Safety Pilot:

Zodiac CH 601 XL: An aircraft under scrutiny

February 1, 2010

In-flight breakups are extremely rare in all models of aircraft when they are operated inside the approved flight envelope. Outside the design limits anything can be broken and no one is surprised. When an airframe breaks unexpectedly, that’s a nasty situation for all concerned. Obviously, the aircraft occupants get the worst outcome, as do their families. The manufacturer, the FAA, and the NTSB have to analyze what happened and determine how to fix it, quickly. Finally, general aviation gets a black eye from negative publicity that portrays our aircraft as flimsy deathtraps, which we know is not the case.

An unusual situation has developed involving a light sport aircraft, the Zenith Aircraft Company Zodiac CH 601 XL. LSAs are lighter and less expensive than most FAA-certificated aircraft, and kitbuilt LSAs may be registered as Experimental (E-LSA) like any other homebuilt or kit aircraft. Under Experimental rules the builder is free to construct anything he’s brave enough to fly in, although the FAA does issue an Experimental airworthiness certificate. Factory-produced versions (S-LSA) must conform to ASTM international standards. Manufacturer compliance is based on the honor system and the aircraft are not FAA certificated or inspected.

The CH 601 XL has been under scrutiny by foreign authorities, the NTSB, and the FAA since it began to experience in-flight breakups in the winter of 2006; there have been nine fatal accidents to date. Four accidents occurred outside the United States, and only limited information about them is currently available. Two of the aircraft were S-LSAs, and the others were kitbuilt E-LSAs. The FAA issued a special airworthiness information bulletin (SAIB CE10-08) for the S-LSAs in November 2009; at the same time, the manufacturer issued a grounding safety alert pending modifications. The FAA also stopped issuing new airworthiness certificates for experimental CH 601 XLs.

The AOPA Air Safety Foundation has been watching LSA safety carefully. The overall record of S-LSA aircraft appears similar to that of FAA-certificated aircraft used in local flight operations: the usual takeoff and landing accidents, a few maneuvering mishaps, very few weather entanglements, and not many mechanical or structural problems. So when this Zodiac began to experience repetitive airframe problems, we were surprised. At press time there had been only one other S-LSA in-flight breakup that did not result from spatial disorientation or apparent overstressing of the airframe. There are no systemic break-up problems within the LSA category as a group.

Several of the Zodiac accident investigations are still in preliminary status, which means there have been no official findings and the circumstances cited here may change:

February 8, 2006; Oakdale, California. As the E-LSA entered the traffic pattern, a witness saw the wings “visibly vibrate” and then the left wing collapsed and folded rearward. A CFI and student pilot were killed. The probable cause was a structural wing failure for undetermined reasons.

November 4, 2006; Yuba City, California. ATC radar showed the S-LSA, built by Aircraft Manufacturing and Design (AMD) of Eastman, Georgia, in level flight at 2,600 feet agl at 106 knots. The airplane entered a climb of about 700 feet per minute to 2,800 feet and then began a rapid descent. The horizontal stabilizer separated from the fuselage, followed by wing separation. The pilot and passenger were killed. The probable cause was structural failure of the horizontal stabilizer and wings for undetermined reasons.

February 5, 2008; Barcelona, Spain. Witnesses reported wing vibration before the right wing folded over the left wing as the E-LSA was in a descent for landing. The pilot and passenger were killed.

May 5, 2008; Pelotas, Brazil. Witnesses reported an apparent in-flight breakup of the amateur-built LSA, but no details.

April 7, 2008; Polk City, Florida. A witness observed the S-LSA, factory built by Czech Aircraft Works in Czechoslovakia, bank left and right several times at significantly steeper angles, then the airplane suddenly yawed right and the right wing folded up. Another witness reported hearing a “pop” sound as the right wing folded back. The pilot was killed.

September 14, 2008; Markermeer, The Netherlands. Witnesses reported the E-LSA in level flight at approximately 1,000 feet agl when the right wing folded up over the fuselage. The pilot and passenger were killed.

March 3, 2009; Antelope Island, Utah. The E-LSA broke up in cruise flight. ATC radar showed a steady flight track traveling at about 112 knots. Winds were reported at 14 knots, gusting to 20 knots, with some turbulence in the area. There were no witnesses. According to the NTSB, “Preliminary examination of the wreckage indicated that the breakup sequence began with the buckling up of the upper spar cap of the left wing followed by the wing folding up and over the fuselage. The buckling is similar to a failure observed during structural tests of a CH 601 XL wing performed in the Czech Republic by CZAW for certification purposes. Many of the features and characteristics of the breakup…are similar to the other accidents.” The commercial pilot was killed.

September 7, 2009; Campo Grande, Brazil. Witnesses reported an apparent in-flight breakup of the amateur-built LSA; no details are available.

November 6, 2009; Agnos, Arkansas. The E-LSA crashed as a result of an apparent in-flight breakup. The debris field was scattered over an area more than 600 feet long. Both wings had separated from the fuselage in flight; the pilot was killed.

