On May 4, 2009, about 1620 Pacific daylight time, a Grumman G-164B, N6628Q, collided with terrain during a forced landing at Colfax, Washington. The owner was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 133. The certificated commercial pilot was not injured. The airplane sustained substantial damage to the firewall, fuselage, and empennage by impact forces. The local aerial application flight was departing when the accident occurred. Visual meteorological conditions prevailed, and no flight plan had been filed.

The operator reported that the accident flight was the 14th flight of the day. The pilot fueled the airplane prior to the 13th flight. The pilot stated that the airplane was about 100 feet above ground level following takeoff when the engine lost power. The pilot landed straight ahead in a muddy dirt field. During the landing roll, the airplane nosed over, and the engine separated from the airframe.

Gustin Aviation, Lewiston, Idaho, examined the engine under the supervision of a Federal Aviation Administration (FAA) inspector. Examination notes are in the public docket. Pertinent parts of the examination follow.

The engine was not runable. Technicians placed it on a stand, and removed the spark plugs. Gaps for all spark plugs were similar with no mechanical deformation. The electrodes for the front spark plugs were oval. Electrodes for the front plugs for cylinders 2, 3, and 6 were gray, which corresponded to normal operation according to the Champion Aviation Check-A-Plug AV-27 Chart. Electrodes for the front plugs for cylinders 1, 4, 5, 7, 8, and 9 were black and sooty, which corresponded to excessive ground idle time or rich idle mixture when compared to the AV-27 chart. Electrodes for the back plugs for cylinders 2, 3, 7, 8, and 9 were gray; 2 and 7 had hard, cylinder like deposits, which indicated lead fouling per the AV-27 chart. Electrodes for the back plugs or cylinders 1 and 4 were black and sooty. Electrodes for the back plugs for cylinders 5 and 6 were wet and oily.

Technicians manually rotated the crankshaft with a wrench. The crankshaft rotated freely, and the valves moved approximately the same amount of lift in firing order.

The oil sump screen and oil filter were free of debris.

The FAA inspector stated that he noted no anomalies during the engine examination that would have precluded normal operation.

The Safety Board investigator-in-charge (IIC) oversaw examination of the Stromberg carburetor (part number nay9e1, serial number 5966358), at S&T Aircraft Accessories in New Braunfels, Texas. It sustained substantial crush damage. Technicians pointed out that the bushings on the throttle shaft were worn, and there was excessive play. The pump stem bushing was worn, and there was excessive play. The economizer linkage was loose. There were fretting signatures on the mating surface to the engine, and fretting on the mounting bolt holes. The carburetor fuel screen was clean. The floats exhibited crush damage, and the accelerator pump was worn.

Following the initial engine examination and release of wreckage, it was sent to Covington Aircraft, Covington, Oklahoma, for a propeller strike inspection. The engine was repaired and returned to the owner who installed it on the airplane. Following a test flight, the owner reported that it was running rough, and he returned the engine to Covington for further examination. They reported that they found a defective exhaust camshaft lobe, and replaced it, which remedied the engine roughness. Covington reported that a defective camshaft lobe could cause roughness, and if ignored, could lead to multiple lobe failure and a loss of engine power. However, they noted that this lobe was not defective to that point, and would not have been a cause for the reported loss of power. They felt that the carburetor was the reason for the loss of engine power.

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