What happened
On April 11, 1964, a pilot conducting a long-distance glider flight initiated an emergency landing near Lüterswil/Biezwil, Switzerland. The flight, which began from Bleienbach airfield as part of a national competition, had lasted approximately seven hours. While flying at an altitude of 1,200 meters, the pilot suddenly experienced a total loss of pressure on the right rudder pedal. The sudden mechanical failure caused the DFS-Weihe HB-484 to enter a spiral dive, though the pilot managed to recover the aircraft's attitude. Due to the severely restricted controllability, the pilot performed an immediate emergency landing in a field. Upon touchdown, the aircraft veered left, crossed a road, and overturned over an embankment before coming to a halt against trees. The pilot escaped the crash uninjured, but the aircraft sustained heavy damage.
The investigation
Investigators examined the wreckage of the HB-484 to determine the cause of the control failure. The investigation focused on the rudder control system, which utilizes two independent steel cables and return springs. The examination of the debris revealed that the right rudder cable had snapped approximately 30 cm from the rudder connection near a guide pulley. The fracture zone showed significant crushing and abnormal wear, with roughly half of the cable strands displaced and tangled. Furthermore, the investigation identified a 2 mm deep notch on the trim tab fork, which had caused a corresponding wear point on the right rudder cable. The investigation also looked into the assembly process of the stabilizer (stabilo) performed by the ground crew prior to the flight.
Findings
way The investigation established that the partial loss of control was caused by the fracture of the right rudder cable due to improper assembly of the stabilizer. During the assembly of the stabilizer, the trim tab fork was incorrectly positioned between the fuselage wall and the right rudder cable, rather than between the two cables. This misalignment caused the cable to rub against a sharp edge on the steel console of the guide pulley. The continuous movement of the cable during flight led to progressive fraying and the eventual snapping of the strands. While it could not be determined if this specific error occurred immediately before the flight or during a previous assembly, the investigation noted that the lack of precise assembly instructions and poor visibility within the fuselage inspection port made such errors difficult to detect.