History of Flight
On May 31, 2014, about 0944 eastern daylight time, an experimental amateur-built Vans RV-6, registration N6RP, operated by a private individual, was substantially damaged near Suffolk, Virginia. The private pilot sustained minor injuries. Visual meteorological conditions prevailed for the personal flight conducted under 14 CFR Part 91. The flight originated from William Tuck Airport (W78) in South Boston, Virginia, at 0830, destined for Suffolk Executive Airport (SFQ).
According to the pilot's written statement, after departure he climbed to 3,700 ft msl and leveled off for cruise. He later descended to 2,000 ft msl about 10 nm from the destination. At the first waypoint, he turned to a heading of 100 degrees magnetic. The engine rpm then went to redline, thrust was lost, and a reddish residue resembling hydraulic fluid from the propeller covered the windscreen. The pilot pitched the nose up, pulled the throttle, and turned into the wind. He selected an emergency landing site but, upon attempting to add power, the engine yielded no thrust. Realizing he would not make the site, he slowed the aircraft. Just above the trees, he pulled the control stick aft; the airplane stalled before contacting the treetops, then fell through trees, impacted the ground, and came to rest nose first, resulting in substantial damage to the wings and fuselage.
Personnel Information
The pilot held a private pilot certificate with single-engine land rating and about 1,418 total flight hours, of which 242 were in the accident airplane make and model. His most recent third-class FAA medical certificate was issued September 3, 2013.
Aircraft Information
According to FAA records, the RV-6 was a two-seat, all-metal, low-wing airplane with tricycle landing gear. It received an experimental airworthiness certificate on August 25, 2006. Built from kits by Vans Aircraft and Eggenfellner Aircraft, Inc., it was equipped with an Eggenfellner Subaru H4 modified automobile engine with a Gen 3 V4 propeller speed reduction unit (PSRU) gearbox driving a Vari-Prop VP-01 propeller assembly with three hydraulically actuated wooden blades. The propeller was a closed, airless hydraulic system independent of other aircraft systems, rated at 165 HP and 6500 rpm. The airplane had 158 total flight hours when the Vari-Prop assembly was installed; blades were replaced at 221 hours.
The propeller system allowed in-flight pitch adjustment. Power transferred from the gearbox through a propeller drive shaft bolted at the aft end to the gearbox output and inserted at the opposite end into a female key slot in the propeller hub. Blade pitch was adjusted via a non-rotating slave piston cup and rotating blade pin spool separated by a needle thrust bearing. Hydraulic fluid from a separate reservoir fed an expandable chamber in the slave piston, moving the blade pin spool fore and aft. The blade pin spool had two bolts with nuts providing a mechanical low pitch stop in case of hydraulic pressure loss. According to the builder/pilot, the stop nuts were tightened so washers were slightly loose when blade pitch angles provided required static rpm. The builder positioned blade angles to achieve 2520 rpm stationary.
At the time of the accident, the airplane had 247 total flight hours. The propeller was last inspected and serviced about 28 hours earlier when the pilot/builder cleaned and lubricated the bearings with Sig Wurth 3000 grease, a highly viscous lubricant, or a mixture of Honda Moly 60 and Sig Wurth 3000. The pilot reported using only Sig Wurth 3000 on the needle thrust and slave servo bearings. According to the lubricant manufacturer, Sig Wurth 3000 is advertised for fleet and heavy equipment in automotive/trucking industries. The manufacturer stated a policy not to sell products for flying applications.
Meteorological Information
The 0935 automated observation at SFQ included wind from 020° at 7 knots, visibility 10 statute miles, sky clear, temperature 21°C, dew point 15°C, altimeter 30.16 inHg.
Wreckage and Impact Information
The propeller, wings, and tail section were accounted for at the accident site. The airplane came to rest nose-down in an area surrounded by trees. Photographs showed the airplane supported upright by the left wingtip and nose, with the fuselage against a tree. Both wings had significant leading edge damage; a tree was embedded in the right wing about mid-span, eight inches from the leading edge. The left wing had horizontal crush damage on the outboard leading edge. The vertical stabilizer and rudder had compression wrinkles. All control surfaces remained attached. All three propeller blades separated from the hub; only two were accounted for at the site.
Examination at a recovery facility revealed severe thermal damage in the servo unit. The non-rotating portion of the slave servo had seized to the rotating portion at the needle thrust bearing interface, causing the entire unit to rotate with the propeller. The hydraulic hose support bracket was fractured; the hydraulic fitting on the non-rotating portion was also fractured, allowing hydraulic fluid to escape. The needle thrust bearing was severely heat-damaged; its three components had become fixed and rotated as one. No grease was present, and needle rollers were separated and displaced. Burned material samples were consistent with Sig Wurth 3000 grease. Significant heat damage was present throughout the needle thrust bearing and the slave servo collar, which contained no lubrication. The Delrin bushings over the pins in the propeller hub were melted.
Additional Information
The Vari-Prop VP-01 installation manual (Jan 12, 2010) instructed to "lightly grease the bearings on the blade/bearing assembly" and to grease the thrust bearing between the spool and piston, but did not specify a lubricant. A Vari-Prop representative recommended Lubriplate for the needle thrust bearing. The pilot/builder stated the manufacturer provided no additional guidance for lubricating the bearings.