Amateur-built VANS RV6 N6RP accident near Suffolk, Virginia

Probable cause

The pilot/builder's decision to use a lubricant that was not designed for high-speed aviation applications, which resulted in the failure of the propeller needle thrust bearing due to the loss of lubrication from overheating, a total loss of aerodynamic thrust, and a subsequent forced landing.

NTSB analysis

According to the pilot, who was also the owner/builder of the experimental amateur-built airplane, while operating about 2,000 ft mean sea level, the engine rpm went to redline, the airplane lost thrust, and a red fluid covered the windscreen. The pilot attempted a forced landing to a field, but the airplane impacted trees, which resulted in substantial damage to the wings and fuselage.

Postaccident examination of the engine revealed no mechanical malfunctions or anomalies to the engine or gearbox. Examination of the propeller showed that the fine pitch stop nuts were missing. The propeller needle thrust bearing and mating parts were galled, burned, and damaged from rotational and thermal wear. The heat damage likely initiated at the needle thrust bearing and progressed both fore and aft to the blade pin spool and slave piston collar, respectively. The Delrin bushing melted after the heat reached the blade pin spool, which allowed the propeller pins to make bare metal contact with the spool pin slots. With the absence of stop nuts, the propeller blade angle could decrease below a 0-thrust angle, and the melted Delrin bushings allowed the propeller blade angles to decrease an additional 6 degrees in the negative direction. The cumulative effect of both events was a total loss of aerodynamic thrust.

About 28 flight hours before the accident, the pilot/builder used a viscous lubricant to grease the bearings in the propeller assembly. According to the lubricant manufacturer, the lubricant has a high viscosity and was designed for low-speed automotive and trucking applications not for aviation use. Lubricants with high viscosities can cause the bearing to skid, which can result in rapid overheating and failure of the bearing.

Text from the NTSB. Open the NTSB docket for ERA14LA272 for the full record.