NTSB: Planes may break apart.
Zodiac XL. Photo by Zenith Aircraft Comany.
In a letter to the Federal Aviation Administration (FAA), the National Transportation and Safety Board (NTSB) has asked that all flights of Zenith Aircraft Company’s (Zenair) Zodiac CH-601XLs be prohibited until a few urgent safety issues are resolved. After evaluating six accidents, the NTSB determined the kit-built plane was susceptible to in-flight break ups. No, I’m not talking about some insignificant piece of airplane falling to the ground leaving you with a gaping hole on your fuselage; what the NTSB is saying is that you’ll be flying along on your Zodiac and suddenly the wing of the thing folds over the fuselage or the tail breaks off. The NTSB advised the FAA to prohibit the flights and also sent a letter to American Society for Testing and Materials (ASTM) International, the organization responsible for determining design, manufacturing, and flight standards of light sport aircraft (LSA) regarding changes they think should be made to the LSA certification standards. The CH-601XLs are mostly certified as LSA now, but used to get an experimental airworthiness status before.
According to the NTSB, the flight control surfaces of the airplanes can develop aerodynamic flutter under certain conditions that, if left unchecked can, basically, vibrate the aircraft apart. Aerodynamic flutter is just what it sounds like and refers to when something (in this case the control surfaces such as the ailerons, elevator, or rudder) in the airplane begins to flutter as a result of a certain flight condition or pilot control input. As the fluttering or vibrations get more pronounced, the allowable load factors on the airplane’s structure are exceeded and the aircraft breaks apart. The video above shows the effects of flutter on a remote control airplane and let me assure you that the results will be similar on a regular airplane if it develops a violent flutter condition.
This isn’t the first time the 601XLs have been called out for this aerodynamic flutter; in October of last year, Zenair’s European division alerted pilots to this problem and stated that it was primarily due to improper tension on the flight control cables. The improper tension would fail to dampen the fluttering and result in the in-flight break-ups.
Since 2006, there have been at least six accidents involving 601XLs that were due to in-flight break-ups and ten people are dead as a result. The accidents are as follows:
1) February 8, 2006 - N105RH - crashed near Oakdale, California after the wings collapsed soon after the airplane entered a traffic pattern at an airport. Two people were killed.
2) November 4, 2006 - N158MD - broke up while cruising near Yuba City, California. Two people were killed when the horizontal stabilizer and then the wings broke off the plane.
3) February 5, 2008 - EC-CMJ - the wings folded up just before the airplane was supposed to land. Two people were killed and witnesses reported seeing the wings vibrate before the crash.
4) April 7, 2008 - N357DT - aircraft broke up in flight near Polk City, Florida. The right wing folded over the plane as the pilot was unable to correct for oscillating flight controls. One person was killed.
5) September 14, 2008 - airplane was in level flight when the right wing folded over the fuselage, killing two people.
6) March 3, 2009 - N3683X - airplane broke up in flight while cruising near Antelope Island in Utah. The pilot was killed.
An in-flight break-up is about as serious as a problem can get for a pilot; it’s an irrecoverable flight condition that virtually guarantees death. As was said by NTSB’s chairman Mark V. Rosenker, if the issue is not addressed immediately, more people will die. The only time your wing is supposed to be folding over the fuselage is if you’re parking the thing in an aircraft carrier (and the plane has wings that intentionally fold). The CH-601 XLs propensity for flutter in its design isn’t the only thing the NTSB pointed out with the aircraft. The organization also pointed out that the stick force gradient (basically a measure of how the pilots inputs translate to forces on the flight control surfaces) in the airplane was uneven as the stick was moved through its range of motion. The uneven force through its range of motion could also lead to structural failure conditions. To top it all off, the NTSB said that the airspeed indicating system on the aircraft commonly displayed the wrong airspeed for the airplane. This is a serious problem as there are airspeeds that should not be exceeded and passing them can also lead to structural failure in the aircraft.
As I mentioned before, most of the newer CH-601XLs are certified under LSA requirements. The advantage of LSA is that it has stimulated the aircraft manufacturing sector by allowing airplanes to be designed and manufactured under lower standards, which lowers the cost of developing an aircraft program. There are limitations as to what planes may be LSA certified, but suffice it to say there are many LSA designs out there today. Of course, it is hard for the general public, who perceives that it’s a miracle light planes can stay up in the first place, to understand why the FAA would allow a “lowered” certification standard, but LSA requirements aren’t actually all that lacking.
LSA aircraft airworthiness is determined by a set of standards determined and published by ASTM International. The NTSB sent ASTM a letter regarding the standards and addressing the need that they be strengthened. The recommendations specifically targeted the flutter issue on flight control surfaces, as well as the stick force gradient and airspeed indicator problems.
The NTSB is not a regulatory agency, so it’s unable to issue its own rules and requirements. The task of addressing the problem with the CH-601XL falls upon the FAA and the airplane’s manufacturer. While the FAA is known for putting some safety issues on hold, I’m sure something will be done to address this problem with the CH-601XL. It is not often that the NTSB issues this kind of urgent, ground-all-planes warning and I’m sure the FAA will not take it likely. Will there be a lot of upset owners out there who will be severely ticked off about not flying their planes or that their kits are defective in some way? Of course, but this is a safety issue and it should be addressed sooner rather than later. The responsibility will fall upon Zenair to quickly solve the problem with their design (design-wise, flutter isn’t all that difficult to stop). If the company really cares about its pilots it will be working on this already and not on stopping the NTSB recommendation (after all, Zenair’s lawyers may be busy with the slew of lawsuits these findings will probably cause). For Zenair, this will be a lesson similar to what other aircraft companies have dealt with in the past as they matured. CH-601XL pilots will just have to suck it up and wait for the modifications to come through. They need to understand that while they may be willing to fly an airplane with a known severe flutter possibility, this is a risk that may not be understood by their passengers and may even put in jeopardy people and property on the ground that have nothing to do with the pilot’s decision. Still, you have to wonder what will happen to flight schools that use this plane or other “for-money” operations. What will they do in the meantime?
Some may say this problem highlights problems in the LSA certification process and it does. However, it should not be used to thwart the continued use the LSA program and its aircraft, but rather to strengthen the process by which LSA aircraft are produced and certified and plugging the “leaks” in ASTM’s standards. I’ll do my best to keep you informed of how the FAA, Zenair, and ASTM International react to all of this.
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