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FAA's Zodiac 601/650 Aircraft Report

A closer look

By Patrick Panzera, EAA 555743, ppanzera@eaa.org

Pat Panzera

Introduced in 2001, the Zenith Zodiac CH 601 XL first showed signs of in-flight structural failures in 2006. A series of six total in-flight structural failures of the Zodiac CH 601 XL (occurring in the United States between February 2006 and November 2009) led the agency to stop issuing airworthiness approvals to those Zodiac 601s and the structurally similar 650 models in late 2009. These aircraft are available as fully constructed light-sport aircraft from Aircraft Manufacturing and Design Inc. (AMD) as well as amateur-built plans or kits from Zenith Aircraft. These accidents prompted the FAA to conduct a thorough investigation.

The Accidents

The first aircraft in the series of accidents, an experimental, amateur-built CH 601 XL was observed to be on the 45 at 800-1,000 feet above ground level (agl), and the wings were seen to “vibrate” while observers heard the engine rpm become high-pitched and erratic. With the left wing collapsing, the plane entered a spin. After one full revolution, the other wing folded at the root, both wings remaining intact after impact with the ground. Although upon inspection of the Sensenich propeller it was noted that one of the two blades was missing, the “probable cause” of the accident was “the structural failure of the wings for undetermined reasons.” The FAA investigation of this first 601 XL accident caused them to visit Zenith (kit manufacturer) to help identify a root cause, the results being inconclusive. Read the full narrative

The second in the chain, a factory-built special light-sport aircraft (S-LSA) version built by AMD was tracked by radar and reported going through a 2,000-foot-per-minute rate of descent prior to coming apart. Unlike the first accident, this accident included the loss of the horizontal stabilizer, and it’s noted that “The separation of the wings and horizontal stabilizer was in a downward direction.” The probable cause is listed as being “in-flight structural failure of the horizontal stabilizer and wings for undetermined reasons.” Read the full narrative

The third U.S. aircraft in the series of 601 accidents, another amateur-built version lost its wings in-flight while flying into “deteriorating weather,” which is also described in the FAA report as 2 statute miles visibility, 800 feet broken and “heavy thunderstorms,” with temperature and dew point both being 59 degrees. The 176-hour sport pilot had 18 months of experience. The probable cause is listed as “the pilot’s improper pre-flight planning and decision to depart into deteriorating weather conditions, which resulted in a loss of control and subsequent in-flight break up.” Other noteworthy details: The seatbelt was found to be unlatched, the canopy was unlocked, and the beginning of the debris field contained cockpit contents such as “pieces of paper and a section of the airplane’s checklist.” Read the full narrative

In the fourth accident, an S-LSA version built by the Czech Aircraft Works (CZAW) and specified as a CH 601 XL RTF came apart after witnesses observed it “banking and yawing.” During the last yaw, the left wing failed in an upward (or positive) direction, which was determined to be due to a fracture of the lower left wing spar chord. The probable cause is reported to be “the in-flight failure of the left wing for undetermined reasons.” Of special note is the fact that the canopy was never found. The report makes no mention of flutter. Read the full narrative

The fifth accident happened in the spring of 2009 over mountainous terrain. Radar showed the airplane in steady flight traveling north at about 112 knots. Prevailing wind was generally from the south at about 14 knots, gusting to 20 knots. Turbulence was reported in the area at the time of the accident. Maneuvering speed (VA) is reported to be less than 80 knots. Both horizontal stabilizers were bent in a downward direction, and there is no mention of flutter in the report. At this writing, the National Transportation Safety Board (NTSB) website doesn’t indicate a probable cause. Read the full narrative

And the last accident, which happened well over a year ago, doesn’t have much published information that is accessible to the public. What could be found basically says that the S-LSA came apart in flight and that both wings came off before hitting the ground. Read the full narrative


Event Date



N Number

Probable Cause  


Oakdale, California

Hooker Zodiac 601 XL


Probable Cause  


Yuba City, California

Aircraft Mfg. & Dev. Co.(AMD) CH 601 XL S-LSA


Probable Cause  


Canadian, Texas

Walker Zenith Zodiac 601 XL


Probable Cause  


Polk City, Florida

Czech Aircraft Works CH 601 XL RTF




Antelope Island, Utah

Stephenson Zodiac 601 XL




Agnos, Arizona

Zenith Aircraft Co. Zodiac 601


The FAA’s investigation noted that two “similar accidents” occurring in Spain and the Netherlands prompted the United Kingdom, the Netherlands, and Germany to ground the CH 601 XL in October 2008. However, the German authority later opted for a reduced flight envelope, allowing the airplane to continue to fly. At that time, the FAA concluded the accidents were due to improper operations, such as flying beyond the weight and airspeed limits published in the pilot operating handbook (POH), or improperly operating in turbulent conditions.

