Brawley, CA, USA
N8520V
ROCKWELL INTERNATIONAL S-2R
The pilot was en route to perform an aerial application pass when the airplane’s engine lost partial power. He made a forced landing in a nearby pasture; however, the airplane struck a railroad track during the landing roll, and a postcrash fire ensued. Examination of the radial engine revealed that the connecting rods for the pistons in cylinders 3 and 8 had failed, resulting in significant internal engine damage and the partial loss of power. Examination of the remains of the connecting rods for both cylinders revealed wear signatures that were mostly consistent with engine operation after failure. Because the engine continued operating after the failure, most features on the connecting rods fracture surfaces were obliterated, which made it difficult to identify the initial failure. However, further examination of the inside diameter surfaces of the wrist pin bearing of the number 3 cylinder connecting rod revealed wear signatures consistent with normal engine operation. Based on this finding, it is most likely that the event was initiated by the failure of the number 3 piston rod because it was not affected by the continued operation of the engine. The design of the engine's fuel priming system was such that cylinder number 3 was prone to piston hydraulic lock due to its low position within the priming system. Hydraulic lock is caused by excessive fuel being injected into a cylinder prior to engine start, and can be caused by either excessive priming, or a failure in the plunger or check valve assemblies, either of which could result in fuel trickling from the lowest nozzle within the system during non-operation. Postaccident fire damage precluded a thorough examination of the plunger and check valve assemblies. Hydraulic lock can result in a bent connecting rod and subsequent rod failure. The airplane was a low wing type, with integral wing fuel tanks located below the engine, and as such, it is unlikely that fuel had leaked into cylinder number 3 during non-operation.
HISTORY OF FLIGHT On June 23, 2011, about 0700 Pacific daylight time (PDT), a Rockwell International S-2R, N8520V, struck a railroad track during a forced landing near Brawley, California. Farm Aviation, Inc., was operating the airplane under the provisions of Title 14 Code of Federal Regulations (CFR) Part 137 as an aerial application flight. The certificated commercial pilot sustained serious injuries. The airplane sustained substantial damage during the impact sequence, and was consumed by post crash fire. The flight departed Brawley Municipal Airport, Brawley, about 0630. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that the airplane was loaded with powdered sulfur, and that while en route to the application area, he heard a bang, followed by a vibration and a total loss of engine power. He attempted to force land the airplane in a field, and during the landing roll, the airplane struck a railroad track. The collision breached the hopper, and the airplane came to rest intact about 60 feet beyond the initial impact point. The fixed-gear, tailwheel-equipped, low-wing airplane, serial number 2280R, was manufactured in 1976. The operator reported that at the time of the last annual inspection, dated June 11, 2011, the airplane had accrued a total time of 6,427 flight hours. The airplane was powered by a Pratt and Whitney R-1340-59 engine, serial number 11588, rated at 600 horsepower. The engine was a single-row, nine-cylinder, air-cooled radial type, with an integral centrifugal supercharger (blower). Maintenance logbooks indicated that Younkin Aviation, Inc., West Fork, Arkansas, overhauled the engine in May 2011, 21 flight hours prior to the accident. The records did not indicate the engines total time at overhaul. TESTS AND RESEARCH The engine was examined by the NTSB investigator-in-charge (IIC). The blower gearbox case had sustained impact damage, and had become separated from the aft section of the engine. All cylinder heads remained attached to the crankcase, all pushrods were straight, and the case was free of perforations or cracks. Removal of the rocker covers revealed the springs, valve stems, and rocker arms to be intact and wet with oil. The forward spark plugs were removed and examined. The electrodes were of the massive electrode type, and were dark grey in color, with wear signatures consistent with normal operation and a short service time when compared to the Champion Aviation check-a-plug chart AV-27. The crankshaft was seized, and could not be rotated. The engine was subsequently transported to the facilities of Younkin Aviation, Inc., for examination under the supervision of a Federal Aviation Administration (FAA) inspector. Disassembly revealed that the connecting rods for all cylinders except numbers 3 and 8 sustained bending damage, and remained attached to their respective pistons and the crankshaft/master rod assembly. The piston rod for cylinder 8 exhibited a slight curve, remained attached to its piston, and had become separated from its link pin end, which was fragmented. Piston number 3 exhibited a slight curve, remained attached to its link pin, and had separated from its piston wrist pin, which was fragmented. Piston number 3 exhibited damage to its skirt consistent with piston rod contact. Fragmented sections of the cylinder number 3 and 8 connecting rods, link pins, bearings, and rod fragments were sent to the NTSB Office of Research and Engineering Materials Laboratory Division for examination. The fracture surfaces of the number 3 connecting rod, wrist pin end, and connecting bearings were battered such that the mode of fracture could not be determined. The piston end of the number 8 connecting rod was deformed and fractured through the I-beam and through the wrist pin bore. All fracture surfaces on the connecting rod were battered such that the mode of fracture could not be determined except for a small area, which exhibited features consistent with an overstress fracture. The inside diameter surfaces of the cylinder number 3 wrist pin bearings were free of grooves and scratches; all other recovered bearing and link pin contact surfaces exhibited deeply grooved wear scars consistent with low-stress abrasions. The complete examination report is contained within the public docket for this accident. ADDITIONAL INFORMATION The engine's fuel priming system consisted of a plunger, check valve, and distribution assembly, which supplied fuel to primer nozzles within the inlet ports of cylinders 1,2,3,8, and 9. The position of the nozzle for cylinder number 3 was at the lowest point in the priming system, with its intake manifold being the only one that flows in a downward direction to the intake valve. The Pratt and Whitney Maintenance manual applicable to the R1340 series engine states that excessive priming can cause fuel to drain to the lower cylinders, resulting in hydraulic lock and subsequent piston rod damage. Postaccident fire damage precluded an accurate assessment of the priming system prior to the accident.
The partial loss of engine power shortly after takeoff, which likely resulted from the failure of the piston connecting rod in cylinder number 3 due to hydraulic lock in that cylinder.
Source: NTSB Aviation Accident Database
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