Light Sport Breakups

April 15, 2009 by Bruce Landsberg

This just in – The NTSB has issued an unusual request to FAA to immediately ground the Zenair CH601XL, a special light sport aircraft (S-LSA) . NTSB identified 6 fatal accidents involving in-flight breakups and resulting in 10 fatalities since 2006. Two of the breakups occurred outside the U.S.

This is a sobering development in the burgeoning LSA market and one that the “special” designation was hoping to address. “Special” means that the aircraft are built in a factory to ASTM consensus design standards (originally American Society for Testing and Materials). This organization was chosen as a simple, less expensive and extensive alternative to FAA certification. A cynic might say that we should stick with tried and true methods. Of course, if that were the case airplanes might not have been invented and I point out that there have been some spectacular failures in FAA-certificated aircraft. Several of the aircraft were kits so there’s a mix between the factory vs. homebuilt and it appears that if the designer’s specs are not followed exactly, bad things can happen.

We need to learn more. NTSB pointed to flutter and control forces as areas of interest. According to NTSB, “The stick force gradient – a measure of the force applied to the control stick and the increase in lift that results – was not uniform throughout the range of motion, particularly at high vertical accelerations or Gs. The lessening of the gradient at high Gs could make the airplane susceptible to being inadvertently over-controlled by the pilot, which could create a condition in which the airplane is stressed beyond its design limits leading to an in-flight structural failure.”

The NTSB does not have the authority to ground the aircraft but the FAA does. Six breakups in seemingly routine flight does not instill confidence so it’s time to get to the facts – quickly and unemotionally.

The safety record of S-LSA aircraft has been reasonable up to this point with no particular aircraft failure modes. ASF has tallied many landing accidents but this seems to relate to pilot skill as opposed to hardware issues.

This a setback for Zenair, and their family of owners and pilots. We encourage a thorough and timely investigation to resolve this.

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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  • Bob Davidson

    That ought to reduce the price on existing models of Zenair. Nows the time to buy at rock bottom prices and wait for the fix.

  • Stephen Wilson

    It’s apparent that if you fly one of these, you’re a credulous test pilot.

  • Rick Lindstrom

    This is not a new issue, we Zenith 601 builders/pilots have been actively researching this “problem” for several years. At this time, there is no identifcation of one single causal factor for the wing failures when the 601 airframe is overstressed in flight. The FAA has concurred with the manufacturer that the aircraft is safe to fly as long as the aileron cable tensions are within spec, and the airframe is operated within its designed flight envelope. The recent NTSB report, unfortunately, relies on hearsay and “eyewitness observations” for many of it conclusions, with very little scientific data.

    No one wants to fly a “deadly” (as Mr. Wilson calls it) airplane. His bias for Cessna products is profoundly displayed here, and his ill-chosen comments do nothing but harm the image of all LSA aircraft in general, and the 601 specifically. (BTW, Steve, the Skycatcher crashes wouldn’t have the happy endings they did without the full deployment of the BRS systems that saved the pilot’s bacon in each instance. Why didn’t mention that very pertinent fact?)

    Personally, I’m glad the NTSB has brought this issue back to the forefront. As a 601 pilot, I’d like to see some concrete conclusions reached after all the data is gathered and distilled. In the mean time, however, I’ll check my cable tensions, fly the airplane as it was designed, and enjoy the magic of flight.

  • Mack Kreizenbeck

    This problem will soon pass — resulting in the CH601XL being the most tested and safest LSA out there. This, in itself, will give current and future owners a higher degree of confidence in the bird.
    Cessna is having its problems and Beech (doctor and attorney killer), Ercoupe, as well as others, have had their turn at remedying similar opportunities for a cure.
    I have utmost faith in the airplane and the factory.

  • Terry Bounds

    I am a long time fan of the Zenair line and a retired Blacksmith. I shied away from building a 601 when I was informed that the design does not have aileron hinges but rather used the uper wing skins as a sort of, flexing hinge. I am not an engineer but my experience tells me that constant flexing of any metal will cause cracks. I could imangine that partial loss of an aileron (the same reason we check the aileron hinges in pre-flight) could cause structural failure of the wing. If I were an owner, I would consider re-engineering the wing to include hinges.

  • Steve Look

    The flex aileron hinge is probably the most thoughly tested part of the 601XL. A convention hinge option is available for the sceptical. I have not seen anything to suggest the hinge has any part in this investigation. There is already quite enough of unfounded speculation as it is.

  • David Hiser

    I am building a 601XL at this time. I purchased the kit knowing there was some history involved, however there was never conclusive proof what was going on and appeared to be based on rumors from back yard engineers. I decided to move forward thinking that the FAA would step in if the facts reflected the need. I am a bit bummed for myself and Zenith, but in the long run it will be best for the safety of the 601 fleet as well as all LSA’s. In the mean time I have my tail feathers done and 75% of my wings. I will move ahead slowly as we follow this.

  • Isaac

    Just proceed safely, the FAA said fly inside the evelope and its a good design.

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