# NTSB WPR13LA404: Bell 47G5, N7836S, King City, CA, September 11, 2013

- Type: NTSB accident investigation
- Classification: Accident · No injuries · Limited investigation · Final report
- Date: September 11, 2013, 07:15 local (14:15 UTC)
- Place: King City, CA
- Coordinates: 36.191944, -120.942222 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=36.1919&lon=-120.9422&mi=10
- Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping)
- Defining event: [Landing: Hard landing](https://openairdocs.org/ntsb-events/092)
- Highest injury: No injuries
- Injuries: 1 uninjured
- Status: Completed, report August 14, 2014
- Report: Approved by Office Director
- NTSB office: Western Pacific Region
- Parties: Federal Aviation Administration
- NTSB case key (mKey): 88025
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR13LA404
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/88025/pdf
- NTSB docket: https://data.ntsb.gov/Docket?ProjectID=88025
- Page: https://openairdocs.org/ntsb/WPR13LA404

## Probable cause

The failure of the helicopter’s center frame tube due to a fatigue crack and corrosion originating from a welded surface. Contributing to the accident was maintenance personnel’s inadequate inspection of the helicopter during its most recent 100-hour inspection.

## Findings

NTSB's coded findings; those marked are part of the probable cause.

- [Fuselage: frames (main fuselage)](https://openairdocs.org/ntsb-findings/01035311): Aircraft › Aircraft structures · Fatigue/wear/corrosion (probable cause); cases with it: [Bell 47G-5, 1 case](https://openairdocs.org/ntsb-findings/01035311/cases.md?model=1181032), [All aircraft, 8 cases](https://openairdocs.org/ntsb-findings/01035311/cases.md)
- [Maintenance/inspections: scheduled maint checks](https://openairdocs.org/ntsb-findings/01010520): Aircraft › Aircraft handling/service · Inadequate inspection (probable cause); cases with it: [Bell 47G-5, 1 case](https://openairdocs.org/ntsb-findings/01010520/cases.md?model=1181032), [All aircraft, 76 cases](https://openairdocs.org/ntsb-findings/01010520/cases.md)
- [Maintenance: scheduled/routine maintenance](https://openairdocs.org/ntsb-findings/02062010): Personnel issues › Task performance · Maintenance personnel (probable cause); cases with it: [Bell 47G-5, 1 case](https://openairdocs.org/ntsb-findings/02062010/cases.md?model=1181032), [All aircraft, 320 cases](https://openairdocs.org/ntsb-findings/02062010/cases.md)

## Sequence of events

The events NTSB coded, in order, by phase of flight. The defining event best describes what happened.

- [Maneuvering-low-alt flying: Aircraft structural failure](https://openairdocs.org/ntsb-events/337); cases with it: [Bell 47G-5, 1 case](https://openairdocs.org/ntsb-events/337/sequence.md?model=1181032), [All aircraft, 290 cases](https://openairdocs.org/ntsb-events/337/sequence.md)
- [Maneuvering-low-alt flying: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [Bell 47G-5, 2 cases](https://openairdocs.org/ntsb-events/440/sequence.md?model=1181032), [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md)
- [Landing: Hard landing](https://openairdocs.org/ntsb-events/092) (defining event); cases with it: [Bell 47G-5, 2 cases](https://openairdocs.org/ntsb-events/092/sequence.md?model=1181032), [All aircraft, 2,102 cases](https://openairdocs.org/ntsb-events/092/sequence.md)

## Analysis

The pilot reported that he was flying low over a field when he heard a "loud bang." He initiated a precautionary landing, during which, the helicopter started to spin. The pilot lowered the collective, and the helicopter subsequently landed hard on a road. Postaccident examination of the helicopter structure revealed that one of the center frame tubes, located just aft of the cabin, had separated due to a fracture. Examination of the fracture surface revealed that a fatigue fracture initiated in the tube at the root of a fillet weld associated with a reinforcing fillet welded axially along the tube. The fatigue crack propagated through the thickness of the tube and circumferentially around the tube from both sides of the weld root. Darker corrosion product found on the fracture face indicated that the preexisting fatigue crack had been exposed to atmospheric elements. The tube fracture occurred in single-sided bending consistent with in-flight airframe loads. The helicopter's most recent 100-hour inspection occurred about 1 month before the accident. According to manufacturer's guidance, the 100-hour inspection required, in part, an inspection of all structural tubing and fittings for cracks, cuts, bends, corrosion, distortion, and damage. If maintenance personnel had adequately inspected this area, they might have identified the degradation of the tube.

