WPR11IA213 · Cessna 421B

The airplane entered the downwind leg of the traffic pattern high, and, in an effort to descend, the pilot extended the landing gear and deployed full flaps. About the time the flaps reached their maximum extension, the right flap experienced an instantaneous retraction, and the airplane simultaneously rolled about 80 degrees to the right. The pilot countered with almost full left aileron control input for the remainder of the flight. He began troubleshooting steps, but was unable to extend the right flap or retract the left flap. He diverted to another airport and, for the remaining 35 minutes of flight, employed the assistance of a passenger to help with maintaining left aileron control deflection. The landing was made without further incident.

Postincident examination of the flap control system revealed that the right wing flap extend cable had failed in the area where it made contact with the inboard flap pulley, an area where the cable had experienced multiple bending cycles throughout its life. The failed cable strands exhibited fatigue signatures, and similar frays and failures were observed in the area of the outboard pulley. The corresponding left flap cable also exhibited similar strand failure features in the inboard and outboard pulley contact areas. The cables were installed when the airplane was manufactured, 36 years prior to the incident. Over this period, the airplane had accumulated 4,832.1 total flight hours.

The flap cable was not life limited, and the airplane manufacturer's maintenance manual did not require the removal of flight control cables during inspection. The mechanic who performed the most recent inspection reported that he examined the cables utilizing the methods prescribed in the manufacturer's service manual but did not detect any damage. He further stated that the damage was only obvious once the cables had been removed and subsequently flexed and looped by hand.

A review of Federal Aviation Administration Service Difficulty Reports for the airplane series revealed 33 instances of similar flap cable wear or failure on 25 separate airplanes. About half of the reports indicated flap cable failures occurring during flight; all were during the critical landing approach phase. The failures resulted in asymmetric flap deployment, and some resulted in a violent departure from controlled flight. In a few instances, the damage caused by the cable separation prevented the retraction of the remaining extended flap, and, therefore, the pilot had to maintain very high opposing aileron control inputs in order to control and land the airplane. A common finding noted in the reports was that the cable damage could not be readily observed unless the cables were removed.

A service bulletin is in development by the airplane manufacturer concerning the inspection procedures and replacement criteria for the flap cables in the airplane series.

Verbatim NTSB analysis and probable cause from the NTSB dataset