WPR17LA071

The commercial pilot of the helicopter reported that, while conducting an aerial application flight, he heard a loud bang followed by a total loss of engine power. The pilot initiated an autorotation to a plowed field; however, during the landing sequence, the main rotor blades struck the tailboom. Postaccident examination revealed that, when power was applied, the fuel quantity gauge indicated about 15 gallons of fuel when the actual amount present was about 6.5 gallons. This erroneous fuel level indication led the pilot to believe that more fuel was available than was actually in the fuel tanks. The helicopter flight manual indicated that, due to possible fuel sloshing in unusual attitudes or out-of-trim conditions, and one or both fuel pumps inoperative, the unusable fuel is 10 gallons. The engine was subsequently installed on a test stand and several test runs revealed no anomalies. Further testing revealed that the aft fuel boost pump was inoperative. The thermal fuse was found open, consistent with overheating due to the lack of fuel being transferred through the pump. It is likely that, during the flight, the attitude of the helicopter was slightly nose-down. This allowed the fuel to transfer to the forward portion of the tank, which resulted in a lack of fuel flow through the aft fuel pump; the subsequent overheating triggered the thermal fuse, which caused the aft fuel pump to become inoperative. It is likely that the total loss of engine power was the result of fuel exhaustion.

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 the provisions of Title 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed for the local aerial application flight, which originated from a nearby staging area about 1 minute prior to the accident. The pilot reported that during an agricultural aerial application flight, he was flying about 5 ft over the crop at an airspeed of 70 mph, when he heard a loud bang followed by a loss of engine power. The helicopter shook violently as he rolled down the throttle twist-grip to the detent stop. As he reached over with his right hand to press the throttle detent switch, he inadvertently rolled on the throttle slightly and heard the engine begin to spool up in RPM, at which time he was able to reach the throttle detent switch to close the throttle. An autorotation was executed and upon touchdown in a freshly plowed soft field, the main rotor blade struck the tailboom and the helicopter came to rest upright. The pilot stated that he had refueled the helicopter about 20 minutes prior to the accident with 20 to 25 gallons of fuel. He estimated that at the time of the accident, he had about 10 gallons of fuel on board. Postaccident examination of the helicopter by the pilot revealed that the tailboom was separated forward of the tail rotor gearbox. The helicopter was recovered to a secure location for further examination. Examination of the helicopter by representatives of Bell Helicopter and Rolls Royce under the supervision of a Federal Aviation Administration inspector revealed that when power was applied to the helicopter, the fuel quantity gauge indicated about 15 gallons of fuel. The fuel tank was drained and contained about 6.5 gallons of fuel. When the fuel was poured into the fuel tank, the fuel quantity gauge displayed about 6 gallons. The forward boost pump was found operational; however, the aft boost pump was inoperative. No preimpact anomalies were observed with the main rotor assembly, main rotor drive system, or flight controls. The engine was removed and subsequently sent to Keystone Aviation for further examination. The engine was installed on a test stand and subsequently run. Throughout 6 test runs, the engine appeared to run normally with the exception of various chip detector lights throughout about 33 minutes of total run time. The engine was disassembled to gain access to the gearbox. Damage was noted to the forward face of the separator on the #3 bearing, which could have been sustained during replacement of the double lip seal with a single lip seal to accommodate the engine test run. No additional anomalies were noted with the engine. Examination of the fuelpump revealed that the thermal fuse was open consistent with over-heating. Representatives from Parker Hannifin stated that the fuel pump thermal fuse is designed to shut power off to the fuel pump when the fuel pump casing builds excess heat due to being run without fluid (fuel). 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.