## Factual narrative

On September 11, 2013 about 0715 Pacific daylight time, a Bell 47G5 helicopter, N7836S, sustained substantial damage during a precautionary landing about 6 miles southeast of King City, California. The pilot, the sole occupant, was not injured. The helicopter's lower center frame and tailboom sustained substantial damage. The helicopter was registered to and operated by Wilbur-Ellis Corporation as an agricultural spray flight under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed for the flight and no flight plan was filed. The pilot reported that he was spraying a field about 3-4 feet above the ground when he heard a loud bang. The pilot elected to make a precautionary landing; during the landing flare, the helicopter started to spin. The pilot lowered the collective and the helicopter landed hard onto a road; subsequently, the helicopter's skids spread, and the lower center frame and tailboom were bent. Postaccident examination by a Federal Aviation Administration (FAA) inspector revealed that one of the helicopter's center frame tubes, located just aft of the cabin, had fracture separated. A senior metallurgist at the National Transportation Safety Board (NTSB) materials laboratory reviewed photographs of the fractured surfaces and reported that a fatigue fracture initiated in the tube at the root of a fillet weld associated with a reinforcing filet welded axially along the tube. The fatigue crack propagated through the thickness of the tube and circumferentially around the tube from both sides of the weld root. Darker corrosion product on the fracture face is indicative of a pre-existing fatigue crack that had been exposed to atmospheric elements. The fracture in the tube occurred in single-sided bending consistent with inflight airframe loads. On August 8, 2013 the helicopter underwent a 100 hour inspection. According to manufacturer's guidance, the 100 hour inspection requires in part: to "Inspect all structural tubing and fittings for cracks, cuts, bends, corrosion, distortion and damage."

## Aircraft

- Registration: [N7836S](https://openairdocs.org/aircraft/N7836S), registered now, S/N 7860, built 1967 (OpenAirDocs)
- Reported make and model: BELL 47G5
- Registry model: [Bell 47G-5](https://openairdocs.org/model/bell-47g-5) (registry code 1181032) (OpenAirDocs)
- Registry engine: [Lycoming VO-435 series](https://openairdocs.org/engine/lycoming-vo-435-series) (registry code 41525, reciprocating) (OpenAirDocs)
- Category: Helicopter
- Serial number: 7860
- Year built: 1967
- Airframe hours: 16798
- Engines: LYCOMING VO-435 SERIES 260 horsepower reciprocating
- Damage: Substantial damage
- Flight: Aerial application, Part 137
- Operator: Wilbur-Ellis Co
- Registered owner: Wilbur-Ellis Co
- Airworthiness certificate: Special, Restricted (Special); Standard, Normal
- Last inspection: Continuous airworthiness, September 5, 2013, 10 hours before
- Departed: KIC, King City, CA
- Destination: KIC, King City, CA
- Airport: KIC, Mesa Del Rey Airport

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 58; injuries: none; flight instructor, commercial; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 2, with waivers/limitations; last flight review April 6, 2012; seat: left; restraint: 3-point; flight hours: 30,000 total, 20,000 in this make and model

## Weather

- Conditions: Visual (VMC), Day
- Wind: 320° at 4 knots
- Visibility: 10 statute miles
- Lowest ceiling: 1,700 ft overcast
- Temperature: 63 °F, dew point 55 °F
- Altimeter: 30.00 inHg
- Observed at: SNS, 40 nm away

## FAA reports on this event (1)

The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs).

- [Accident 20130911022579A](https://openairdocs.org/incident/20130911022579A), Sep 11, 2013: Bell 47G-5, King City, CA, Substantial damage. During aerial application activity, the pilot stated he heard a loud bang and made a hard landing on a dirt road 6 miles se of King City, CA.

## Docket (9 files)

The files NTSB released with the investigation on March 14, 2014, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=88025. 2.5 MB in all.

- Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 256 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40406721&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF
- Photographs of the Fracture Surface - Courtesy of the Federal Aviation Administration (item 2, .PDF, 418 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40406861&FileExtension=.PDF&FileName=Photographs%20of%20the%20Fracture%20Surface%20-%20Courtesy%20of%20the%20Federal%20Aviation%20Administration-Master.PDF
- Maintenance Records (item 3, .PDF, 805 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40408013&FileExtension=.PDF&FileName=Maintenance%20Records-Redacted.PDF
- Excerpt from Manufacturer's 100 Hour Inspection Maintenance and Overhaul Instructions (item 4, .PDF, 390 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40408011&FileExtension=.PDF&FileName=Excerpt%20from%20Manufacturer%27s%20100%20Hour%20Inspection%20Maintenance%20and%20Overhaul%20Instructions-Master.PDF
- Materials Laboratory Report (item 5, .PDF, 61 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40410638&FileExtension=.PDF&FileName=Materials%20Laboratory%20Report%20%20-Master.PDF
- Photo 1. Overall view of the structural tube frame and the location of a fracture in a tube. (item 6, .PDF, 174 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40410639&FileExtension=.PDF&FileName=Photo%201.%20Overall%20view%20of%20the%20structural%20tube%20frame%20and%20the%20location%20of%20a%20fracture%20in%20a%20tube.-Master.PDF
- Photo 2. Closer view of the factured tube pieces. (item 7, .PDF, 131 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40410640&FileExtension=.PDF&FileName=Photo%202.%20Closer%20view%20of%20the%20factured%20tube%20pieces.%20%20-Master.PDF
- Photo 3. Fracture surface of the failed tube (forward piece). (item 8, .PDF, 150 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40410641&FileExtension=.PDF&FileName=Photo%203.%20Fracture%20surface%20of%20the%20failed%20tube%20(forward%20piece).%20%20-Master.PDF
- Photo 4. Fracture surface of the failed tube (aft piece) mating to the fracture surface showin in photo 3. (item 9, .PDF, 108 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40410642&FileExtension=.PDF&FileName=Photo%204.%20Fracture%20surface%20of%20the%20failed%20tube%20(aft%20piece)%20mating%20to%20the%20fracture%20surface%20showin%20in%20photo%203.%20%20-Master.PDF

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

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