Middle Grove, NY, USA
CONSOLIDATED AERONAUTICS INC LAKE MODEL 250
The pilot stated that, shortly after takeoff, the engine experienced uncommanded rpm changes, so he turned the airplane to return to the departure airport. He then heard a "loud bang" and the engine running roughly, and it subsequently lost power. When the pilot realized that he would be unable to land at the airport, he maneuvered the airplane for a gear-up forced landing in a field. With only 1/4 of the length of the field remaining, he intentionally yawed the airplane right to avoid obstructions ahead; the airplane came to rest upright with the engine displaced to the left. Oil was covering the engine pylon, the top portion of the empennage, and the leading edges of the vertical and horizontal stabilizers. Postaccident examination of the engine revealed a hole in the crankcase adjacent to the No. 6 cylinder. The No. 6 cylinder connecting rod was fractured and separated from the crankshaft, and one nut from the No. 6 cylinder connecting rod bolt was not present and was found inside the oil sump. Metallurgical examination of the components of the No. 6 cylinder connecting rod revealed that the connecting rod was fractured through one side of the strap at the crankshaft end of the connecting rod and that the strap segment on the other side of the connecting rod was deformed inward. The fractured strap exhibited fatigue cracks from multiple origins, and the split line face on the deformed segment of the strap exhibited fretting damage. The connecting rod likely fractured due to insufficient clamping force across the split line at the crankshaft end of the connecting rod. Although the No. 6 cylinder had been removed about 110 hours before the accident flight, the work performed at that time was not associated with the No. 6 cylinder connecting rod. Therefore, given that the engine was overhauled only about 153 hours before the accident flight, it is likely that insufficient torque was applied to the connecting rod nut during the overhaul.
On August 27, 2013, about 1519 eastern daylight time, a Revo, Incorporated Lake Model 250, N522WB, registered to Terell Enterprises LLC, operated by a private individual, was landed hard during a forced landing in a field near Middle Grove, New York. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 Code of Federal Regulations (CFR) Part 91 personal flight from Saratoga County Airport (5B2), Saratoga Springs, New York, to a body of water called Blue Mountain Lake, located in Indian Lake, New York. The airplane sustained substantial damage and the private pilot, the sole occupant sustained minor injuries. The flight originated about 7 minutes earlier from 5B2.The pilot stated that earlier that day he performed a preflight inspection of the airplane which included checking the engine oil noting it had just over 9 quarts, then flew the airplane on an uneventful 28 minute flight from Blue Mountain Lake to 5B2 for the purpose of avionics work. After landing, he personally fueled the airplane, and before departure for the return flight, he performed a standard walk around inspection of the airplane noting there was no evidence of oil leakage on the ground. He taxied to runway 05 where he performed an engine run-up; no discrepancies were reported. During takeoff the engine developed full rpm (2,575), and about 3 minutes into the planned 28-minute flight while climbing through 2,000 feet with the mixture control full rich and the throttle full forward, the engine rpm went from 2,575 to 2,625, then decreased, then increased to 2,675. He turned to return to 5B2, while slowly continuing to climb to 2,200 feet above ground level (agl), and heard a loud bang followed by a rough running engine. Prior to that point the engine had been running smoothly. He pulled back the throttle, but the engine rpm did not decrease; the engine did not feel like it was developing power. He set to maintain best glide airspeed (74 knots), and while descending at 1,200 feet-per-minute, he realized he was unable to reach 5B2. He set up for a base to final approach for an open field, and while on final approach pushed the nose forward after clearing trees. He secured the fuel and electrical, and forced the airplane to the ground where it bounced 20 feet. He then pushed the nose down and flew above the field touching down intentionally gear-up with only ¼ of the length of field remaining. Realizing there were obstructions ahead he intentionally caused the airplane to yaw to the right in an effort to avoid obstructions ahead. After coming to rest he exited the airplane and called 911 to report the accident, but reported that neighbors had already called 911. Inspection of the airplane by an FAA inspector revealed the engine was displaced to the left, and oil was covering the engine pylon, top portion of the fuselage aft of the engine, and also the leading edges of the vertical and horizontal stabilizer. The right engine cowling was opened and oil was coating the interior surface. Closer inspection of the engine revealed a hole in the engine crankcase adjacent to the No. 6 cylinder. The airplane was recovered for further examination of the engine. Examination of the engine revealed the No. 6 cylinder connecting rod remained connected to the piston via the piston pin; however, the connecting rod was separated from the crankpin journal. The connecting rod cap was not attached to the connecting rod, and the No. 6 cylinder connecting rod was deformed at the yoke. The shank and threaded portion of one connecting rod bolt remain trapped in the deformed portion of the