Los Banos, CA, USA
N475AL
BELL 47G-3B-2
During an agricultural application flight, the helicopter had just departed from the elevated landing pad when it experienced a loss of lift, which the pilot thought was a loss of engine power. The pilot attempted to arrest the descent but was unsuccessful before the helicopter hit the trees. Examination of the engine revealed no pre-impact mechanical malfunctions that would have prevented normal operations. Postaccident examination noted that the compressor vanes and both main rotor blades were layered with a substance that had the consistency of peanut butter. As a result of the contamination, the compressor would not operate at its optimum level, which affected the power output performance of the engine. The material on the main rotor blades would also have affected the helicopter’s lift performance. The examination did not reveal what the substance was, but the airplane flew through agricultural products, which may have led to the substance building up on the blades and compressor over time. Had the pilot done a preflight inspection, he may have noticed the contamination in the engine compressor section and the main rotor blades. Because the helicopter was departing from an elevated land platform, after liftoff, the helicopter was immediately out of ground effect, which resulted in an immediate descent that the pilot was unable to arrest.
HISTORY OF FLIGHT On August 14, 2011, about 1245 Pacific daylight time (PDT), a Bell Helicopter, 47G-3B-2 Soloy, N475AL, impacted trees following a loss of lift near Los Banos, California. Double Springs Helicopters was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 137. The commercial pilot was not injured; the helicopter sustained substantial damage to the tailboom and to the main rotor blades. The agricultural flight had just departed from the tender truck about 1245. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that just after takeoff from the elevated landing pad located on top of the tender truck, he observed the rotor rpm decaying. He attempted to regain the rpm by lowering the collective; however, the rpm did not recover, and due to the low altitude operation, he was forced to land into the almond grove. The helicopter was recovered from the accident site for further examination. The helicopter and engine were inspected on August 18, 2011, at the operator’s facility. No evidence of pre-impact abnormalities with the helicopter’s flight controls were noted. The engine had impact damage to the fuel control unit, which resulted in a fuel leak from the control arm shaft. No other mechanical abnormalities were noted that would have precluded normal operation. It was noted that the compressor stator vanes were very dirty from what appeared to be an agricultural product. The representative of the engine manufacturer noted that there is a need to complete compressor rinses more frequently when the engine is operated in a corrosive environment such as agricultural operations. The main rotor blades were noted to be contaminated with a unknown agricultural product. The entire surfaces of both blades were frosted with a layer of an unknown material, which had the consistency of peanut butter.
The pilot’s inadequate preflight, and contamination of both the engine compressor section and the main rotor blades, which resulted in reduced performance of the engine and rotor system.
Source: NTSB Aviation Accident Database
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