The US Federal Aviation Administration (FAA) has launched a special review of the structural design and flutter characteristics of certain Zenair Zodiac CH-601 XL aircraft models in response to several recommendations on the type issued by the National Transportation Safety Board (NTSB) in April.

The agency however stopped short of the NTSB’s call to ground the experimental and factory-built light sport aircraft (S-LSA) versions of the low-wing two-seater.

In a 13 July letter to the NTSB, FAA administrator Randolph Babbitt writes that the FAA “lacks adequate justification” to ground the CH-601XL models, per the NTSB’s request. “Data indicates the CH-601XL has a safety record similar to other S-LSA and appears capable of safe flight and operations if maintained according to the manufacturer’s recommendations,” Babbitt continues.

NTSB investigators have determined that flutter and other related factors are suspected as factors in six CH 601 XL crashes that have killed 10 people in Europe and the USA since 2006.

Along with grounding the experimental and factory-built versions of the aircraft until the FAA determines that the CH-601 XL has “adequate protection from flutter”, the NTSB is also requesting that light sport aircraft consensus design standards, issued by ASTM, be changed to reduce the potential for aerodynamic flutter, minimise inadvertent over-controlling of the aircraft through stick forces and ensure accurate airspeed indications, all elements identified as potentially problematic in the investigations.

Babbitt says the special review team will complete an assessment of the design and “determine a proper course of action”, a statement which would not preclude later grounding the aircraft.

The review team plans to perform flight tests in an S-LSA version of the aircraft, gathering information that includes control stick forces. In addition to flutter concerns, the NTSB wants to know if the aircraft’s light stick forces could cause pilots to inadvertently over-control, thereby overstressing the airframe.

Source: FlightGlobal.com

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