Cessna 421 N2349S incident near Sugar Land, Texas

Probable cause

The improper installation of the pitch trim cable which resulted in it making improper contact with a guard pin that led to the fatigue failure of the cable.

NTSB analysis

The airplane was on an autopilot descent when the pilot noticed the elevator pitch trim was rotating continuously in the nose-up position. He disconnected the autopilot and immediately heard a popping sound. The movement of the pitch trim stopped, and the pilot could manually turn the pitch trim wheel without force or resistance. He then hand flew the airplane using spoilers, 15 degrees of flaps and exerted approximately 10 to 20 pounds of back pressure on the yoke to keep the airplane in level flight. The pilot executed an emergency landing without further incident. A postincident examination of the airplane revealed that the pitch trim cable had fractured and separated in the area where it passed over the rearmost pulley in the tail. A subsequent materials laboratory examination revealed the pitch trim cable displayed evidence of permanent curved deformation near the fracture location, with localized smearing wear. Several of the fractured cable wires showed evidence of fatigue fracture and some showed the presence of cadmium. One of the cadmium plated pulley guard pins showed evidence of rubbing contact with the cable. The smearing wear and the presence of the cadmium on the cable wires is consistent with a misrouting of the cable over the cable guide pins instead of the proper routing over the cable pulley. Examination of partial aircraft maintenance records did not show entries for any maintenance events when the pitch trim cable was rerouted or reinstalled. The circumstances leading up to the improper misrouting of the pitch trim cables could not be determined.

Text from the NTSB. Open the NTSB docket for CEN09IA578 for the full record.