Pilot Debrief

BELL 47G5 near King City, CA — 2013-09-11

Final reportWPR13LA404
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Date
2013-09-11
Location
King City, CA, USA
Airport
KIC
Aircraft
BELL 47G5
Registration
N7836S
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Maneuvering

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.

NTSB narrative

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.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Inspect structural tubing for cracks during the 100 hour inspection.

NTSB coding

Evidence available

  • Photos
  • 9 docket documents
View NTSB final reportView NTSB docket

Docket documents9

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