Owners and pilots of Zodiac aircraft may want to keep them on the ground for awhile.

On Nov. 7 the FAA issued a Special Airworthiness Information Bulletin (SAIB) on variants of Zodiac CH601XL and CH650 airplanes because of several in-flight structural failures. In response, Zenith Aircraft Co. has developed an upgrade package to address the concerns in the SAIB, which affects all serial numbers and variants of the aircraft, including S-LSA models, which are produced by AMD LLC in Eastman, Ga.

According to the National Transportation Safety Board, there have been five in-flight structural failures of the CH601XLin the United States since 2005.

Since April of this year the NTSB and the FAA have been conducting a special review of the Zodiac CH601XL and the similarly designed CH650 to evaluate design and operational details of the aircraft. The FAA’s review noted: “We made a determination that these accidents did not clearly indicate a single root cause. Instead, it implicated the potential coupling of design and operational aspects of the aircraft.”

Officials at Zenith Aircraft Co. have been working with the FAA to address the issues raised by the investigation, in particular the possibility of wing flutter as a contributing cause of the accidents.

In April, the company posted a response to the NTSB’s concerns on its website, noting: “We continue to believe wing flutter will not occur if the control cables are adjusted properly. Nonetheless, we are carefully considering the points raised in the memo, including whether the Zodiac CH-601XL is susceptible to wing flutter.”

On Nov. 8 the company updated its website with a question and answer page to address concerns, and included procedures required to resume normal operations, including upgrades to address the potential issues outlined in the SAIB.

“An Upgrade Package will be made available to all builders and owners of kit versions of the aircraft, including detailed step-by-step installation instructions,” company officials said. “We are committed to working with all builders and owners so that they may resume building and operating their aircraft.”

The FAA’s analysis cited that the bending loads used to design the wing structure were “non-conservative” and “do not appear to meet the intent of the ASTM standards for a 1,320 lb. airplane, given the current flight envelope in the Pilot Operating Handbook.”

The review also noted buckling in the wing structure and, based on photographs of the accident aircraft, suggests that flutter may have contributed to the in-flight failures. In addition the FAA determined that calibration procedures do not appear to adequately account for basic static pressure source error data because of the location of the static port, noting that “the situation could lead to the potential of operating the airplane above the maneuver speed and or the design cruise speed, potentially leading to structural failure.”

The FAA recommends that S-LSA owners contact AMD LLC and Zenith Aircraft Co. to acquire a modification kit and follow the manufacturer’s instructions for assembly and adjustment.

For more information: ZenithAir.com, NewPlane.com, FAA.gov