Accident Details
Probable Cause and Findings
Maintenance personnel’s failure to safety wire the throttle control adapter per maintenance instructions when the carburetor was replaced and to detect that the adapter was not safety wired during the annual inspection, which resulted in the throttle control disconnecting and subsequent loss of engine control.
Aircraft Information
Registered Owner (Current)
Analysis
On March 30, 2014, about 1130 central daylight time, a Bell 47G-3B helicopter, N147WT, rolled over during an autorotation near Oglesby, Texas. The pilot and passenger were not injured. The helicopter was registered to and operated by Lazy 7 Agricultural Services LLC as a 14 Code of Federal Regulations Part 91 aerial observation flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from the Gatesville Municipal Airport (GOP), Gatesville, Texas at 1100.
The pilot reported the purpose of the flight was to spot wildlife. He stated that during the flight he initiated a climb to transit from one area to another. As he increased the throttle he noticed a "slight resistance." Shortly thereafter, the throttle freed up, but there was no response to the throttle movements. The pilot stated that both the engine and rotor rpm were indicating in the green, but the rotor and engine rpm decayed when he increased the collective. When he stopped increasing the collective, the rotor and engine rpm increased rapidly and continued to accelerate. The pilot initiated an autorotation, during which time he cutoff the fuel. During the final collective application to cushion the landing, the helicopter suddenly yawed to the left and simultaneously rolled to the right on touchdown.
A postaccident examination of the helicopter revealed the throttle control adapter had disconnected from the serrated shaft on the carburetor. The retention screws on the throttle control adapter were not safety wired to the carburetor idle stop lever as required in the Bell Helicopter maintenance manual.
Service Bulletin 131SB was issued on December 2, 1960, which addressed the reworking the adapter screw holes and safety wiring of the adapter to the carburetor idle stop lever. This Service Bulletin addressed the Bell 47G-2 and 47J-2 models. The maintenance manual for the 47G-3B was later revised to reflect the information in the Service Bulletin. Maintenance records show the carburetor was removed and replaced on September 25, 2012, and the last annual inspection was performed November 11, 2013.
Data Source
Data provided by the National Transportation Safety Board (NTSB). For more information on this event, visit the NTSB Records Search website. NTSB# CEN14LA174