Bell 47G 3B N147WT accident near Oglesby, Texas

Probable cause

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.

NTSB analysis

The pilot reported that, during an en route climb in the helicopter, he increased the throttle and noticed a "slight resistance." Shortly after, the throttle freed up, but it was ineffective; there was no corresponding change in engine power when he moved the throttle control. Both the engine and rotor rpm were indicating in the green, but they began to decrease when he adjusted the collective. When he stopped moving the collective, the engine and rotor rpm increased rapidly and continued to accelerate. The pilot initiated an autorotation and cut off the fuel. The pilot then increased the collective, and the helicopter yawed left, touched down, and rolled over to the right, which resulted in the tailboom separating.

A postaccident examination of the helicopter revealed that 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 helicopter manufacturer's maintenance manual. A review of maintenance records revealed that the carburetor was removed and replaced about 1 1/2 years before the accident and that the helicopter's last annual inspection was conducted about 4 1/4 months before the accident.

Text from the NTSB. Open the NTSB docket for CEN14LA174 for the full record.