ERA18LA135 · Heard Alan E MOSQUITO XE

The day before the accident, the pilot conducted flight tests in the experimental, amateur-built helicopter with the newly-installed engine autotuner in order to determine the proper air-to-fuel ratio (AFR); however, the pilot inadvertently failed to complete the autotuner setup, and the autotuner reverted back to a lean air-fuel ratio setting. The next day, while departing on a local flight about 75 ft above ground level (agl), the engine started to backfire and the engine power began to surge. The pilot decided to land straight ahead, but the main rotor rpm was low, which precluded entering an autorotation, and the engine subsequently lost total power about 50 ft agl. The helicopter entered an uncontrolled descent and impacted the ground; a postcrash fire ensued. Postaccident examination of the thermally-damaged engine revealed no evidence of preimpact failure or malfunction of the powertrain or lubrication system. Although the autotuner was destroyed in the postcrash fire, the in-flight separation of one air induction filter, the coloration of the spark plugs, and the backfiring engine were consistent with the engine operating in an excessively lean air-fuel condition. The partial, then total, loss of engine power was likely due to the engine computer (ECU) detecting the backfiring and putting the engine into a self-preserving partial, then idle, power mode. It is likely that the pilot's failure to maintain rotor rpm following the loss of engine power resulted in the uncontrolled descent and hard landing.

Verbatim NTSB analysis and probable cause from the NTSB dataset