Bell is working with the FAA on a revised and shorter inspection schedule for key components of its 407 helicopter series tail booms. A related service bulletin and airworthiness directive could be issued this week.
The move comes after the National Transportation Safety Bureau (NTSB) took the extraordinary move of issuing a public appeal to mandate such inspections as it continues to investigate the June in-flight tail boom separation on a Bell 407 in Hawaii that seriously injured three of the six aboard. The NTSB also opened the public docket of that investigation on October 31st.
Bell told AIN via email that the company “is unable to comment about the referenced accident as it is still under investigation by NTSB. The Bell 407 does not have a history of events of this nature. In the meantime, Bell is working with our regulators on an inspection procedure to ensure fleet safety while the NTSB completes its investigation.”
Several bulletins and ADs have been issued since 2007 related to Bell 407 tail booms with regard to the aft fuselage top skin and bulkhead, replacement attachment hardware and torque checks, installation of external strap doublers on the upper-left longeron, and repair instructions for cracked longerons. One of those, (AD) 2012-18-09, requires recurrent torque checks of the boom’s attachment hardware every 300 hours.
However, the accident helicopter had been torque-checked 114 hours prior to tailboom separation and no anomalies were found. Post-accident, the operator, Paradise Helicopters, elected to replace the tail boom attachment hardware on its remaining five Bell 407s. Before hardware removal, the operator torque-checked the attachment hardware and found no evidence of loose hardware, but eddy-current nondestructive inspection on the aft fuselage longerons uncovered cracks on two of these five 407s.
Those facts prompted the FAA FAASTeam to issue a special advisory reminding all operators of the 300-hour requirement and subsequently prompted the FAA to issue a new, overarching AD on November 27th that in part requires a tail boom inspection regime and mandatory retirement of certain model 407 tail booms after 5000 hours time in service. The AD was scheduled to take effect on Jan. 23, 2023.
The NTSB found this insufficient and late last week issued a public appeal for immediate and more frequent inspections of all Bell 407s based on its investigation of the Hawaii accident to date.
According to the NTSB, “An examination of the helicopter wreckage revealed that the upper left attachment hardware, installed in one of four fittings that attaches the tail boom to the fuselage, was missing and could not be located at the accident site. The remaining three fittings and hardware were found with the tail boom, one fitting with multiple fatigue fractures and two fittings with overload fractures.”
The safety board said it was “concerned that there may be additional Bell 407 helicopters with missing or fractured tail boom attachment hardware, and the potential for catastrophic failure warrants immediate and mandatory action.” The NTSB urged the FAA and Transport Canada (the 407 is manufactured at Bell’s plant in Mirabel, Quebec) to “require Bell 407 operators to conduct an immediate inspection of the tail boom attachment hardware and to reduce the inspection interval from 300 hours to a more conservative number to increase the likelihood of detecting fractured attachment hardware before a catastrophic failure can occur.”
Tail boom fatigue fractures have been a well-known issue with the 407 and its military variant, the OH-58, for decades and the aircraft has been used as the test subject for evaluating new fracture-detecting technologies and methods at Penn State’s Applied Research Laboratory and the University of Maryland.