Kalea, HI, USA
N402SH
BELL 407
The pilot reported the first part of the air tour flight around the island was normal. About 35 minutes into the flight, during cruise flight at 1,500 ft above ground level (agl), the helicopter experienced a violent upset, followed by an uncontrolled spin to the right. The helicopter entered a rapid descent, its airspeed decreased, and it continued to spin uncontrollably. The pilot attempted to recover control of the helicopter, but it impacted a lava field and came to rest on its left side. Postaccident examination of the helicopter revealed the tail boom separated in flight. Further examination revealed there were no malfunctions or failures with the rotor systems, flight control systems, and engine before the tail boom separation. Examination of the tail boom revealed the upper-left tail boom attachment bolt was not present in the wreckage and was not found. Circumferential gouge and impression marks within the upper-left attachment bolt holes indicated the bolt was installed before the accident but that it had fractured and migrated out. After the bolt fractured, the structural load increased on the remaining three tail boom attachment points. As a result, multiple origin fatigue cracks, as well as fatigue cracking originating from a single corrosion pit, initiated on the lower-left attachment fitting. Once the fatigue cracks grew to a critical length, all three remaining attachment fittings failed in overload, resulting in the tail boom separation. The examined tail boom attachment fittings and attachment hardware met manufacturing requirements, which were not a factor in this accident. Due to the missing upper-left attachment hardware, the fracture mode and the duration of crack growth of the upper-left bolt could not be determined. The presence of sealant at the faying surface between the upper-left longeron and aft fuselage bulkhead indicated there was a small gap between those two structures. This gap was likely introduced during the replacement of the upper-left longeron and aft fuselage bulkhead, per manufacturer technical bulletins (TB), about 8,050 flight hours before the accident. The manufacturer reported a previous occurrence (on a different Bell 407) of a fractured upper-left attachment bolt that was attributed to abnormal loading of the bolt due to a gap and misalignment between the upper-left longeron and aft fuselage bulkhead. Similarly, the gap between the two structures on the accident helicopter likely applied abnormal loads on the upper-left attachment bolt that, over time, resulted in crack initiation on the bolt shank and the subsequent bolt failure. Due to structural deformation caused by the accident, it could not be determined if any axial misalignment was present between the upper-left longeron, the aft fuselage bulkhead, and the tail boom. A 300-hour recurring inspection, which included a torque check of the four tail boom attachment bolts and visual inspection of the fittings, occurred about 114 flight hours before the accident and resulted in no anomalous findings. It is unlikely the bolt had fractured or had a crack of sufficient size to fail during this last torque check. Therefore, it is likely the upper-left attachment point continued to carry load during the last 300-hour torque check and that the multiple-origin fatigue cracking on the lower-left attachment fitting had not yet initiated. However, it is possible that the fatigue crack had already initiated from the single corrosion pit on the lower-left attachment fitting at the time of the last 300-hour recurring inspection, but that it was too small to visually detect. In summary, examination of the wreckage revealed the accident occurred due to the in-flight separation of the tail boom during cruise flight, which resulted in the pilot’s inability to control the helicopter. The tail boom separated due to the abnormal loading and fracture of the upper-left longeron attachment hardware. The presence of a gap between the upper-left longeron and the aft fuselage bulkhead, introduced during the accomplishment of manufacturer technical bulletins, likely led to the hardware failure.
