The National Transportation Safety Board (NTSB) is expressing new concerns over the safety of the Zodiac CH601 XL amateur-built airplane as a result of another fatal crash.

On Nov. 6, 2009, a Zodiac CH601XL (N538CJ) experimental light sport airplane, designed by Zenith Aircraft Company, was destroyed when it impacted terrain, following an in- flight breakup 1.5 miles south of Agnos, Arkansas. Both wings had separated from the fuselage in flight.

Visual meteorological conditions prevailed at the time of the accident. The pilot was fatally injured. The cross-country flight had departed Sharp County Regional Airport (KCVK), Ash Flat, AR.

According to the Arkansas State Police, the pilot departed KCVK and was en route, either to Flippin or Mountain View, AR, with the intention of returning to KCVK. A local rancher located the Zodiac wreckage in a field.

The debris field was scattered over 600 feet on a magnetic bearing of 35 degrees. The right wing assembly initiated the debris field, coming to rest in a pond. The left wing assembly came to rest approximately 200 feet from the right wing. The left wing spar, left wing fuel tank, various cockpit items, and personal effects were located in the debris field. The fuselage, empennage, engine, and propeller assembly came to rest, inverted, approximately 600 feet from the right wing. There were no ground scars identified between the right wing and the main wreckage that could be associated with the left or right wing, empennage, or fuselage.

This past April, the Safety Board called on the FAA to ground the Zodiac CH601XL 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 since 2006.

The Zodiac CH601X is a fixed-gear single-engine, two-seat low-wing plane made from kits .

In its urgent safety recommendation, the Safety Board cited four accidents in the United States and two in Europe in which the CH-601XL broke up in-flight.

The CH-601XL was certified as a Special Light Sport Aircraft (S-LSA) by the FAA in 2005. This type of certification does not require that the FAA approve 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 of ground and flight tests.

The Safety Board believes the FAA should prohibit further flight of the Zodiac CH601XL until they can determine that the airplane is no longer susceptible to aerodynamic flutter. The Safety Board's investigations of the U.S. accidents point to a problem with the design of the flight control system, which makes the airplane susceptible to flutter.

The NTSB does not often recommend that all airplanes of a particular type be grounded. But the Safety Board in April said: "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 FAA replied in July that they lacked "adequate justification to take immediate certificate action to ground the entire fleet."

On Nov. 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 CH601XL/CH650 airplanes comply with a Safety Alert/Safety Directive issued by the manufacturer, Aircraft Manufacturing and Design, LLC.

The Safety Alert/Safety Directive advises all owners of the light sport aircraft models to make structural modifications to the airplane and add aileron counter-balances before further flight.

The designer, Zenith Aircraft, 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 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 ongoing.