Gordon, AL, USA
N933CW
WALLER CHARLES R RANS S12-XL
As the pilot leveled the airplane after departure and accelerated to cruise airspeed, the airplane's elevator suddenly began to flutter violently, shaking the airplane. The pilot reduced airspeed in an attempt to stop the elevator from fluttering, but the elevator continued to flutter. As a result of the flutter, the control column was moving rapidly forward and aft about 6 inches, which would correspond to an elevator up and down movement of about 20 degrees. The pilot decided to make an emergency landing in a cow pasture because he was worried about a structural failure. Upon touchdown on the cow pasture, the nosewheel dropped into a hole, and the airplane nosed over and came to rest inverted. Postaccident examination revealed that the fuselage was substantially damaged, and the trim push wire that actuated the elevator trim tab was fractured. Examination of the trim push wire revealed that it had been secured to the trim tab by a wire swivel/screw stop that was attached to the trim horn. Further examination revealed that the trim push wire was bent and had fractured just forward of the wire swivel/screw stop. The fracture face displayed corrosion and striations consistent with fatigue. Examination of the trim horn on which the wire swivel/screw stop was mounted also revealed wear, scratching, and gouging. The observed damage was consistent with the wire swivel/screw stop binding during operation and being unable to rotate under load. Airplane manufacturer personnel indicated that they observed a similar failure when spectators at an airshow leaned against a trim tab on an airplane that was on display, bending the trim push wire, which subsequently failed in flight.
On November 10, 2013, at 0843 Central Standard Time, an experimental amateur built RANS S-12XL, N933CW was substantially damaged during a forced landing near Gordon, Alabama. The sport pilot and his passenger were uninjured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight conducted under Title 14 Code of Federal Regulations Part 91 which departed a private airstrip near Gordon, Alabama, destined for a private airstrip in Ashford, Alabama According to the pilot, he departed from a private turf airstrip near Gordon, Alabama. After climbing to an altitude of 350 above sea level he leveled off. Approximately one mile north of the airstrip, while cruising at an indicated airspeed of 60 miles per hour (mph), the airplane's elevator suddenly began to flutter "violently" shaking the whole airplane. The pilot reduced his airspeed to 50 mph in an attempt to stop the elevator from fluttering but was unsuccessful. He then decided to make an emergency landing in a cow pasture as he was worried that structural failure could occur as the flutter kept rapidly moving the control stick forward and aft approximately 6 inches and the elevator was rapidly moving up and down about 20 degrees. Upon touchdown on the cow pasture the nosewheel dropped into a hole and the airplane nosed over and came to rest inverted. The passenger and the pilot then undid their seatbelts, fell out of their seats, and exited the airplane. PERSONNEL INFORMATION According to Federal Aviation Administration (FAA) and pilot records, the pilot held a sport pilot certificate and repairman certificates for the RANS S-12 and S-14. He reported that he had accrued 2,000 total hours of flight time with 221.9 hours in the accident airplane make and model. AIRCRAFT INFORMATION The accident airplane was a high wing, pusher configured, monoplane. The cockpit area was constructed of welded 4130 steel tubing and the rear fuselage was constructed of bolted 6061-T6 aluminum tubing. The wings were also built of aluminum tubing and aluminum ribs. The fuselage, wings, and tail surfaces, were covered in Dacron fabric. The airplane was equipped with wing mounted flaps, and tricycle landing gear. It was powered by a 100 horsepower, Rotax 912 ULS engine, driving a Warp Drive, 3-bladed, carbon fiber propeller. According to FAA and maintenance records, the airplane received its experimental airworthiness certificate in 2002. Its last conditional inspection was completed on August 3, 2012. At the time of the accident the airplane had accrued 221.9 total hours of operation. METEOROLOGICAL INFORMATION The reported weather at Dothan Regional Airport (DHN), Dothan, Alabama, located 21 nautical miles northwest of the accident site, at 0853, recorded: winds 100 degrees at 3 knots, 10 miles visibility, clear sky, temperature 17 degrees C, dew point 7 degrees C, and an altimeter setting of 30.38 inches of mercury. WRECKAGE AND IMPACT INFORMATION Post accident examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the nose wheel landing gear assembly had collapsed, the tubing of the forward fuselage structure was substantially damaged, and one of the three propeller blades was broken. Control continuity for the control surfaces was able to be established to the cockpit however, it was discovered the cable that actuated the elevator trim tab was fractured. TESTS AND RESEARCH Mechanical Trim Tab System The RANS Mechanical Trim Tab System featured a trim tab actuated by a push-pull wire known as the "trim push wire". By turning the trim wheel, the wire would move the trim tab. The trim tab was constructed of sheet aluminum and could be adapted to the trailing edge of many experimental aircraft. The trim tab was sized for both low & high speed aircraft. Examination of the system revealed that the trim push wire was not braided and was of a solid core design. It rode inside of a trim cable housing until it reached the right elevator trailing edge spar, at which point it exited the end of the trim cable housing and was secured to the trim tab by a wire swivel/screw stop that was attached to the trim horn. Further examination of the system revealed that the trim push wire was bent, and had fractured just forward of the wire swivel/screw stop. Examination of the fractured end of the trim push wire revealed that it displayed corrosion, and striations which were indicative of fatigue. Examination of the faces of the trim horn that the wire swivel/screw stop was mounted on also revealed wear, scratching, and gouging which was indicative of the wire swivel/screw stop, binding when in operation and being unable to rotate under load. Previous Failure After recovery of the airplane, photographs of the fractured trim push wire were forwarded to the airplane manufacturer for further examination. They advised the NTSB that they also believed the condition of the fractured face of the wire indicated that the wire had suffered a fatigue failure. They also advised that they had observed a similar failure when spectators at an airshow had leaned up against a trim tab on an airplane that was on display bending the trim push wire which then failed in flight. ADDITIONAL INFORMATION In order to improve safety, on May 24, 2013, RANS Design Inc., issued Operational Alert 173 (OA-173), requiring mandatory compliance with the alert for all models of the manufacturer's airplanes which utilized a push-pull wire trim system, recommending in part to: • "Inspect during pre-flight. Look closely at the wire where it terminates at the wire swivel. This should be secure and properly assembled with appropriate hardware. The intent of the design is to capture the wire with the wire swivel, while allowing the wire swivel to pivot freely in the trim tab horn. IMPORTANT: The wire swivel must pivot freely when the trim wire is final installed with the wire swivel retaining screw fully tightened." • "Replace any suspect push-pull wire with proper spec wire." • "If the push-pull wire has been bent, it needs to be replaced." • "Consider installing an electric trim servo."
A fatigue failure of the elevator pitch trim cable, which resulted in aerodynamic flutter of the elevator in flight and the airplane’s subsequent nose-over during the resultant precautionary landing.
Source: NTSB Aviation Accident Database
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