Government response

The Netherlands Civil Aviation Authority issued an emergency grounding airworthiness directive in October 2008 after the September 14 accident. The British CAA grounded its CH 601 XLs shortly afterward. In April 2009 the NTSB asked the FAA to immediately ground the CH 601 XL and sent a letter to ASTM advising the organization to reconsider its standards relative to flutter. After the November 2009 accident, the FAA issued a special airworthiness information bulletin grounding the CH 601 XL and CH 650 S-LSAs, and giving those flying experimental models (E-LSA) a very good reason to park their aircraft until appropriate modifications could be made.

There was considerable discussion within the FAA on how to handle the E-LSAs. Experimental aircraft are just that, and unless they pose a hazard to people on the ground or other aircraft, a different standard applies to them than to factory-built aircraft.

At this writing it’s impossible to determine whether the builders of the experimental accident aircraft complied with the plans or appropriate construction standards, although this type of mishap is unusual in the kitbuilt world. Although Zenith has noted that there is no common thread, the aircraft all share essentially the same airframe design.

Aerodynamic flutter appears to have been a factor or the cause in several accidents. A question is whether flutter caused the wing to fail, or if there was a structural failure within the wing that led to flutter—the outcome is the same but the fix is different.

The FAA’s Small Airplane Directorate conducted a thorough review of the aircraft and, among other concerns, determined that the wing design loads were 20 to 30 percent too low and modification was essential.

The NTSB defines aerodynamic flutter as “when aerodynamic and structural forces interact in such a way that energy from the airflow around an airplane gives rise to an unsafe structural vibration. These vibrations can quickly lead to structural failure if not sufficiently damped. Whether wing or aileron flutter develops and continues also depends on the mass distribution of the ailerons. Mass-balanced ailerons greatly decrease susceptibility to flutter as they are less likely to deflect because of dynamic bending or twisting of the wing.”

The original CH 601 XL design called for aileron cable tension rather than mass balancing to counteract flutter. Zenith advised owners to recheck cable tension but that apparently did not solve the problem in the Antelope Island accident, where the builder stated, “The tensions of the control cables were set according to the instructions provided by Zenith and that the cable tensions were checked again at eight flight hours [six flight hours before the accident flight] and had not changed.” A Zenith survey found that even in November 2008, after owners had been notified of the importance of cable tension, 12 of 14 aircraft reviewed did not meet design specifications.

The NTSB and FAA noted that accuracy of calibrated airspeed relative to indicated airspeed was in question—meaning that what pilots saw wasn’t necessarily what they were getting. That also could lead to an inadvertent overstress. Additionally, light stick force gradients—especially at higher G loadings—could lead to overstressing the airframe.

Zenith response

Zenith’s owner and chief designer, Chris Heintz, has a long record of successful plans and kits. Approximately 1,200 sets of CH 601 XL plans have been sold, but I was unable to determine how many of the model were flying as experimental aircraft. Additionally, there appear to be fewer than 100 AMD S-LSAs of varying design on the FAA’s registry.

Zenith is under tremendous pressure from various government entities, owners, and the families of lost aircraft. It is understandably careful in its description of the crisis and proposed solutions.

There is a detailed question and answer series on the company’s Web site along with new drawings, pictures, and a description of the “upgrade kit” as Zenith describes it. The kit is relatively inexpensive at $352 but the labor, which the company recommends be done by a certificated mechanic, is extensive. Some experienced observers estimated it would be well more than $10,000 for what amounts to a wing rebuild.

Some observations

Monday morning quarterbacking offers no shortage of opinion on what should have been done, by whom, and by when. One accident investigator noted that investigating experimental aircraft accidents is, to borrow from the wisdom of Forrest Gump, like a box of chocolates—you never know what you’ll find. Frequently, no two aircraft are the same. Builders modify, designers change plans, stuff happens. That might have accounted for some of the delay. A countervailing view is that when a series of catastrophic failures occur to similar, if not identical, airframes it might be better to ground first and fly later. There are no easy answers.

Grounding experimental aircraft is nearly unprecedented because they are, well, experimental. Protection for the public and, to a lesser extent, passengers, is the FAA’s primary concern. The agency also has the option of issuing limitations that might include carrying no passengers, avoiding densely populated areas, or reducing airspeed or weights. There’s a fine line between overreaching and being unresponsive. There have only been two widespread groundings of experimental designs in past decades.

The loss of life is tragic and goes beyond the immediate families, although in a much different way. Owners are left to wonder about the safety of their aircraft and the investment that they’ve made. Zenith is the target of lawsuits in addition to experiencing a loss of revenue because of depressed consumer confidence. With the possible exception of the attorneys, there are no winners here.

ASTM is already reviewing its standards and there are likely to be some changes to improve both the products and the process. Is this an indictment of the LSA movement? Absolutely not! The statistics show that if the aircraft are operated intelligently, within design limits, they are safe. Zenith notes that after the modifications are made to the Zodiac CH 601 XL, it will be the most thoroughly reviewed and tested LSA in history. But, it’s a painful lesson for all concerned.

Special acknowledgements go to the NTSB, FAA’s Small Airplane Directorate and Aircraft Certification Division, Chris Heintz and Zenith, the EAA, and AOPA’s Government Affairs Division for providing information for this article.