Models 336/337; Skymaster; Asymmetric Flap Extension; ATA 2750

Models 336/337; Skymaster; Asymmetric Flap Extension; ATA 2750

This aircraft experienced an asymmetric flap extension as the result of a failed flap cable. Excessive wear at the right hand inboard bellcrank caused this failure. The wear occurred at the tight radius of the bend and was not be noticeable until the cable was removed.

The maintenance manual requires a 100-hour inspection of the flap cable system. It is recommended that maintenance personnel treat this bellcrank and all others in this system as a “critical fatigue area” as defined in AC 43.13-1B, Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair, chapter 7, section 8, paragraph 7-149(b). These areas require a close visual inspection, which can only be accomplished by removing the cable from the bellcrank. As recommended in AC 43.13-1B, the suspect cable should be removed and inspected for wear and broken wires. It is recommended that the cable be bent to check for broken wires on the interior of the cable.

The state of the cable should be compared against the requirements of the maintenance manual and/or AC 43.13-1B to determine if the cable should be replaced.

Part total time-2,893 hours.

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