ENG23LA045 · Bell 407

The upper-left tail boom attachment bolt failed due to fatigue fracture and subsequent overload. Fatigue cracking initiated on the upper-left tail boom attachment bolt due to abnormal loading of the bolt shank caused by the presence of a gap and misalignment between the aft fuselage upper-left longeron and the tail boom. The gap and misalignment were likely introduced during installation of a new upper-left longeron in 2014, about 3,858.9 flight hours prior to discovery of the fractured upper-left attachment bolt. After the upper-left attachment bolt fractured, the load normally carried by the upper-left attachment fitting was redistributed to the remaining three tail boom attachment fittings. As a result, multiple-origin fatigue cracking initiated on the lower-left longeron fitting. This incident was the third known occurrence involving a fractured upper-left tail boom attachment bolt due to a gap and misalignment of the upper-left longeron. (In NTSB investigation ANC22FA041, one of the three occurrences, the upper-left tail boom attachment bolt was not found, but the upper-left tail boom attachment bolt likely fractured based on the available information.) In the three occurrences, each involving a different repair facility, the upper-left longeron was installed in accordance with Bell Technical Bulletin (TB) 407-12-96. Bell TB 407-12-96 did not contain steps to verify the absence of gaps and misalignment between the upper-left longeron and the aft fuselage bulkhead at the end of their installation instructions. The lack of these verification steps was a factor in the nondetection of the gap and misalignment that were introduced during the upper-left longeron installation for these three occurrences. On June 8, 2022, a Bell 407 helicopter, N402SH, experienced an inflight separation of its tail boom (NTSB investigation ANC22FA041). During the N402SH investigation, the NTSB issued safety recommendations A-22-29 and A-22-31 to the Federal Aviation Administration (FAA) and Transport Canada Civil Aviation (TCCA), respectively, to require subsequent torque checks of the tail boom attachment hardware be performed at a more stringent interval until the causal factors of the N402SH accident could be determined and corrective actions enacted. The FAA and TCCA did not take the recommended action and the tail boom attachment hardware torque check interval remained unchanged at 300-hours. As a result, the incident operator did not discover the fractured upper-left attachment bolt until a scheduled 300-hour inspection, by which time a fatigue crack on the lower-left tail boom attachment fitting had already initiated and grew. The combination of a loss of the upper-left tail boom attachment load path and fatigue crack growth on the lower-left longeron fitting substantially increased the risk of an inflight tail boom separation. Had the FAA and TCCA adopted the NTSB’s previously recommended action, the operator of this incident would have had more opportunities to detect the fractured upper-left attachment bolt, mitigating the risk of an inflight tail boom separation.

Verbatim NTSB analysis and probable cause from the NTSB dataset