Although the full narratives aren’t available to this writer at this time, the NTSB states that in February of 2008, a CH 601 XL, built by the pilot, collided with terrain near Barcelona, Spain, after its wings folded up during a descent for landing. Witnesses reported that “the right wing folded over the left wing above the airplane.” Some witnesses observed the wings vibrating prior to folding.

In September of 2008, a CH 601 XL, built by the pilot from a kit manufactured by CZAW, crashed in the Netherlands. Witnesses reported that the airplane was in level flight at approximately 1,000 feet agl when the right wing folded up over the fuselage. The airplane entered a steep dive before impact.

The Report

During this detailed review, the FAA worked closely with the NTSB at a technical level since they had also identified concerns with the airplane specifically related to aircraft structure, flutter, proper airspeed calibration, and stick forces. NTSB concerns focused primarily on evidence that seemed to indicate flutter was a root cause for some, if not all, of the accidents. They did this while at the same time proving that loose aileron cables are the root cause of the flutter, which is of course a builder issue, not necessarily a design flaw.

The FAA on the other hand, through their preliminary analysis of the accidents, found that a common root cause for the structural failure accidents was not evident and it would seem that a “perfect storm” of just the right set of circumstances needs to prevail, and also recognized that proper aileron cable tension is paramount. In addition to flutter the FAA report also identified issues with the wing structure and carry-through, airspeed calibration, stick force gradients and operating limitations. Read the NTSB safety recommendations here.

At the end of the special review, published in earlier March, the FAA pointed to several factors as being the cause for Zodiac CH 601 XL aircraft in-flight accidents, including a wing structure that doesn’t meet ASTM consensus standards for light-sport aircraft. (Read the full 40-page report here. Read the Appendix here.) Please note that no CH 650s were involved in any accident. However, due to the similar—if not identical—wing structure and attachment, the 650 is included in all actions.
Wing Structure

An important finding in the FAA study was regarding the wing structure which didn’t meet the approved ASTM International standards to handle the required loads and stresses for a 1,320-pound aircraft. Specifically, the manufacturer’s structural analysis included that portion of the wing planform covered by the fuselage as being as effective in producing lift as an adjacent portion of exposed wing planform. The FAA concluded that this assumption may not be conservative and may be noncompliant with the ASTM F 2245 § requirement that “…these loads must be distributed to conservatively approximate or closely represent actual conditions.”

This may seem trivial, but it does dictate just how the weight is distributed to the inverted wing during static load testing.

In addition to calculating the center section as producing unaffected lift, the manufacturer’s analysis used an assumption that the lift distribution has a purely elliptical shape.

A rendition of the 601 with an elliptical lift distribution superimposed above the starboard wing and a lift distribution over the port wing that is commonly accepted as one for a wing with a mild-tapering planform, like with the 601 XL, and with zero lift from the center section.

According to the FAA, this assumption is often used in academic evaluations but is incorrect for compliance to ASTM F 2245 §, stating that there are several, more appropriate methods for estimating the distribution of wing lift, referencing advisory circular (AC) 23-19A, Airframe Guide for Certification of Part 23 Airplanes.

Additional issues with the load calculations, which seem minimal but may be cumulative, include:

  • Assuming a 15-gallon fuel tank in each wing when the manufacturer’s drawings and advertising materials show a 12-gallon tank as standard.
  • The manufacturer’s analysis incorrectly defines the maximum zero fuel weight (MZFW) as the maximum takeoff weight minus full fuel rather than the airplane weight is at a maximum but with the wing fuel tanks only partially full.
  • The manufacturer’s analysis was based on estimates of the airfoil-pitching moment characteristics obtained from the airfoil designer. The manufacturer then assumed a reduction in the magnitude of the airfoil-pitching moment without providing substantiating test data to support it.
  • The manufacturer’s inaccurate interpretation of ASTM F 2245 § X1.3.3.3, assuming the intent being to multiply both torsion and wing-bending loads by 75%. This standard is intended to combine the wing-bending loads at ¾ of the design limit load factor, or 3g, with 100% of the torsion loads occurring at design diving speed (VD). This could impact the resulting torsional stiffness of the wing structure, which in turn could impact flutter characteristics.

