Bell 47G N96195 accident near Arlington, Indiana

Probable cause

The inflight separation of the upper transmission due to the thread failure of transmission housing attachment bolts, which were installed on the helicopter with thread damage from a prior installation.

NTSB analysis

The commercial helicopter pilot was conducting aerial applications work on a corn field. The helicopter did not return to the loading platform when expected, and a subsequent search for the helicopter ensued. The wreckage of the helicopter was located near the south edge of a field being sprayed. The upper transmission, mast, and main rotor separated from the helicopter and were located about 75 ft north of the main wreckage. Ground scars, damage to the crop, and damage to the helicopter was consistent with an inflight separation of the upper transmission, mast, and main rotor. A postimpact fire ensued and damaged the cabin and engine of the helicopter.

Postaccident examination of the lower transmission housing found elongation of four of the bolt holes and tearing of two of the bolt holes; two of the bolt holes were unremarkable.

The upper main rotor transmission housing pieces and gears separated from the lower transmission housing due to shear fractures of the threads in six of the eight bolts attaching the housing pieces. The fractures likely occurred due to the fracture of threads on the two bolts that were weakened by thread damage from a prior installation using those bolts as discussed below.

Specifically, these bolts showed damage on intact threads adjacent to the sheared threads. The damage had circumferential markings and a profile matching the nut threads with deformation to the remaining thread, consistent with an over-torque installation of a nut on the bolt. However, because the nuts likely did not back off from their installed position, the thread damage would have occurred during a prior installation. Thus, these two bolts were likely installed on the accident helicopter with damaged threads that were not detected at the time of installation.

Additionally, one of the bolts had thread profile anomalies indicating that the bolt was likely manufactured with an incompletely formed thread. The deviations from the standard thread profile could have contributed to an improper fit with the installed nut and might have contributed to the thread damage on that bolt during a prior installation.

For the other six bolts, the fit between the nut and the bolt might have contributed to threads shearing. The major diameter of threads on one of these six bolts was below minimum specification, which would limit the thread engagement to produce a shear fracture of the thread, and the major diameter of another of these bolts was close to the lower limit. No evidence of prior thread damage was observed on either of these bolts, indicating the failures were likely secondary to the failure of bolts Nos. 2 and 3.

The housing assembly attachments showed evidence of poor maintenance practice. Deviations from the assembly drawing included the use of bolts with an incorrect number of washers and incorrect length. Furthermore, the damage to bolts 2 and 3 from a prior installation was not detected when the bolts were installed on the accident helicopter.

Limited maintenance records were available for the helicopter. According to the available records, the main rotor transmission was replaced in 2009, and the transmission bolts were retorqued in 2014, about 73.6 flight hours before the accident. The investigation was unable to determine if any other maintenance was conducted on the transmission fasteners, when the maintenance was performed that initially damaged the threads of the two of the bolts, or when the damaged bolts were installed on the helicopter.

No other mechanical anomalies were noted that would have precluded normal operation. The inflight separation of the upper transmission was due to the weakened bolt threads as a result of damage during a prior installation which caused the threads on those bolts as well as the threads on other bolts to fracture. However, the prior installation damage could not be associated to any particular maintenance action.

Text from the NTSB. Open the NTSB docket for CEN18FA258 for the full record.