Safety Pilot: No one wins

June 1, 2009

Bruce Landsberg was named president of the AOPA Air Safety Foundation in January 2009.

The NTSB recently issued an urgent request to the FAA to immediately ground all Zenair CH 601XL light sport aircraft. Highly unusual, and it’s bound to raise more than a few eyebrows about more than just the CH 601XL. Some in the community openly oppose the concept of a factory-built Special Light Sport Aircraft (SLSA). The blogosphere has been active with all manner of suppositions and slurs. Mark Twain suggested getting the facts first and then distorting them as much as you like. So, let’s look at what is known at this point.

There have been six Zenair in-flight breakups since 2006 resulting in 10 fatalities. On February 8, 2006, an experimental amateur-built CH 601XL crashed near Oakdale, California, after its wings collapsed as it entered the traffic pattern. A witness saw the wings “visibly vibrate” before the left wing collapsed and folded back against the fuselage. The aircraft spun to the right. The probable cause was wing structural failure for undetermined reasons.

On November 4, 2006, a factory-built CH 601XL broke up in flight while cruising near Yuba City, California. Radar showed level flight at 2,600 feet agl and 106 knots groundspeed when the aircraft began to climb at 700 feet per minute to 2,800 feet agl and descended rapidly. Probable cause was structural failure of the horizontal stabilizer and wings for undetermined reasons.

On February 5, 2008, a homebuilt version crashed near Barcelona, Spain, in descent just before landing. The right wing failed and witnesses observed the wings vibrate prior to folding. This accident is still under investigation. On April 7, 2008, a factory-built version broke up in flight near Polk City, Florida. A witness saw the aircraft bank slightly left and right followed by significantly steeper angles when it suddenly yawed right and the right wing folded up. Another crash in the Netherlands on September 14, 2008, involved a kitbuilt aircraft. The airplane was in level flight at 1,000 feet agl when the right wing folded up over the fuselage.

The most recent loss was a homebuilt version on March 3, 2009, which broke up in flight cruising near Antelope Island, Utah. Radar showed the airplane in steady flight at 112 knots. The winds were estimated at 14 knots, gusting to 20 with turbulence reported in the area. There are no known observers of the accident. While the accident is still under review, according to the NTSB, “preliminary examination of the wreckage indicates that the breakup sequence began with the buckling 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-601XL wing performed in the Czech Republic by CZAW for certification purposes.”

Without going too deeply into the engineering, the NTSB suspects that these aircraft may have experienced aerodynamic flutter. The NTSB notes that, “Aerodynamic flutter is a type of dynamic aeroelasticity that occurs 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 in the airplane. These vibrations can quickly lead to structural failure if not sufficiently damped.”

The NTSB has voiced other concerns on the design and construction of this model, and the FAA will likely review a variety of areas.

Light sport aircraft must weigh less than 1,320 pounds with a maximum cruise speed of 120 knots. They can be kit-built or factory-built. Special Light Sport Aircraft (SLSA) are the factory-constructed version and must comply with an American Society of Test and Measurements (ASTM) consensus standard. The industry, in collaboration with the FAA, sets the rules for construction. According to the FAA, “The expected level of safety of an LSA is not the same as Part 23-certificated products, nor is the level of FAA oversight the same.”

That doesn’t mean they are unsafe by any means, but the FAA does not issue a type or production certificate. It does, however, require that the aircraft conform to the ASTM standards; the manufacturer must submit in writing that it complies and be able to show that it has tested the aircraft to meet flight performance and construction requirements. The FAA is in the process of assessing various manufacturers, domestic and abroad, for compliance. The rule provides a middle ground of basic aircraft at reduced cost and complexity that should be safe to fly.

Going beyond the Zenair accidents, safety in the light sport area is doing reasonably well. Landing mishaps seem to be in higher proportion, although fatal weather-related accidents are quite a bit lower than Standard or Part 23-certificated aircraft. That is exactly in line with the category’s expected use of staying close to home with relatively little exposure to weather on cross-country flights. With the recent exception of the CH601XL there has been little to suggest any systemic problems.

This is a setback for Zenair, but we certainly shouldn’t write off the whole LSA concept based on these mishaps. Students of history know that even certificated aircraft, some built by the most experienced manufacturers in the business, also can sustain catastrophic failure. In 1978 Cessna introduced the Conquest II or C441 twin turboprop, but a design flaw that did not come out in flight testing crept into the production model. A demo aircraft was lost with seven people on board when a trim tab actuator failed and the elevator went into flutter mode, resulting in the loss of the tail. Cessna grounded the fleet and added a second trim tab actuator. However, there was more to it, and shortly after the first fix another demo pilot sensed an unusual vibration and was able to land safely. The tail needed additional strengthening and was significantly redesigned. Problem solved, but it took several tries.

Beech built V-tail Bonanzas for decades before it was discovered that there was too much flex in the ruddervators, which resulted in the loss of several aircraft. In several cases the aircraft were already in an unusual attitude and possibly outside the approved flight envelope when the failure occurred. An airworthiness directive to strengthen the tail was published, and that problem was resolved.

There are other examples of emergency ADs being issued to correct critical flaws that wriggled through a rigorous design and certification process. Many designs suffer early in the production run while for others it takes awhile for the glitch to appear. Obviously, both certification systems and design concepts don’t always work as intended. The marketplace tends to be harsh if the parties involved respond grudgingly. Usually, it’s time to get back to the drawing board and the shop floor to reengineer the misbehaving parts. ASTM, the industry, and the FAA will also want to look at the consensus standard guidance to see that it’s sufficiently comprehensive. No one wins when we wipe out customers, aircraft models, or a category.