The FAA completed its load analysis and provided its comments to the manufacturer in September 2009. Based on these comments and agreeing with the FAA’s above-mentioned concerns on the distribution of lift along the wing span, the fuel tank capacity, and the interpretation of ASTM F2245 § X., Zenith revised their analysis, retested the static strength of the wing structure, and provided the FAA with the updated results. However, the manufacturer didn’t agree with and didn’t incorporate into its analysis the FAA position on the distribution of lift between the wing and the fuselage, MZFW, airfoil-pitching moment data, as well as the design-maneuvering speed (VA).

The manufacturer successfully tested the modified wing structure to loads higher than determined in their revised analysis and the wing structure didn’t fail at the ultimate test loads.
Despite disagreeing with the FAA on several issues deemed “significant” by the FAA, the maximum bending moment achieved in the static test was only a few percent below the FAA-estimated maximum required bending moment. The FAA assumes that it is entirely possible that had Zenith continued the static load test to the point of structural failure, the ultimate load may have equaled or exceeded FAA estimates factoring all concerns.

Zenith has since developed modifications for the Zodiac wing structure and is distributing free plans as well as a materials kit (at cost) to aircraft builders and owners in an upgrade package, which was approved by the FAA in November of 2009. As previously mentioned, airworthiness approvals had been suspended until those modifications were made, because of “known safety concerns,” in accordance with FAA regulations. During testing, the modified structure didn’t fail at the ultimate load (as already noted); however, the wing-attach bolts were bent and the bolt holes were elongated, indicating there was permanent damage to the structure at these loads. At the time of the FAA report, the manufacturer was finalizing design details of the modifications to the wing structure. Owners and operators should contact Zenith or AMD for the latest information on these modifications. The FAA report suggests that the manufacturer perform additional structural testing once a final design is reached, to demonstrate the final wing design meets the ASTM standards.
Subsequent additions to the upgrade package provide an even more robust aircraft than what had been approved by the FAA in November, says Mathieu Heintz, vice president of Zenair. “We went way, way overboard on an upgrade kit package, and part of the reason we did that is we felt that if we just applied the bare minimum, it would not be enough to reassure the people. So it is important to understand that the upgrade we are now offering is significantly more than what was put on the aircraft when the static load test was done.”

The upgrade/modification includes adding angles to the inboard portions of the wings, the addition of a center wing-spar doubler, adding wing-to-fuselage joint attachment reinforcements, adding structural support to the wing rib at the aileron bell-crank connection, and adding reinforcements to the rear spar at the aileron control rod aperture.
As an aside, even though there were accidents involving the structural failure of the elevator, no part of the FAA report addresses this.
In addition to the wing structure itself, the FAA had issues with the following:

  • Errors in the airspeed indication system. The report goes into great detail in this area, but as far as homebuilt aircraft is concerned, making sure your airspeed instruments read properly falls on the shoulders of the builder, not the designer, and should be sorted out during phase one flight testing.
  • Stick forces: The FAA gives the 601 and 650 a clean bill of health in this area but then goes on to say, “flight test data from foreign authorities indicates that 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.”

    “Though the CH 601 XL does not have to comply with 14 CFR Part 23, the data does indicate that stick force gradients may not be acceptable for aircraft that are intentionally or unintentionally flown outside the POH center of gravity limits.”

    The FAA’s assertion begs the question, why would a designer bother to ensure that their plane maintain acceptable stick force gradients for operations outside the center of gravity limits.

A Potential for Flutter

Although the FAA found the available flutter data results were inconclusive, they stated that it’s clear from the evidence (from aircraft involved in structural failure accidents) that flutter was a “causal factor.” Further stating that it’s not possible to determine whether flutter was the root cause of the structural failure or a secondary cause after some initial structural deformation of the wing, the FAA is looking at the seemingly diminished structural stiffness influencing flutter while also noting that improperly rigged aileron cable tensions and improperly installed flap stops have certainly been the culprits in noncatastrophic flutter incidents.