HISTORY OF FLIGHTOn June 8, 2022, about 1726 Hawaii-Aleutian standard time, a Bell 407 helicopter, N402SH, sustained substantial damage when it was involved in an accident near Kalea, Hawaii. The pilot and two passengers sustained serious injuries, and three passengers sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 on-demand air tour flight. According to the operator, Paradise Helicopters, about 1701, the helicopter departed the company base at Ellison Onizuka Kona International Airport at Keahole (PHKO), Kona, Hawaii, to the south for an air tour flight around the island. Automatic dependent surveillance-broadcast and company flight track data showed the helicopter traveled southeast toward an area known as South Point. The pilot stated that the first part of the flight was normal. About 35 minutes into the flight, at an altitude of 1,500 ft agl and an airspeed of 130 knots, the helicopter experienced a violent upset, followed by an uncontrolled spin (yaw) to the right. A passenger, seated in the aft-left forward-facing seat, reported that as the helicopter continued to spin she observed something fall off the helicopter; however, she was not able to identify the specific part. The pilot communicated two mayday calls and attempted to recover helicopter control as it continued to spin uncontrollably and rapidly descend. The helicopter subsequently impacted a lava field and came to rest on its left side (see figure 1). After the impact, about 1728, a passenger placed an emergency call to report that the helicopter had crashed. Simultaneously, the US Coast Guard Honolulu Joint Rescue Coordination Center received an emergency locator transmitter alert and initiated search and rescue operations. Figure 1. Main wreckage in a lava field. AIRCRAFT INFORMATIONAirframe Structure Description The helicopter fuselage was composed of three primary structural assemblies: the forward fuselage, the intermediate fuselage, and the tail boom. The intermediate fuselage begins at the rear of the passenger compartment and extends to the front of the tail boom. The aft portion of the intermediate fuselage is referred to as the aft fuselage. Four aft fuselage longerons—upper-left, upper-right, lower-left, and lower-right—are attached to the aft fuselage bulkhead. The tail boom is installed to the intermediate fuselage with fasteners (bolts, washers, and nuts) to connect the four aft fuselage longerons to four corresponding longerons located at the forward end of the tail boom. According to Bell, the tail rotor thrust produces tension loads on the two left attachment points and compression loads on the two right attachment points. The weight of the tail boom as well as aerodynamic forces from the horizontal stabilizer applies tension loads on the two upper attachment points and compression loads on the two lower attachment points. As a result, the upper-left tail boom attachment point has the highest tension loading of the four attachment points. The lower-left attachment point carries the next highest load, followed by the upper-right and the lower-right attachment points. Helicopter Maintenance Records A review of the accident helicopter’s maintenance records revealed that the most recent 300-hour torque check of the tail boom attachment fasteners was completed on May 4, 2022, at a total airframe time of 22,891.4 flight hours. On June 6, 2022, 2 days before the accident, the helicopter had a total airframe time of 23,005.6 flight hours (114.2 flight hours since the torque check), 30,490 total flight cycles, and no additional maintenance had been conducted to the attachment fasteners since the torque check. The tail boom (part number [P/N] 407-030-801-205D and serial number BP-1598) was installed on August 23, 2009, at a total airframe time of 5,780.0 hours. The tail boom was last removed and reinstalled on July 3, 2014, to facilitate longeron and frame repairs. According to the work order, a repair station assisted the operator accomplishing Bell TB 407-12-96 Revision A (Rev A). The subject of TB 407-12-96 Rev A was the installation of a redesigned aft fuselage upper-left longeron assembly. TB 407-07-78, dated September 19, 2007, introduced an improved machined aft fuselage bulkhead, P/N 407-030-027-101. This was the P/N of the bulkhead installed on the accident helicopter. A later bulletin, TB 407-07-78 Rev A, issued In August 2020, introduced a new aft fuselage bulkhead that had a reinforced inside flange in the upper left area. The helicopter record list contained no entries regarding TBs 407-07-78 and 407-12-96. Bell Technical Bulletins for the Aft Fuselage Bell TBs 407-07-78 and 407-12-96 Rev A utilize an aluminum drill plate, manufactured locally in accordance with instructions provided in both TBs, to verify the location and planar alignment between the four tail boom attachment fittings and their bolt holes. Within TB 407-12-96 Rev A, before the final installation of the upper-left longeron and aft fuselage bulkhead to the aft fuselage, Step 23, a note recommends installation of the new improved machined aft fuselage bulkhead per TB 407-07-78. Additionally, this note states it is acceptable to reinstall the existing sheet metal aft fuselage bulkhead. Bell TB 407-07-78 provides instructions in three parts for the installation of the new improved machined aft fuselage bulkhead. Part I contains instructions for the removal of the aft fuselage bulkhead and positional verification of the longeron