History of Flight
On March 3, 2009, about 0840 mountain standard time, an experimental amateur-built Stephenson Zodiac 601XL, registration N3683X, collided with mountainous terrain on Antelope Island, approximately 12 miles southwest of Syracuse, Utah. The commercial pilot, the sole occupant, was killed. The airplane was operated as a visual flight rules personal flight under Part 91. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from Tooele, Utah, about 15 minutes prior, with an intended destination of Bountiful, Utah.
Family members alerted the FAA after the airplane was overdue, and an alert notice was issued. Search and rescue personnel discovered the wreckage later that evening in a remote area on the western side of Antelope Island. The wreckage came to rest on sloping terrain about 1,600 feet below a ridgeline.
FAA radar data showed a target departing Tooele and proceeding north toward Antelope Island; altitude data was not available. The final radar target was over Antelope Island at 0838, about 0.5 miles south of the wreckage location. Calculations using radar and meteorological data indicate the flight was steadily progressing northward at 113 knots calibrated airspeed.
Pilot Information
The 37-year-old pilot held a commercial pilot certificate with single-engine, multi-engine, and instrument ratings. His most recent FAA second-class medical certificate was issued on June 19, 2006. His logbook showed approximately 371 hours of flight experience, with 9.1 hours in the accident airplane. The first logbook entry for the accident airplane was dated November 8, 2008, noting the airplane's maiden flight.
Aircraft Information
The airplane was an experimental amateur-built Zodiac 601XL, serial number 6-5136. It was an all-metal, two-place, low-wing monoplane with fixed tricycle landing gear and a 120-horsepower Jabiru six-cylinder engine. According to the pilot's logbook, the airplane had completed nine flights totaling 9.1 hours. The builder reported that control cable tensions were set before the first flight per Zenith Aircraft instructions and rechecked about six flight hours before the accident, with no change noted.
The airplane was assembled from a kit supplied by Zenith Aircraft, Mexico, Missouri. A designated airworthiness representative issued a special airworthiness certificate on November 3, 2008. The pilot was completing phase one flight testing when the accident occurred.
Meteorological Information
Weather conditions at Salt Lake City International Airport (KSLC), about 17 miles southeast of the accident site, at 0853 indicated wind from 160 degrees at 17 knots, visibility 10 statute miles, few clouds at 7,000 feet, broken clouds at 20,000 feet, temperature 13°C, dewpoint 0°C, and altimeter 29.93 inHg. Surface winds near the flight path were generally from the south at 11 to 14 knots with gusts 18 to 22 knots. Turbulence was reported in the area.
Wreckage and Impact Information
Safety Board investigators responded. All structural components and flight control surfaces were located at the main impact site.
Fuselage: The main wreckage consisted of the fuselage, wings, empennage, and engine. Aft crushing with accordion bending was noted along the longitudinal axis. The propeller hub remained attached; both blades were sheared. The engine remained attached. Extensive fragmentation and impact damage were 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 was bent forward; leading edge skin panel rivets were sheared. The rear spar was bent upward. The bottom rear spar cap was buckled near the aileron/flap junction. The upper spar cap remained attached to the fuselage carry-through structure but exhibited multiple bends and 180-degree rotation. The lower spar cap at the attachment point had fractured in tensile static overload. The rear spar web fractured in static overload. The right wing remained attached; leading edge damage was consistent with ground impact. Both rear spar caps exhibited compression buckling near the aileron/flap junction. The type of damage to the rear spars was consistent with aerodynamic flutter rather than aerodynamic static overload.
Empennage: The empennage was bent forward and canted left. Both horizontal stabilizers were bent downward; spar caps were bent and fractured. Impact damage was noted to the vertical stabilizer.
Control Surfaces: The left aileron bell crank remained attached; the push rod deflected the aileron to full trailing edge down. The right aileron push rod deflected the aileron to full trailing edge up. Over travel evidence was present. The left flap had separated from the hinge; the right flap remained attached with nominal damage.
Landing Gear: Main and nose gear remained attached with impact damage.
All fracture surfaces exhibited static overload features with no evidence of metal fatigue, multiple loading, or cyclic motion. No evidence of pre- or post-crash fire was found.
Tests and Research
Aileron/Wing Design: The ailerons were designed to protect from flutter through high control cable tension. After several accidents in Europe, the UK Light Aircraft Association designed and flight tested ailerons with counter balances as a more direct mitigation strategy.
Previous Accidents: The NTSB investigated four other Zodiac 601XL accidents involving structural failure: February 8, 2006 (Oakdale, CA); November 4, 2006 (Yuba City, CA); April 7, 2008 (Polk City, FL); and March 3, 2009 (Antelope Island, UT, this accident).
Structures Study: Extensive examinations were conducted, including flight and static testing, and results from US, UK, Netherlands, and German authorities.
Accident Rates: The fatal accident and in-flight breakup rates for Zodiac 601XLs were compared to general aviation. The in-flight breakup rate was about 200 to 500 times greater. The 601XL group showed distinct evidence of flutter, while general aviation breakups typically involve loss of control or metal fatigue.
Medical and Pathological Information
Toxicological testing detected no ethanol or drugs. An autopsy attributed the cause of death to "Blunt Force Injuries."
Additional Information
On April 14, 2009, the NTSB issued eight recommendations to the FAA, including an urgent recommendation to prohibit further flight of the Zodiac CH-601XL (both S-LSA and experimental) until adequate flutter protection was determined. The Board recommended considering mass-balanced ailerons and addressing cable tension adequacy.
On November 6, 2009, a Zodiac 601XL breakup in Agnos, AR, was attributed to flutter.
On November 7, 2009, the FAA issued Special Airworthiness Information Bulletin (SAIB) CE-10-08 recommending that owners install the manufacturer's structural modification kit to prevent potential catastrophic structural failure.
Also on November 7, 2009, Aircraft Manufacturing and Design, LLC, released a Safety Alert/Directive requiring mandatory upgrades for S-LSA 601XLs before next flight, including wing structure stiffening and aileron counterbalance installation. Experimental amateur-built aircraft were not required to comply but were advised to do so. The directive also recommended checking control cable tension, aileron and flap free play, airspeed indicator accuracy, canopy latching, and luggage security before every flight.