Two alerts, one from the US Federal Aviation Administration and one from AMD, manufacturer of the Zodiac CH601XL light sport aircraft, came one day too late for an Illinois sport pilot and owner of an experimentally-registered CH601XL.
That pilot, Charles Cummings, died Friday November 6 when his Jabiru-powered Zodiac (N538CJ) broke up in flight and tumbled into a field in Arkansas. According to the NTSB preliminary report, Cummings' was on VFR a cross-country out of the Sharp County Regional Airport (KCVK) at about 0930 that morning and had probably flown to either Flippin or Mountain View, Arkansas as an intermediate destination about 50mi to the southwest. He was likely on his way back to KCVK when the aircraft broke apart near Agnos (about 10mi west of KCVK) at around 1100h that morning.
According to the NTSB, the right wing assembly that landed in a pond marked the beginning of a 600ft-long debris field oriented on an 035deg magnetic heading. Next came the left wing, 200ft further along and the fuselage, empennage, engine and propeller at the end point. "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," notes the investigator.
The NTSB has taken a special interest in the CH601XL, in both kit and factory-built form based on a series of fatal accidents, as previously reported in this blog.
The FAA and the manufacturer both took decisive action on Saturday, November 7.
For the FAA's part, it issued a special airworthiness information bulletin revealing the results of an airworthiness investigation it launched on the model in April and informing operators to follow a companion safety directive issued by AMD.
Here's what the FAA found, verbatim, in its review:
Wing structure: FAA analysis shows bending loads used to design the wing structure were non-conservative and the basic static strength of the CH601XL/CH650 does not appear to meet the intent of the ASTM standards for a 600kg (1320 lb) airplane, given the current flight envelope in the Pilot Operating Handbook
Structural Stability: Other aviation authorities have noted the presence of buckling in the wing structure, including in the center section. Such structural instabilities can have a significant effect on static strength and flutter characteristics.
Flutter: Our detailed review of available flutter analysis reports was inconclusive. However, accident photos clearly indicated flutter was present during the in-flight structural failures. The FAA believes flutter may either be a first order root cause of in-flight structural failure or a secondary cause after some initial wing structural deformation or twisting.
Airspeed calibration: Calibration procedures do not appear to adequately account for basic static pressure source error due to the location of the static port. This could lead to potential airspeed indication anomalies, particularly since the CH601XL/650 derivatives can be delivered/built with several different airspeed indicators installed or without an indicator at all. 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.
Stick force characteristics: Flight test data from foreign authorities indicates at aft center of gravity the stick forces become very light. The FAA believes this may be a contributing factor in structural failure accidents if coupled with operations over gross weight, at speeds higher than VA, and/or for aircraft loaded improperly. In such conditions, it would be very easy to dynamically load the CH601XL/CH650 wing beyond its design structural load limit.
"In order to prevent potential catastrophic structural failure, we strongly recommend that all owners and operators of Zodiac CH601XL/CH650 comply with actions outlined in a forthcoming Aircraft Manufacturing & Design, LLC (AMD) Safety Directive / Safety Alert to address the above-referenced concerns before further flight," the FAA concluded.
AMD's companion directive calls for several modifications to the aircraft "before further flight" includes installing aileron counterbalance weights and verifying certain aspects of the airspeed indicator.