Accident Details
On March 1, 2017, about 1030 Pacific standard time, a Bell 206B helicopter, N211CS, was substantially damaged during a forced landing following a loss of engine power near Firebaugh, California. The commercial pilot was not injured. The helicopter was registered to and operated by West Valley Aviation Inc., under 14 CFR Part 137 for agricultural aerial application. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from a nearby staging area about 1 minute prior to the accident.
Pilot Report
The pilot reported flying about 5 ft over the crop at 70 mph when he heard a loud bang followed by a loss of engine power. The helicopter shook violently as he rolled the throttle twist-grip to the detent stop. He reached over with his right hand to press the throttle detent switch and inadvertently rolled on the throttle slightly, hearing the engine spool up in RPM. He then reached the throttle detent switch to close the throttle. An autorotation was executed; upon touchdown in a freshly plowed soft field, the main rotor blade struck the tailboom, and the helicopter came to rest upright.
Postaccident Findings
The pilot had refueled about 20 minutes prior with 20-25 gallons of fuel, estimating about 10 gallons remained at the time of the accident. Postaccident examination revealed the tailboom separated forward of the tail rotor gearbox. When power was applied, the fuel quantity gauge indicated about 15 gallons; the tank was drained and contained about 6.5 gallons. When fuel was poured back, the gauge displayed about 6 gallons. The forward boost pump was operational; the aft boost pump was inoperative. No preimpact anomalies were observed with the main rotor, drive system, or flight controls.
Engine and Fuel Pump Examination
The engine was removed, installed on a test stand, and run through 6 test runs totaling about 33 minutes, appearing normal except for various chip detector lights. Disassembly revealed damage to the forward face of the separator on the #3 bearing, possibly sustained during seal replacement for the test run. No additional anomalies were noted. Examination of the fuel pump found the thermal fuse open, consistent with overheating. Parker Hannifin stated the thermal fuse is designed to shut off power when the pump casing builds excess heat from running without fluid.
Manual Reference
The Bell Helicopter Flight Manual, section 3, page 3-8, states: "Due to possible fuel sloshing in unusual attitudes or out of trim conditions, and one or both fuel pumps inoperative, the unusable fuel is ten gallons."
Probable Cause
The investigation did not provide a stated probable cause.