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NTSB Final Report: Arlington, IN

 

Location:

Arlington, IN

Accident Number:

CEN18FA258

Date & Time:

07/06/2018, 1720 EDT

Registration:

N96195

Aircraft:

Bell 47G

Aircraft Damage:

Substantial

Defining Event:

Part(s) separation from AC

Injuries:

1 Fatal

Flight Conducted Under:

Part 137: Agricultural


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.

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be:
 

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.

 

Findings

Aircraft

Aircraft propeller/rotor - Incorrect service/maintenance (Cause)

Fasteners - Incorrect service/maintenance (Cause)

Factual Information

History of Flight

Maneuvering-low-alt flying

Part(s) separation from AC (Defining event)

Uncontrolled descent

Collision with terr/obj (non-CFIT)

On July 6, 2018, about 1720 eastern daylight time, a Bell 47G-2 helicopter, N96195, was substantially damaged when it impacted a corn field near Arlington, Indiana. A postimpact fire ensued. The commercial pilot was fatally injured. The helicopter was owned and operated by Central Indiana Ag Services, LLC as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions prevailed and no Federal Aviation Administration (FAA) flight plan had been filed for the flight. The local flight departed about 1655 from a loading platform at a farm located 1 mile northwest of the accident location.

According to the co-owner of the helicopter, the pilot had been flying since 0945 that morning and had completed between 10 and 15 spray runs. The pilot was applying a fungicide to corn crops and each run was averaging 20 minutes. When the pilot did not return after 30 minutes, they initiated search operations. The wreckage was located later that evening.

Figure 1 shows the flight track for the accident flight based on GPS latitude and longitude data recovered from a Lowrance AIRMAP 2000c device installed on the helicopter. The device contained data from three flight tracks; one of which was the accident flight. The data did not contain altitude or time data, so the time lengths of the track are not known. The last data point of the final flight recorded was consistent with the wreckage location.

There were no witnesses to the accident.