FAA Mulls Grounding Of Zodiac CH-601XL
Frances Fiorino fiorino@aviationweek.com 
The FAA, already reviewing design issues for all variants of the Zodiac light sport aircraft (LSA), is now weighing a decision to ground the CH-601XL version as the NTSB urgently recommended on Apr. 15.
Since 2006, 10 people lost their lives in six in-flight breakups of the CH-601XL, four in the U.S. and two in Europe. As a result, the NTSB urged the FAA to order the grounding until investigators deem the two-seat, single-engine LSA is no longer susceptible to aerodynamic flutter--which the NTSB suspects led to the breakups.
Aerodynamic flutter refers to sudden vibrations of aircraft control surfaces, which if unmitigated, could lead to catastrophic structural failure, according to the safety board.
The NTSB is looking at a flight control system design as the possible cause of the aircraft's susceptibility to the flutter, at least in the case of the four U.S. accidents.
In its recommendation letter to the FAA, the NTSB noted its probes revealed several areas in which "design standards" for the CH-601XL were "deficient." For example, the CH-601XL's stick force gradient, or amount of force applied to the control stick, was not uniform throughout the range of motion, particularly in high vertical accelerations. At high gs, the non-uniformity could lead to inadvertent over-controlling and stressing the aircraft beyond design limits--which could lead to structural failure.
The NTSB also identified problems with the aircraft's airspeed indication system, specifically, that the actual and indicated airspeeds did not correlate. This could result in the aircraft operating at airspeeds exceeding design limits, it said. The board noted that these problems were not directly linked to any of the accidents, but rather are a safety of flight issue that should be corrected.
(The CH-601XL was certified by the FAA in 2005 as a Special-Light Sport Aircraft (S-LSA) and as such, its design does not require FAA approval. Rather, the agency issues an airworthiness certificate if the manufacturer asserts the aircraft meets industry design standards and successfully completes ground and flight tests. Zenair Ltd. is headquartered in Ontario, but Zodiac aircraft are also manufactured in the U.S., Europe, and South America.)
Following the accidents, the FAA formed a team of experts, including British and Dutch investigators, to examine Zodiac design and manufacturing issues, says agency spokesman Les Dorr. (Including all variants, the Zodiac has been involved in nine fatal accidents, according to FAA.)
On Apr. 16, the FAA began its evaluation of the NTSB's call to ground the CH-601XL, with the team reviewing design characteristics of the entire Zodiac family of aircraft (which includes the 601 HD, 601HDS and 601 UL.).
The safety board's letter to the FAA contained seven other non-urgent recommendations, including asking the agency to make a comprehensive evaluation of the wing and aileron system to identify design or operational changes that would reduce the aircraft's potential for flutter. For complete NTSB recommendation letter, go to: http://www.ntsb.gov/recs/letters/2009/A09_30_37.pdf
The NTSB also issued separate recommendations ASTM International (formerly known as the American Society for Testing and Materials), a group that sets design standards, asking it to incorporate requirements into LSA standards that provide for additional strategies to reduce flutter. For the full NTSB recommendation letter to ASTM, go to http://www.ntsb.gov/recs/letters/2009/A09_38_40.pdf.
Photo credit: Zodiac
