Incident Overview
A pilot reported that a normal engine runup was completed before takeoff. During the takeoff roll, the aircraft accelerated through 75 knots when a loud bang was heard from the left side. The takeoff was promptly aborted and a rollout was accomplished without further incident.
Examination Findings
Post-incident examination of the left engine revealed that one of three left propeller counterweights was missing. Each counterweight is secured by a single bolt and locking nut. Examination of the bolt that had secured the missing counterweight showed evidence of pre-load stress.
Service Bulletin Reference
A service bulletin had been issued on March 3, 1982, stating that to reduce pre-load stress on the counterweight bolt, the torque specification had been reduced from 105-100 foot-pounds to 65-60 foot-pounds. The relationship between this bulletin and the missing counterweight is not detailed in the available information.