Incident Discovery
On September 21, 2023, during scheduled inspections that included a torque check of the four tail boom attachment bolts, the operator of a Bell 407 helicopter, registration N406TD, discovered a fractured upper-left tail boom attachment bolt. The operator, Helicopter Flying Services (doing business as HeliNY), subsequently inspected the aft fuselage structure and found that the lower-left tail boom attachment fitting exhibited a crack emanating from its bolt hole. After removal of the tail boom, the operator determined that the upper-left longeron fitting was not in plane with the remaining three fuselage-side tail boom attachment fittings, resulting in a gap of at most about 0.015 inches at its outboard side. Additionally, a notable quantity of corrosion preventive compound (CPC) was found on the contact surfaces of the upper-left tail boom attachment point, whereas the remaining three attachment points did not have CPC on their contact surfaces.
Examination Findings
In November 2023, Bell requested that the operator ship the upper-left longeron, along with the upper-left bolt and lower-left longeron, to the Bell Field Investigations Lab in Hurst, Texas, for detailed examination. The fracture on the upper-left attachment bolt was located on the bolt shank about 1.20 inches below the bolt head. An area of fretting and wear was visible on the bolt shank and co-located with the fracture. The fracture face exhibited fatigue cracking emanating from multiple origins, growing about 0.37 inches before transitioning to overload. Corrosion damage precluded striation and band counting to estimate fatigue growth duration. Examination of the crack on the lower-left longeron fitting found evidence of fatigue emanating from multiple origins from a 0.0002-inch deep machining line. Band counting revealed about 149 bands, with higher concentrations near the crack origin. Both components' materials and dimensions conformed to drawing requirements.
Maintenance History
The incident helicopter was manufactured in 2007 with serial number 53745. It was involved in a hard landing accident on May 30, 2013 (then registered as N407HC), documented in NTSB investigation ERA13TA261. Postaccident repairs were performed by Summit Aviation in Middletown, Delaware, and included structural repairs by USCAN Limited in Ontario, Canada, where the aft fuselage upper-left, lower-left, and lower-right longerons were replaced. The new upper-left longeron was installed per Bell Technical Bulletin (TB) No. 407-12-96 Revision A. The repairs were completed on December 5, 2014, at an aircraft total time (ATT) of 1,929.6 hours. According to HeliNY, the tail boom had not been removed since installation after repairs, so the attachment bolts accumulated about 3,858.9 hours from installation in 2014 to the discovery of the fractured bolt in 2023.
Related Events and Recommendations
NTSB investigation ANC22FA041, involving an inflight tail boom separation on another Bell 407 (N402SH), found two occurrences of lost upper-left load path due to a fractured bolt, leading to fatigue cracking of the lower-left fitting and eventual overload failure. Based on these findings, the NTSB issued Safety Recommendations A-22-28 through A-22-31 on December 1, 2022, to the FAA and TCCA. Bell issued Alert Service Bulletin (ASB) No. 407-22-128 on December 8, 2022, for a one-time torque check and visual inspection. TCCA and FAA subsequently issued Airworthiness Directives (AD) requiring these actions. For N406TD, the operator completed ASB 407-22-128 on December 16, 2022, at ATT 4,867.9 hours, with no anomalous findings. Thereafter, the operator continued 300-hour recurrent torque checks, completing three without anomalies before the fractured bolt was found during the fourth on September 21, 2023, at ATT 5,788.5 hours.
On February 12, 2024, Bell issued ASB No. 407-24-134, a three-part interim action requiring recurrent inspections based on whether the upper-left longeron was original or replaced per specific TBs. As of the report, neither the FAA nor TCCA had mandated this ASB. Bell also issued revisions to TBs 407-17-125, 407-07-78, and 407-12-96 to clarify in-plane condition verification.