB called on the FAA to ground the Zodiac CH-601XL after the Safety Board linked six accidents involving that aircraft model to aerodynamic flutter, a phenomenon in which the control surfaces and wings of the airplane can suddenly oscillate and lead to catastrophic structural failure. Those accidents killed a total of ten people. Preliminary investigation of the November 6 accident in Arkansas reveals a failure mode similar to that seen in the earlier crashes.
The Safety Board's urgent recommendation to the FAA was to "prohibit further flight of the Zodiac CH-601XL, both special light sport aircraft and experimental, until such time that the FAA determines that the CH-601XL has adequate protection from flutter." The FAA replied in July that they lacked "adequate justification to take immediate certificate action to ground the entire fleet."
The Zodiac is available as a ready-to-purchase airplane (classified as a special light sport aircraft), which is manufactured by Aircraft Manufacturing and Design, LLC, and as an amateur-built plane from a kit (classified as an experimental aircraft) available from the designer, Zenith Aircraft Company.
On November 7, one day after the accident in Arkansas, the FAA issued a Special Airworthiness Information Bulletin strongly recommending that all owners and operators of Zodiac CH-601XL/CH650 airplanes comply with a Safety Alert/Safety Directive issued by the manufacturer, Aircraft Manufacturing and Design, LLC. The Safety Alert/Safety Directive requires all owners of special light sport aircraft models to make structural modifications to the airplane and add aileron counter-balances before further flight. Since the directives of the manufacturers of special light sport aircraft must be complied with, those aircraft not in compliance are effectively prohibited from further flight.
The designer, Zenith Aircraft Company, has asked the owners of the kit-built experimental airplanes to make the same modifications, but there is no requirement that the modifications be completed before further flight is attempted.
"We are pleased that the FAA and the manufacturer have acted on the safety-of-flight issues that we identified with the Zodiac special light sport airplane. We are troubled, however, that the no modifications are required on the amateur-built planes," said NTSB Chairman Deborah A.P. Hersman. "We are very concerned that a lack of required compliance may lead to more accidents like the one in Arkansas, and others we've already seen," she said.
The Safety Board's investigation of the November 6 accident is on-going.