Aircraft: Rans S12-XL. Injuries: None. Location: Gordon, Ala. Aircraft damage: Substantial.
What reportedly happened: The pilot reported that the takeoff was normal, but as he accelerated to cruise airspeed the elevator suddenly began to flutter violently, shaking the airplane.
He reduced airspeed, but the elevator continued to flutter. The control column was moving rapidly forward and aft about six inches, which would correspond to an elevator up and down movement of about 20°.
The pilot made an emergency landing in a cow pasture because he was worried about a structural failure. Upon touchdown, the nosewheel dropped into a hole, and the airplane nosed over.
The post-accident examination revealed that 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. 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 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.
Probable cause: 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.
NTSB Identification: ERA13LA052
This November 2012 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.