Cessna 207 N6908M accident near Dillingham, Alaska

Probable cause

Maintenance personnel’s failure to detect and correct improper rudder control system alignment, the resulting interference with the pulley’s rotation, and the accelerated corrosion and wear of the control cable, which ultimately led to its failure in flight.

NTSB analysis

The pilot reported that, during approach to landing, he became aware that the left rudder pedal was not responsive. He declared an emergency and landed uneventfully. Examination revealed that the left rudder cable fractured due to corrosion and severe cable wear at the left pulley location. Nearly all wires on the cable were worn to a chisel-like point, leaving a small number of wires in the core strand to fracture in ductile overstress during final separation. The cable corrosion and wear were likely accelerated due to sliding contact with the left pulley.

Issues with the rudder control system installation were evident on both pulleys. Rubbing damage on the face of the left pulley indicated that the pulley was contacting adjacent structure at the outer diameter. This contact likely interfered with pulley rotation, leading to the accelerated corrosion and wear of the control cable from sliding contact within the pulley groove. Although no rubbing damage was observed on the right pulley, the cable contact pattern was offset from the groove centerline, indicating an alignment problem in the rudder control system at the right pulley. Review of the maintenance logs revealed that the last inspection of the flight controls was completed about 8 months before the accident, and an entry about 3 months later noted a stuck pulley and corrosion of the left rudder cable. The maintenance response to the entry stated, “upon further inspection, determined cable not corroded. . . No defects.” Given the findings of the postaccident examination, it is likely that the accident would have been prevented had maintenance personnel taken more proactive action at that time.

Text from the NTSB. Open the NTSB docket for ERA22LA358 for the full record.