WPR09FA141

The in-flight failure of both wings due to aileron flutter. The aileron flutter was the result of inadequate wing stiffness and the lack of aileron counterbalances.

Radar data and a performance study depicted the airplane flying on a steady northerly course adjacent to mountains at 113 knots calibrated airspeed. The pilot changed course slightly to fly close to or over the mountains. The airplane subsequently experienced an in-flight breakup. All of the airplane's structural components and flight control surfaces were located at the main impact site. There was no evidence of excessive airspeed or maneuvers that would lead to a structural overload and failure of the wings. Turbulence was present in the area, although it was not believed to be severe and likely did not contribute to the failure. An examination of the airplane wreckage revealed that the left wing spar had buckled upward near the fuselage and the left wing had wrapped around the fuselage near the cockpit. There was compression buckling of the lower spar cap of the left wing's rear spar and compression buckling of the upper and lower spar caps of the right wing's rear spar. The compression damage to both rear spars and the upward buckling of the left wing's main spar are evidence of upward and downward bending of both wings. The upward and downward bending of the wings is consistent with aerodynamic flutter. The structural loading at the inboard section of the left wing was further increased as the trailing edges of the outboard sections moved up and down. Ultimately the left wing failed as it bent upward near the root. Aerodynamic flutter can occur when there is insufficient stiffness in the structure or when the flight controls are not mass-balanced. Counterbalanced flight controls can protect less stiff surfaces at higher airspeeds. The ailerons did not have counterbalances, which would have offered direct protection from aerodynamic flutter.

