Bell 47G-3B-2 N475AL accident near Los Banos, California

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for WPR11LA379 for the full record.