What happened
On September 15, 1967, a Champion-7 GCB, registration HB-UAL, was performing a glider tow operation near Schänis Airfield. After the glider successfully released at an altitude of approximately 400 meters, the pilot noticed a significant loss of control pressure. The elevator became unresponsive to pull inputs, responding only to momentary nudges.
The pilot immediately recognized that an elevator control cable had severed. To maintain a controlled descent and prevent a stall, the pilot utilized the aircraft's trim and high engine power. While attempting to clear overhead telephone lines, the pilot had to reduce power, causing the nose to drop sharply. To prevent a ground impact, the pilot applied full power and maximum nose-up trim. This maneuver resulted in the main landing gear striking a small embankment, causing the left gear to be torn away and the aircraft to slide 75 meters along the runway. There were no injuries to the pilot.
The investigation
SUST investigators examined the wreckage and the aircraft's maintenance history. The investigation focused on the mechanical failure of the elevator control system and the adequacy of the maintenance performed by the Alpine Segelflugschule Schänis AG. The inquiry reviewed the aircraft's logbooks, noting that while 100-hour inspections were conducted by a certified facility, the intermediate 33- and 66-hour inspections were performed by the operator's maintenance person, who lacked official inspector credentials.
Findings
Technical analysis revealed that the left elevator control cable had snapped due to wear caused by chafing against a metal retaining bracket on a pulley. This bracket was found to be movable, unlike the fixed bracket on the right-side cable, which allowed the cable to rub against the sharp edge of the bracket. The investigation also noted that the European-manufactured cable used in the aircraft was less flexible than the original American-made cables, which may have contributed to the friction.
Furthermore, the investigation identified significant maintenance lapses. The maintenance person had admitted to not specifically checking the control cables for chafing during the 31- and 66-hour inspections. Additionally, the maintenance person had not followed instructions to clean and inspect the pulleys between major inspections, despite warnings that dirt could cause the pulleys to seize and lead to cable chafing. The use of maintenance forms intended for a different aircraft type (Piper Super Cub) also meant that specific inspection requirements for the Champion model were not being strictly followed.