fittings. Part II contains instructions for the installation of the bulkhead using existing pilot holes on the bulkhead and is considered the preferred method of installation. Part III contains instructions for drilling new pilot holes in the bulkhead (the existing pilot holes are not used). Part III is considered the alternate method for installing the bulkhead. A note within TB 407-07-78, before Step 10, which accomplishes the final ream for the upper-left bolt hole, states not to drill the upper left upper longeron/fitting hole at this point if the aft fuselage bulkhead installation is accomplished in conjunction with the installation of a replacement upper-left longeron per TBs 407-12-96 or 407-17-125. According to TB 407-07-78, the new aft fuselage bulkhead is initially secured to the four longeron fittings and the drill plate via four 3/16-inch diameter bolts that are torqued to 50 inch-pounds. At the four bolt hole locations, bushings are used to fill the gap between the 3/16-inch diameter bolt and the larger diameter bolt holes of the longeron fittings and the drill plate. According to TB 407-12-96 Rev A, after removal of the existing longeron and a first-fit installation of the new upper-left longeron, the drill plate is temporarily secured to the upper-right, lower-left, and lower-right longeron fittings using the existing fasteners and torqued to 50 inch-pounds. The new upper-left longeron, whose fitting has a 3/16-inch diameter pilot hole, is secured to the drill plate using a 3/16-inch diameter bolt. Unlike TB 407-07-78, TB 407-12-96 does not specify instructions to use bushings for the installation of the 3/16-inch diameter bolt to secure the upper-left longeron to the drill plate. After securing the new upper-left longeron to the drill plate, the instruction states to verify that no gap exists between the longeron assembly and the drill plate and/or the spacer. A note preceding this step states that it is acceptable to hold the new upper-left longeron to the drill plate using a C-clamp until it is completely riveted to the fuselage. The last step in Part I of TB 407-07-78 is to verify the four longeron fittings are in plane within 0.002 inches. A drill plate is attached to the four longeron fittings using the existing hardware that is torqued to 50 inch-pounds. The TB instructs that a gap check should be conducted between the drill plate and the aft face of each longeron fitting. If a gap is more than 0.002 inches on only one longeron fitting, the affected longeron must be replaced before the installation of the new aft fuselage bulkhead. If a gap is more than 0.002 inches on more than one longeron fitting, the affected longerons must be replaced, which requires installing the helicopter on a Bell-approved fuselage fixture. According to TB 407-12-96 Rev A, before the installation of rivets for the upper-left longeron and aft fuselage bulkhead, sealant should be applied to these items. Specifically Step 23 of TB 407-12-96 Rev A, states the following: Apply a coat of sealant (C-251) to faying surfaces of longeron assembly (2), aft fuselage bulkhead (6), the splices joining affected bulkhead sections and if needed the shims made in Step 20 before installing in place with drill plate (8). Secure longeron assembly (2) and bulkhead (6) with applicable rivets wet with sealant (C-251). Do not install rivets common to oil cooler fairing retainers (1, 2, Figure 2), the side skin panel (16, Figure 7) and the top skin (1, Figure 6) at this time, but secure any applicable shims made earlier with clecos. According to Bell, if TB 407-07-78 is to be accomplished in conjunction with TB 407-12-96, then the aft fuselage bulkhead would be installed immediately before Step 23 of TB 407-12-96. Additionally, according to Bell the engineering drawing for the aft fuselage bulkhead installation does not require sealant between the contact surfaces of the aft fuselage bulkhead and the upper-left longeron. In TB 407-07-78, both Parts II and III contain instructions to apply sealant to the faying surfaces of the shim and aft fuselage bulkhead. AIRPORT INFORMATIONAirframe Structure Description The helicopter fuselage was composed of three primary structural assemblies: the forward fuselage, the intermediate fuselage, and the tail boom. The intermediate fuselage begins at the rear of the passenger compartment and extends to the front of the tail boom. The aft portion of the intermediate fuselage is referred to as the aft fuselage. Four aft fuselage longerons—upper-left, upper-right, lower-left, and lower-right—are attached to the aft fuselage bulkhead. The tail boom is installed to the intermediate fuselage with fasteners (bolts, washers, and nuts) to connect the four aft fuselage longerons to four corresponding longerons located at the forward end of the tail boom. According to Bell, the tail rotor thrust produces tension loads on the two left attachment points and compression loads on the two right attachment points. The weight of the tail boom as well as aerodynamic forces from the horizontal stabilizer applies tension loads on the two upper attachment points and compression loads on the two lower attachment points. As a result, the upper-left tail boom attachment point has the highest tension loading of the four attachment points. The lower-left attachment point carries the next highest load, followed by the upper-right and the lower-right attachment points. Helicopter Maintenance Records A