connecting rod, and the head portion of the bolt was separated. The head portion of the bolt was bent, fractured, and tapered. Approximately the last 3 to 4 threads of the bolt visually appeared undamaged, while numerous threads towards the shank portion of the bolt were damaged. The associated nut was not present on the bolt. The inside of the crankcase adjacent to the Nos. 5 and 6 cylinders had extensive scoring and gouge marks. The oil sump was removed and found to contain approximately four ounces of oil and foreign material, which consisted in part of the engine crankcase, part of the piston, part of a main bearing, and the nut that was absent from the No. 6 cylinder connecting rod bolt; no other bolt was definitively identified. The No. 6 cylinder was cut between the barrel and the head to facilitate removal of the piston. Examination of the other cylinder connecting rods and main bearings revealed all were unremarkable; oil was present on all surfaces. The connecting rod cap was not identified. The No. 6 cylinder piston, fractured connecting rod, fractured connecting rod bolt, and nut from the No. 6 cylinder connecting rod that was found in the oil sump were removed and retained for further examination. Also retained were the portable GPS receiver and the J.P. Instruments EDM 700 engine monitor. An engine examination report is contained in the NTSB public docket. Examination of the components from the No. 6 cylinder was performed by the NTSB Materials Laboratory located in Washington DC. The connecting rod was fractured through one side of the strap at the crankshaft end of the connecting rod, and the strap segment on the other side of the connecting rod was deformed inward. The deformed segment of the strap was cut to facilitate cleaning and further examination of the surface features. Relatively smooth fracture features with curving crack arrest marks consistent with fatigue from multiple origins were observed on the fracture surface; the fatigue extended across nearly the entire fracture surface. The split face of the deformed strap of the connecting rod exhibited fretting damage. Examination of the fractured connecting rod bolt revealed the features were consistent with ductal overstress fracture, while examination of the recovered connecting rod nut revealed it was impacted and deformed mainly on the clamping face and the opposite face of the nut. The nut remained intact around the circumference, and intact threads were observed on the interior of the nut. Review of the maintenance records revealed the engine was overhauled last on December 15, 2011, and was installed in the airplane on December 26, 2011, at hour meter reading 1515.3. On January 6, 2012, approximately 12 hours since the overhauled engine was installed, the engine oil was drained and the oil filter was cut open and inspected. A new oil filter was installed and the engine was serviced with 10 quarts of Phillips 20W50 mineral oil; the airplane was returned to service. On June 12, 2012, at approximately 43 hours since the overhauled engine was installed, or hour meter reading 1558.7, during a 100-Hour inspection of the engine, an entry in the logbook indicates, "#4 & #6 cylinder thru bolts leaking, replace push rod seals"; the engine was approved for return to service. On June 21, 2013, approximately 141 hours since the overhauled engine was installed, or hour meter reading 1,656.6, the engine oil was drained, the oil filter was cut open and inspected, and the engine was serviced with 9 quarts of shell 100Wplus oil, along with installation of a new oil filter; the engine was approved for return to service. There were no further entries in the engine maintenance records. The engine had accumulated approximately 153 hours since the last major overhaul. Excerpts of the maintenance records are contained in the NTSB public docket. According to personnel of the facility that discovered oil leakage on January 6, 2012, the leakage was attributed to be from the cylinder through bolts; however, the only maintenance they performed included lubrication of the threads and nuts associated with the through bolts, and reinstallation of the cylinders with new push rod seals and o-rings at the base of each cylinder. The invoice and paperwork documenting the work is contained in the NTSB public docket. The global positioning system (GPS) receiver and J. P. Instruments EDM 700 were submitted to the NTSB Vehicle Recorders Division for readout. The internal time of the EDM 700 was noted to be two minutes behind Universal Coordinated Time (UTC), so a two minute offset was performed. The engine monitor begins recording during the accident flight about 1 minute 30 seconds after takeoff, and the oil temperature was noted to decrease slightly over the next 2 minutes 40 seconds, while during the same time the rpm, exhaust gas temperate (EGT) and cylinder head temperature (CHT) readings were steady. At about 1516:50, the oil temperature began to increase, and between 1517:02 and 1517:08, the rpm dropped from 2,630 to 2,365, and then 6 seconds later at 1517:14, the rpm was at 1,234. At that same time the readings for all EGT and CHT began to decrease. A copy of the report and data downloaded from the EDM700 and GPS receiver are contained in the NTSB public docket.
Maintenance personnel’s application of insufficient torque to the No. 6 cylinder connecting rod bolts during overhaul, which resulted in the fatigue fracture of the connecting rod and a subsequent total loss of engine power.
Source: NTSB Aviation Accident Database
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