HISTORY OF FLIGHTOn August 8, 2012, about 1230 mountain daylight time, an experimental amateur-built Santana PA-18 Replica, N3AK, departed the paved surface of runway 34 after touchdown at the Ronan Airport (7S0), Ronan, Montana. The private pilot/owner, the sole occupant, was not injured. The airplane sustained substantial damage to the left wing. The personal flight was operated under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the local area flight, and no flight plan had been filed.

According to the pilot, he took off from runway 34, and flew a left-hand traffic pattern for a landing back onto the same runway. He reported that he encountered some "convective turbulence" while in the traffic pattern, but the circuit was otherwise normal and uneventful. The pilot made a wheel landing, which was also normal, but immediately after he lowered the tailwheel to the runway, the airplane veered to the right, and exited the paved runway surface. It came to a stop in the grass beside the runway, with the engine still running. The pilot shut down the engine and exited the airplane. The pilot's examination of the airplane just after the accident revealed that both the left and right tailwheel steering springs were disconnected from their respective chains on the left and right lower rudder horns. PERSONNEL INFORMATIONFederal Aviation Administration (FAA) records indicated that the pilot held a private pilot certificate with airplane single-engine land and sea ratings. According to information provided by the pilot, he had approximately 1,964 total hours of flight experience, which included approximately 42 hours in the accident airplane make and model. His most recent flight review was completed in May 2011, and his most recent FAA third-class medical certificate was issued in November 2010. AIRCRAFT INFORMATIONThe airplane was built by the pilot/owner from a kit provided by Backcountry Super Cubs, and was a replica of a Piper PA-18 airplane. The airplane was equipped with a Lycoming O-360 series engine, and a fixed-pitch wood/composite propeller. Cockpit seating was tandem, and the landing gear configuration was conventional (tailwheel). The FAA issued the airworthiness certificate in October 2011. At the time of the accident, the airplane had accumulated a total time in service of about 47 hours. METEOROLOGICAL INFORMATIONAccording to information provided by the pilot, at the time of the accident, the winds were light and variable, with no gusts. The sky was clear at the time, and the pilot reported the visibility was about 10 miles. The pilot estimated the temperature to be about 85 degrees Fahrenheit. AIRPORT INFORMATIONThe airplane was built by the pilot/owner from a kit provided by Backcountry Super Cubs, and was a replica of a Piper PA-18 airplane. The airplane was equipped with a Lycoming O-360 series engine, and a fixed-pitch wood/composite propeller. Cockpit seating was tandem, and the landing gear configuration was conventional (tailwheel). The FAA issued the airworthiness certificate in October 2011. At the time of the accident, the airplane had accumulated a total time in service of about 47 hours. WRECKAGE AND IMPACT INFORMATIONAn FAA inspector examined the wreckage on scene, shortly after the accident. The airplane came to rest upright, facing east, approximately 150 feet off the east side of runway 34. The left main landing gear was crumpled under the airplane. The left wing was bent up about 15 degrees just outboard of the midspan point, and both propeller blades were fracture-separated from the hub just outboard of the spinner. The airplane skid-marks intersected a concrete drain fixture in the grass beside the runway; contact with this fixture collapsed the left main landing gear and resulted in wing and propeller ground contact.

Normally a spring and chain assembly was employed on each side to connect the tailwheel steering arm to its same-side rudder horn; this enabled pilot control of the tailwheel in concert with rudder deflection. Both tailwheel chains were found disconnected from their respective springs. The end link of one chain was fractured, while the end link of the other chain was intact. However, the chains were found to be of different lengths, which could be consistent with the failure and loss of one or more links. The investigation was not provided with, and was unable to subsequently acquire, information regarding which chain exhibited the fractured link, the length difference between the chains, or which chain was shorter. ADDITIONAL INFORMATIONTailwheel Information

According to the pilot, at the time he was building the airplane, the kit manufacturer did not specify any particular tailwheel model or associated hardware. The tailwheel components used by the pilot at the time included an "Alaskan Bushwheel" version of the Scott 1200 tailwheel, and aircraft-grade springs and chains from Wag-Aero. The pilot noted that the tailwheel was always very difficult to "break out" of its center detent or other positions, but because he was not convinced that that condition was abnormal, he did not follow up with any examinations or evaluations.

In late September 2012, subsequent to the accident, the pilot transported the airplane to Alaska, and moved there at some time thereafter. In Alaska, the airplane was repaired and restored to an airworthy condition by a repair facility that specialized in similar airplanes. According to the repair facility owner, when he disassembled the accident tailwheel, its internal pivot mechanism "looked like a bomb exploded." He explained that several springs and internal components were fractured, and that at least one spring was jammed in a position/location, which prevented the tailwheel from pivoting. The repair facility owner was of the opinion that that damage was not the result of the accident; he believed that the damage was the result of undetermined deficiencies or defects with the as-manufactured tailwheel. He was also of the opinion that the damaged and malfunctioning tailwheel was the likely cause for the pilot's loss of control.