HISTORY OF FLIGHT On March 3, 2009, about 0840 mountain standard time, an experimental amateur-built Stephenson Zodiac 601XL, N3683X, collided with mountainous terrain following the in-flight failures of both wings and an uncontrolled descent on Antelope Island, approximately 12 miles southwest of Syracuse, Utah. The commercial pilot, the sole occupant of the airplane, was killed. The airplane was owned by a private party and was being operated by the pilot as a visual flight rules (VFR) personal/pleasure flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the flight. The flight originated from Tooele, Utah, approximately 15 minutes prior to the accident with an intended destination of Bountiful, Utah. Family members alerted the Federal Aviation Administration (FAA) that the airplane was overdue and an alert notice (ALNOT) was issued. Search and Rescue personnel discovered the wreckage later that evening in a remote area on the western side of Antelope Island, Utah. The wreckage came to rest on sloping terrain, approximately 1,600 feet below a ridgeline. According to FAA radar data, a radar target associated with the accident airplane departed Tooele and proceeded north toward Antelope Island; altitude data was not available. The final radar target was depicted over Antelope Island at 0838 about 0.5 miles south of the wreckage location. Calculations using both radar and meteorological data indicate that the flight was steadily progressing northward at 113 knots calibrated airspeed (KCAS). Details can be found in the Antelope Island Performance Study located in the public docket for this accident. PERSONNEL INFORMATION The pilot, age 37, held a commercial pilot certificate with airplane single engine land, multi-engine land and instrument ratings. His most recent FAA second class medical certificate was issued on June 19, 2006. Examination of the pilot's logbook revealed that he had accumulated approximately 371 hours of flight experience, 9.1 hours of which were in the accident airplane. The pilot's first logbook entry for the accident airplane was dated November, 8, 2008. The pilot noted within that entry that it was the airplane's maiden flight. AIRCRAFT INFORMATION The airplane was an experimental amateur built Zodiac 601XL, serial number 6-5136. The all-metal 2-place airplane incorporated a low-wing, flexible skin ailerons, and fixed tricycle landing gear. The airplane was equipped with a 120 horsepower Jabiru six-cylinder engine. According to the pilot's logbook, the airplane had completed 9 flights totaling 9.1 hours of flight time. The builder of the airplane reported that the tensions of the control cables were set before the first flight in accordance with instructions provided by Zenith Aircraft, the kit manufacture. The cable tensions were then rechecked approximately 6 flight hours before the accident flight and no change was noted. The accident airplane was classified as an experimental category amateur-built airplane, and was assembled by the owner from a manufactured kit supplied by Zenith Aircraft, Mexico, Missouri. A designated airworthiness representative (DAR) issued the airplane a special airworthiness certificate on November 3, 2008. The pilot was completing phase one flight-testing when the accident occurred. METEOROLOGICAL INFORMATION The weather conditions reported at Salt Lake City International Airport (KSLC), located about 17 miles southeast of the accident site, at 0853, indicated wind from 160 degrees at 17 knots, visibility of 10 statute miles, a few clouds at 7,000 feet, broken clouds at 20,000 feet, temperature 13 degrees Celsius (C), dewpoint 0 degrees C, and an altimeter setting of 29.93 inches of Mercury. Surface winds in the areas near the airplane's flight path surrounding Salt Lake were generally from the south at 11 to 14 knots, with gusts from 18 to 22 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. WRECKAGE AND IMPACT INFORMATION Safety Board investigators responded to the accident. All of the airplane's structural components and flight control surfaces were located at the main impact site. A complete copy of the examination notes are contained in the public docket for this accident. Fuselage The main wreckage consisted of the fuselage, wings, empennage and engine. Aft crushing with accordion type bending features was noted along the longitudinal axis, from the engine compartment, aft to the empennage. The propeller hub was in place and remained attached to the crankshaft propeller flange. Both propeller blades were sheared at the hub. The engine remained attached to the mounting assembly and associated firewall structure. Extensive fragmentation and impact related damage was noted to the instrument panel and cockpit controls. Wings The left wing was buckled, bent upward, and folded over the cockpit. The upper spar cap, approximately 3 feet from the wing to fuselage attachment point, was bent forward relative to the lateral axis. The upper leading edge skin panel rivets were sheared in multiple locations along the forward spar. The upper spar chord, aft facing flange, was bent down and pressed flat against the aft side of the spar cap and associated spar web. The rear spar was bent upward approximately one foot outboard of the rear spar fuselage attachment bolt. The bottom rear spar cap was buckled near the aileron/flap junction. The upper wing skin exhibited compression wrinkling (mid span) from the upward bend in rear spar, diagonally, outboard and forward, to the front spar. The upper spar cap remained attached to the fuselage carry thru structure. There were multiple bends in the cap with associated 180-degree rotation of the cap sections, in a counter clockwise direction when viewed looking outboard. The spar cap was separated from the spar web. The lower spar cap at the wing to fuselage attachment point consisted of two flat non-flanged spar caps. Both had fractured in tensile static overload about 2-inches outboard of the attachment point. The fractures corresponded with the location of the outboard sections of the spar. The lower portion of the spar, below the fastener location, exhibited upward and aft bending. The upper portion of the spar, above the fastener location, exhibited upward and aft bending. The aft bending was more pronounced on the lower fracture, below the attachment fastener. The rear spar web remained attached to the fuselage at the rear attach point. The spar web fractured, in static overload, in an upward and outboard direction, at the attachment hole. The portion of the rear spar that fractured remained attached to the wing and was deformed and bent around the head of the rear spar attachment bolt. The right wing remained attached to the fuselage at both the forward upper and lower spar caps. The leading edge exhibited damage consistent with ground impact. The middle and outboard sections of the wing exhibited severe compression damage to the leading edge. The upper and lower wing skins were ballooned outward and separated from the wing ribs. The top and bottom rear spar caps were buckled near the aileron/flap junction. The rear spar web fractured in static overload at the attachment hole in a forward direction. Both rear spar caps exhibited compression damage at the area where the flap and aileron met. The compression buckling on both the top and bottom spar caps of the right rear spar was consistent with the outboard section of the trailing edge bending both upward and downward. The compression buckling on the bottom spar cap of the left rear spar was consistent with the outboard section of the trailing edge bending downward. The bending trailing edge spars introduced large torsion loads int