History of Flight
The pilot completed normal pre-flight procedures and lined up on the center of Runway 26 at Brunton Airfield. The metalled runway surface was dry, 46 meters wide, in good condition, and free of visible debris. To the left of the runway, a six-strand wire fence on wooden posts ran parallel to the edge. Weather was good, with calm surface wind and CAVOK conditions.
During the takeoff roll, the pilot applied power smoothly and continuously. The aircraft accelerated straight with no deviation. The tail wheel lifted off normally, and as the aircraft became light on the main landing gear, the pilot glanced at the airspeed indicator, which read 40 KIAS. At that moment, the aircraft veered to the left, and the pilot could not prevent it from departing the left side of the runway. The left wing struck the fence, causing the aircraft to spin left and continue backwards through the fence before coming to a stop. The pilot then selected the fuel off and vacated the aircraft through the normal port-side door. The aircraft was broken into two pieces; the pilot was uninjured.
Cable Damage
After the accident, the pilot checked the controls and found all except the rudder cables to be serviceable and correctly rigged. The rudder cables had shorteners (length-adjusting devices) fitted at their aft ends near the rudder operating arm. The right cable had failed completely at the cable shortener, and the left cable had partially failed at a similar location.
Metallurgical Examination
The cables, operating arm, and shorteners were examined by the AAIB. All strand failures resulted from tensile failure consistent with mechanical overload, and all failures occurred where the cables had contacted the shorteners. Both cables were tested for tensile strength with and without the shorteners. The tests showed that both cables had an ultimate strength greater than 460 kgf, but the presence of the shorteners reduced the effective strength by about 20%. The manufacturer's guaranteed minimum cable strength was 380 kgf.
Analysis
The pilot had conducted four uneventful takeoffs in the previous two days, including one in a strong crosswind, and was not aware of any change in handling technique. He could not identify any specific event or action that might have led to the accident. The pilot speculated that after leaving the runway, the main wheel might have dug into a rut at the edge of the field, causing the engine compartment to break off to the right due to sideways force on the extended landing gear. Given the cable strengths, it is likely that the damaged right cable failed as the aircraft structure disrupted. The shorteners used were of proprietary manufacture, featuring cable contact over sharp plate edges, which markedly reduced effective strength. The aircraft manufacturer recommends a different type of shortener in which the cable passes around bolts rather than over sharp edges.