CEN15LA170

The airline transport pilot was conducting a cross-country personal flight. The pilot reported that, after climbing the airplane to cruise altitude and leaning the fuel mixture, he noted an unusual engine vibration. He enriched the fuel mixture slightly, and the vibration seemed to lessen. After descending and increasing power to level off, the engine vibration worsened, and he chose to perform a precautionary off-airport landing. During the landing, the landing gear "dug in," and the airplane subsequently nosed over. Testing of the engine's fuel system components revealed that the fuel servo fuel flow was slightly richer than the specified service limits. However, according to the pilot's report, the initial engine vibration did not occur until he leaned the fuel mixture at cruise altitude, indicating that the slightly rich calibration of the fuel servo likely was not associated with the engine vibration described by the pilot. Based on the available evidence, the reason for the engine vibration could not be determined.

On March 10, 2015, about 1210 central daylight time, a Maule M7 airplane, N55000, sustained substantial damage when it nosed over during a precautionary landing near Oregon, Missouri, following a partial loss of engine power during cruise flight. The pilot was not injured. The airplane sustained substantial damage to the vertical stabilizer and rudder. The aircraft was registered to Aardvark Aircraft Acquisition LLC and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not on a flight plan. The flight originated from the North Omaha Airport (3NO), Omaha, Nebraska, about 1125 and was destined for the Rosecrans Memorial Airport (STJ), St Joseph, Missouri.The pilot reported that after climbing to cruise altitude and leaning the engine he noted an unusual engine vibration. He responded by enriching the fuel mixture slightly above normal and the vibration seemed better. After descending and increasing power to level off, the engine vibration became "much worse", and he elected to perform an off-airport precautionary landing. He stated that the landing was normal but during the landing roll, the main landing gear "dug in" and the airplane nosed over. Postaccident examination of the airplane did not reveal any preimpact anomalies. The engine fuel servo, flow divider and nozzles were sent to the manufacturer's facility for testing under the direct supervision of NTSB investigators. The flow divider was tested and met all production test standards. The fuel nozzles were not flow tested but did not have any evidence of obstruction. The fuel servo was tested on the manufacturer's flow bench. The measured fuel flow rates were about 1.5 pounds per hour (1/4 gallon per hour) leaner than specified service limits.