Though changes are being implemented by the manufacturer to mass-balance the ailerons and are included in the plans and retrofit kit being offered, as well as with any new planes and kits, the FAA has suggested the manufacturer perform additional analysis and testing on the revised structure to verify that flutter concerns have been mitigated.
Prior to this report, the Hamburg University of Technology was contracted by Zenith to perform a complete ground vibration test and flutter analysis performed by Prof. Dr.-Ing.Uwe Weltin. His report looked at the typical linear flutter analysis and didn’t show any tendency for flutter within the CH 601 XL flight envelope. However, his analysis indicated that nonlinear flutter analysis could point toward a possibility of flutter or vibration within the flight envelope. Due to the flexibility inherent in the wings (wing skins buckle prior to ultimate load), it may be necessary to consider nonlinear evaluation techniques of the flutter characteristics since the frequency and mode shapes may be affected by post-buckling effects on wing skin structural stiffness.

Here are some links to several reports related to flutter analysis of the CH 601 XL and 650:

Aeroelastic Analysis CH 601 XL
Aeroelastic Analysis CH 601 XL and the CH 601 with a maximum takeoff mass of 600 kg
Aeroelastic Analysis CH 650

The FAA’s concerns for flutter should be mitigated once Zenith has completed testing and analysis to validate the revised design. The changes required by the recent safety directive from AMD include:

  • Structural modification of the wing and center section
  • Aileron modifications designed by the U.K. Light Aircraft Association
  • Verification of flap and other control stop installations
  • Modifications to (reinforcement of) the aileron bell-crank attachment
  • A requirement to check the control cable tensions before each flight.

Proposed Changes to the Rule

Since the entire sport-pilot and light-sport aircraft program is still in what some might consider its infancy, it’s reasonable to expect that practical application of the rules will result in changes being made as the need (or deficiencies) become apparent. The issues with the 601/650 and the self-regulating nature of the consensus standards have made necessary some proposed changes to the ASTM International F 2245-07a as proposed by the FAA:

  • The ASTM standard should incorporate a requirement similar to 14 CFR Part 23 § 23.321 which requires compliance to the flight load requirements for any practicable distribution of disposable load. Section 9 of the ASTM standard should require inclusion of the MZFW in the operating limitations section of the POH.
  • The standard should incorporate the “operating maneuver speed” (VO), similar to 14 CFR Part 23 § 23.1507. VO would replace the value for VA specified in section 9 of the ASTM standard. The ASTM standard’s use of VO would provide common nomenclature between newly types-certificated airplanes and LSA designs. The use of VO would also provide the pilot with the correct maneuvering speed in n1, such as Öcases where the designer has selected a value for VA other than VS from ASTM F 2245 § X1.1.
  • Section should clarify that the designer has the option to define VD in terms of the demonstrated dive speed, VDF.

While it’s certainly understandable that the FAA and the NTSB need to react decisively with issues concerning S-LSA (as it does with certified aircraft), this action can potentially have adverse long-reaching effects on experimental, amateur-built aircraft as well. Currently the FAA has taken unprecedented action in their refusal to issue airworthiness certificates to homebuilt versions of the 601/650 that don’t include the modifications mandated in the Special Airworthiness Information Bulletin issued on November 7, 2009. Their current report contains wording that there is a potential for some changes in the certification and oversight of amateur-built aircraft:

“The special review team believes this to be a pivotal point for defining an appropriate level of safety for experimental amateur-built aircraft and for the LSA community. The situation has galvanized the need for the FAA to clearly define an accepted level of risk for this segment of the market, and consider what means would be needed to achieve that level of safety.”

EAA’s advocacy team continually works with the FAA to ensure any changes or modifications to existing rules are measured and reasoned.

An EAA survey indicates that a majority of owners and operators are well aware of the issues and agree voluntarily that they shouldn’t fly until they have further information from the FAA and the manufacturer.
EAA’s Earl Lawrence, Vice President of Industry and Regulatory Affairs, noting that many of the fatal accidents with the aircraft involved EAA members, added, “Safety is always the top priority, and complete information is the best way to create and maintain the highest standard of safety. Any aircraft accident is a tragedy, and EAA shares the sense of loss with the families and friends of the aviators affected. The FAA’s report contains excellent data and will enhance safety in these aircraft.” With the proposed changes to the standards, this rings true for all light-sport aircraft.

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