Narrative
On August 27, 2013, at approximately 1519 eastern daylight time, a Revo, Incorporated Lake Model 250 (registration N522WB) made a forced landing in a field near Middle Grove, New York. The airplane was operated by a private individual under 14 CFR Part 91 as a personal flight from Saratoga County Airport (5B2), Saratoga Springs, New York, to Blue Mountain Lake in Indian Lake, New York. Visual meteorological conditions prevailed; no flight plan was filed. The private pilot, the sole occupant, sustained minor injuries, and the airplane incurred substantial damage.
The pilot reported that after an uneventful 28-minute flight from Blue Mountain Lake to 5B2 for avionics work, he performed a preflight inspection, including checking the engine oil (just over 9 quarts). He then fueled the airplane and conducted a standard walk-around, noting no oil leakage. After taxiing to runway 05, an engine run-up revealed no discrepancies. During takeoff, the engine developed full rpm (2,575). Approximately 3 minutes into the return flight, while climbing through 2,000 feet with mixture full rich and throttle full forward, engine rpm fluctuated from 2,575 to 2,625, then decreased, then increased to 2,675. The pilot turned back toward 5B2 while slowly climbing to 2,200 feet above ground level (agl), then heard a loud bang followed by rough engine operation. The throttle was pulled back but rpm did not decrease, and the engine did not seem to develop power. The pilot established best glide airspeed (74 knots) and, descending at 1,200 feet per minute, realized he could not reach 5B2. He set up for a base-to-final approach to an open field, pushed the nose forward after clearing trees, secured fuel and electrical, and forced the airplane onto the ground. The airplane bounced about 20 feet; the pilot pushed the nose down and touched down intentionally gear-up with only a quarter of the field remaining. To avoid obstructions ahead, he intentionally yawed the airplane to the right. After coming to rest, he exited and called 911; neighbors had already alerted emergency services.
Examination
An FAA inspector found the engine displaced to the left, with oil covering the engine pylon, top fuselage aft of the engine, and leading edges of the vertical and horizontal stabilizer. The right engine cowling interior was oil-coated. Closer inspection revealed a hole in the engine crankcase adjacent to the No. 6 cylinder. The engine was recovered for further examination.
Disassembly showed the No. 6 cylinder connecting rod remained attached to the piston via the piston pin but was separated from the crankpin journal. The connecting rod cap was missing, and the rod was deformed at the yoke. One connecting rod bolt was trapped in the deformed rod; its head was separated, bent, fractured, and tapered. The nut was absent. The crankcase interior near cylinders No. 5 and 6 had extensive scoring and gouge marks. The oil sump contained about four ounces of oil and foreign material, including crankcase fragments, piston fragments, main bearing pieces, and the nut from the No. 6 connecting rod bolt. The No. 6 cylinder was cut to remove the piston. Other cylinders and main bearings were unremarkable; oil was present on all surfaces. The connecting rod cap was not found.
Components from the No. 6 cylinder were examined by the NTSB Materials Laboratory. The connecting rod fracture surface showed relatively smooth features with curving crack arrest marks consistent with fatigue from multiple origins, extending across nearly the entire fracture surface. The split face of the deformed strap exhibited fretting damage. The fractured connecting rod bolt showed features consistent with ductile overstress fracture. The recovered nut was impacted and deformed on the clamping and opposite faces, with intact threads.
Maintenance History
Maintenance records indicated the engine was last overhauled on December 15, 2011, and installed on December 26, 2011, at hour meter reading 1515.3. On January 6, 2012 (about 12 hours after installation), oil was drained and the filter inspected; a new filter and 10 quarts of Phillips 20W50 mineral oil were added. On June 12, 2012 (about 43 hours after installation, 1558.7 hour meter), during a 100-hour inspection, entry noted "#4 & #6 cylinder thru bolts leaking, replace push rod seals." On June 21, 2013 (about 141 hours after installation, 1656.6 hour meter), oil was drained, filter inspected, and engine serviced with 9 quarts of Shell 100Wplus oil and a new filter. No further entries existed. The engine had accumulated about 153 hours since major overhaul.
According to personnel at the facility that addressed the January 6, 2012 oil leakage, the leakage was attributed to cylinder through bolts. Their maintenance included lubricating threads and nuts of the through bolts, and reinstalling cylinders with new push rod seals and o-rings at the cylinder bases.
Engine Monitor Data
The portable GPS receiver and J.P. Instruments EDM 700 were read out by the NTSB Vehicle Recorders Division. The EDM 700 internal time was two minutes behind UTC (offset applied). Recording began about 1 minute 30 seconds after takeoff. Oil temperature decreased slightly over the next 2 minutes 40 seconds, while rpm, EGT, and CHT remained steady. At about 1516:50, oil temperature began increasing. Between 1517:02 and 1517:08, rpm dropped from 2,630 to 2,365, then six seconds later to 1,234. At that same time, all EGT and CHT readings began decreasing.