review of the accident helicopter’s maintenance records revealed that the most recent 300-hour torque check of the tail boom attachment fasteners was completed on May 4, 2022, at a total airframe time of 22,891.4 flight hours. On June 6, 2022, 2 days before the accident, the helicopter had a total airframe time of 23,005.6 flight hours (114.2 flight hours since the torque check), 30,490 total flight cycles, and no additional maintenance had been conducted to the attachment fasteners since the torque check. The tail boom (part number [P/N] 407-030-801-205D and serial number BP-1598) was installed on August 23, 2009, at a total airframe time of 5,780.0 hours. The tail boom was last removed and reinstalled on July 3, 2014, to facilitate longeron and frame repairs. According to the work order, a repair station assisted the operator accomplishing Bell TB 407-12-96 Revision A (Rev A). The subject of TB 407-12-96 Rev A was the installation of a redesigned aft fuselage upper-left longeron assembly. TB 407-07-78, dated September 19, 2007, introduced an improved machined aft fuselage bulkhead, P/N 407-030-027-101. This was the P/N of the bulkhead installed on the accident helicopter. A later bulletin, TB 407-07-78 Rev A, issued In August 2020, introduced a new aft fuselage bulkhead that had a reinforced inside flange in the upper left area. The helicopter record list contained no entries regarding TBs 407-07-78 and 407-12-96. Bell Technical Bulletins for the Aft Fuselage Bell TBs 407-07-78 and 407-12-96 Rev A utilize an aluminum drill plate, manufactured locally in accordance with instructions provided in both TBs, to verify the location and planar alignment between the four tail boom attachment fittings and their bolt holes. Within TB 407-12-96 Rev A, before the final installation of the upper-left longeron and aft fuselage bulkhead to the aft fuselage, Step 23, a note recommends installation of the new improved machined aft fuselage bulkhead per TB 407-07-78. Additionally, this note states it is acceptable to reinstall the existing sheet metal aft fuselage bulkhead. Bell TB 407-07-78 provides instructions in three parts for the installation of the new improved machined aft fuselage bulkhead. Part I contains instructions for the removal of the aft fuselage bulkhead and positional verification of the longeron fittings. Part II contains instructions for the installation of the bulkhead using existing pilot holes on the bulkhead and is considered the preferred method of installation. Part III contains instructions for drilling new pilot holes in the bulkhead (the existing pilot holes are not used). Part III is considered the alternate method for installing the bulkhead. A note within TB 407-07-78, before Step 10, which accomplishes the final ream for the upper-left bolt hole, states not to drill the upper left upper longeron/fitting hole at this point if the aft fuselage bulkhead installation is accomplished in conjunction with the installation of a replacement upper-left longeron per TBs 407-12-96 or 407-17-125. According to TB 407-07-78, the new aft fuselage bulkhead is initially secured to the four longeron fittings and the drill plate via four 3/16-inch diameter bolts that are torqued to 50 inch-pounds. At the four bolt hole locations, bushings are used to fill the gap between the 3/16-inch diameter bolt and the larger diameter bolt holes of the longeron fittings and the drill plate. According to TB 407-12-96 Rev A, after removal of the existing longeron and a first-fit installation of the new upper-left longeron, the drill plate is temporarily secured to the upper-right, lower-left, and lower-right longeron fittings using the existing fasteners and torqued to 50 inch-pounds. The new upper-left longeron, whose fitting has a 3/16-inch diameter pilot hole, is secured to the drill plate using a 3/16-inch diameter bolt. Unlike TB 407-07-78, TB 407-12-96 does not specify instructions to use bushings for the installation of the 3/16-inch diameter bolt to secure the upper-left longeron to the drill plate. After securing the new upper-left longeron to the drill plate, the instruction states to verify that no gap exists between the longeron assembly and the drill plate and/or the spacer. A note preceding this step states that it is acceptable to hold the new upper-left longeron to the drill plate using a C-clamp until it is completely riveted to the fuselage. The last step in Part I of TB 407-07-78 is to verify the four longeron fittings are in plane within 0.002 inches. A drill plate is attached to the four longeron fittings using the existing hardware that is torqued to 50 inch-pounds. The TB instructs that a gap check should be conducted between the drill plate and the aft face of each longeron fitting. If a gap is more than 0.002 inches on only one longeron fitting, the affected longeron must be replaced before the installation of the new aft fuselage bulkhead. If a gap is more than 0.002 inches on more than one longeron fitting, the affected longerons must be replaced, which requires installing the helicopter on a Bell-approved fuselage fixture. According to TB 407-12-96 Rev A, before the installation of rivets for the upper-left longeron and aft
The in-flight separation of the tail boom as a result of abnormal loading and fracture of the upper-left tail boom attachment bolt due to a gap between the upper-left longeron and aft fuselage bulkhead.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
