Suggests Modifications To Return Aircraft To Airworthy Status

The FAA has released its report concerning the Zodiac CH 601 XL and CH 650 aircraft, which suffered several in-flight failures but which could not be traced to any single root cause. The FAA suspended new airworthiness approvals of both models of the aircraft in November of last year.

"FAA review of the in-flight failures did not indicate a single root cause, but instead implicated the potential combination of several design and operation aspects," the report executive summary states. "Our preliminary assessments focused on the strength and stability of the wing structure. Further analysis during the special review found the loads the manufacturer used to design the structure do not meet the design standards for a 1,320 lb (600kg) airplane. Static load test data verifies our conclusion. The special review also identified issues with the airplane’s flutter characteristics, stick force gradients, airspeed  calibration, and operating limitations."

The FAA looked at a number of factors in the construction of the airplanes. Among their findings:

• FAA analysis of the aircraft showed that the loads used by the manufacturer to design the wing structure did not meet the ASTM standard for a 1,320 lb aircraft.
• The FAA reviewed available flutter data, but the results were inconclusive. However, it is clear from the evidence from aircraft involved in  structural failure accidents that flutter was a causal factor. It is not possible to determine whether flutter was the primary root cause of the structural failure or a secondary cause after some initial structural deformation of the wing.
• AMD and Zenith Aircraft calibration procedures do not clearly explain how to adequately account for basic pressure source position error due to the location of the pitot-static ports. This error is caused by the flow around the airframe  and unless calibrated properly, could lead to potential airspeed indication anomalies; particularly since the CH 601 XL derivatives can be equipped with several different types of pitot-static sources. The situation could lead to operating the airplane above the maneuver speed and/or the design cruise speed, potentially leading to structural failure. Calculations conducted by the FAA show a 1 mph error in airspeed provides approximately a 0.1g load change on the wing at maneuver speed. The FAA recognizes that similar airspeed calibration problems may exist with other light sport aircraft (LSA) and has begun efforts to improve the ASTM standards to address this problem.
• The current ASTM standards simply state, “Longitudinal control forces shall increase with increasing load factors”. Available data indicates the stick force gradient for the CH 601 XL meets this standard, and appears to meet the intent of 14 CFR Part 23 within much of its operating envelope, though this is not required. However, flight test data from foreign authorities indicates at aft center of gravity conditions the stick forces do become light. This may be a contributing factor in structural failure accidents if coupled with operating at speeds higher than VA, especially if flown over gross weight and/or with improperly loaded aircraft.
• During a visit to the manufacturer’s facility in Mexico, MO, the FAA also observed buckling of the local wing skin near the rib containing the aileron bell crank attachment when the ailerons were deflected to their stops. Such structural instabilities can have a significant effect on static strength and flutter characteristics of the wing. The manufacturer has designed modifications to the structure that appear to address the specific areas where buckling has occurred.
• Initial review of available static test data indicated the weight and speed envelopes for the CH 601 XL needed to be substantially reduced to limit the potential risk of exposure to subsequent structural failures.

On November 7, 2009, the FAA issued SAIB CE-10-08 to inform owners and operators of potential safety issues with the CH 601 XL and CH 650. The FAA also issued an action November 12, 2009 to cease issuance of new airworthiness certificates until the safety related issues are addressed. Concurrently, AMD issued a safety directive for the S-LSA versions of the CH 601 XL and CH 650 to address the situation and to communicate details of modifications required before further flight.

Zenith Aircraft also communicated similar information to owners and operators of experimental versions of the CH 601 XL and CH 650. The FAA says the manufacturer was still modifying the design when the report was issued. The FAA concludes that Zenith needs to re-test the modified wing structure once a final design is reached using loads appropriate for the operating envelope of a 1,320lb aircraft to verify the new design. They need to verify through test and analysis the final design is not susceptible to flutter. They need to publish better information regarding proper airspeed calibration, the light stick forces, and the tendency for this airplane to be loaded to an aft center of gravity and overweight condition for pilots that don’t fit the 190 lb ASTM standard for LSA pilots. Finally, they need to provide a statement that the aircraft being altered will still meet the requirements of ASTM F2245 after the changes are made. With type-certificated aircraft, airworthiness standards play an important role in establishing an acceptable level of safety. The special review team recognizes that if this were a 14 CFR part 23 type-certificated aircraft, it is likely the FAA would have taken airworthiness directive action to address an unsafe condition.

FMI: www.faa.gov