TOMS RIVER, NJ, USA
N910X
Grumman G-164-A
After the third aerial application pass, the airplane's engine developed a severe vibration, then seized. The pilot dropped the chemical load, turned the airplane into the wind, and made a forced landing into a cranberry bog. Afterwards, the pilot reported that had he had felt no binding when he pulled the propeller through, to check for hydraulic lock during the preflight. He also stated that the airplane hadn't seemed to be developing full power for about a week prior to the accident. During the post-accident engine teardown, an upper (number 9) cylinder was removed, and a broken connecting rod was found. Metallurgical examination of the connecting rod fragments revealed well-defined crack arrest positions, indicative of fatigue, on the smaller fragment. The fracture features within the fatigue region were relatively uneven, and in several locations, the fatigue crack appeared to have extended in overstress. There were no visible metallurgical defects in the initiation area. The larger fragment contained features typical of an overstress. According Advisory Circular 65-12A, a partial hydraulic lock could cause a 'slightly bent connecting rod,' which could go unnoticed, but would be 'sure to fail later.' Upper cylinders on high wing aircraft have been known to be susceptible to hydraulic lock due to a leaking fuel primer.
On May 25, 2000, about 0900 Eastern Daylight Time, a Grumman G-164-A Ag Cat, N910X, was substantially damaged during a forced landing in Toms River, New Jersey. The certificated commercial pilot was not injured, and visual meteorological conditions prevailed at the time of the accident. No flight plan had been filed for the local flight, from a farmer's airstrip. The aerial application flight was conducted under 14 CFR Part 137. According to the pilot, after the third application pass, and with 30 inches of manifold pressure (36 inches was maximum), the airplane's engine developed a severe vibration. It "quit making power, and emitt[ed] black smoke." The pilot dropped the chemical load, turned the airplane into the wind, then made the forced landing into a cranberry bog. The pilot further stated that earlier, during the preflight, he had pulled the propeller through 11 times, and felt no binding. He also noted that the airplane had not seemed to be developing full power for about a week prior to the accident, and that four different pilots normally flew the airplane. According to a Federal Aviation Administration (FAA) inspector, after touchdown, the airplane flipped over, and the tail broke, just behind the cockpit. During the post-accident examination, the Pratt & Whitney R-1340-AN engine could not be rotated. It was subsequently removed, and forwarded to the facility which had overhauled it in 1989, for teardown under FAA supervision. During the engine's removal, it was not noted whether there had been leakage from the fuel primer. The FAA inspector present during the teardown reported that the number 9 cylinder (one of the upper ones) was removed, and a broken connecting rod was found. A large piece of the connecting rod was lodged between the other connecting rods and the crankcase. In addition, the broken portion of the rod that was still attached to the crankshaft, had been jammed into the crankcase, between cylinders 1 and 9. Further inspection of the number 9 cylinder showed no abnormalities, other than those caused by the broken connecting rod. The cylinder walls were not scored. The rockers and valve springs were removed, and both valves operated smoothly. Both rockers, the lifters and the cam assembly revealed no discrepancies, and the pushrods were straight. Additional inspection of the engine revealed no other damage, except that which was documented from the rod failure. Engine maintenance records did not indicate that there had been any replacement of the number 9 cylinder since overhaul, and the cylinder hold-down nuts had a build-up of rust and dirt. The engine had 1,022 hours of operation since overhaul, in 1989. The Safety Board Materials Laboratory performed metallurgical examination of two connecting rod pieces. According to the factual report, "The fracture surfaces of the two pieces did not mate, indicating that a portion of the I-beam was missing...and presumed to be lost." The report further stated, "The fracture face of the smaller piece contained well defined crack arrest positions indicative of fatigue. The fracture features within the fatigue region were relatively uneven, and in several locations, the fatigue crack appeared to have extended in overstress. The rest of the fracture contained overstress fracture features emanating from the terminus of the fatigue fracture. The examination noted no visible metallurgical defects at the fracture initiation area." In addition, "The larger separated piece of the rod was subjected to post-fracture damage, which almost completely destroyed the fracture features on this piece. Undamaged portions of the fracture contained no evidence of progressive cracking and contained features typical of an overstress." Advisory Circular (AC) 65-12A, the FAA's airframes and powerplants mechanics' handbook, described the dynamics of hydraulic lock, as occurring in a radial engine's lower cylinders and intake pipes only. However, according to a report provided by the engine teardown facility, "Upper cylinders have [also] been known to be susceptible to hydraulic lock due to a leaking fuel primer on high wing aircraft." According to AC 65-12A, "Whenever a radial engine remains shut down for any length of time beyond a few minutes, oil or fuel may drain into...combustion chambers...or accumulate in...intake pipes when the engine starts. As the piston approaches top dead center of the compression stroke (both valves closed), this liquid, being incompressible, stops piston movement. If the crankshaft continues to rotate, something must give. Therefore, starting or attempting to start an engine with a hydraulic lock of this nature...may result in a bent or broken connecting rod. A complete hydraulic lock - one that stops crankshaft rotation - can result in serious damage to the engine. Still more serious, however, is the slight damage resulting from a partial hydraulic lock which goes undetected at the time it occurs. The piston meets extremely high resistance but is not completely stopped. The engine falters but starts and continues to run as other cylinders fire. The slightly bent connecting rod resulting from the partial lock also goes unnoticed at the time it is damaged but is sure to fail later. The eventual failure is almost certain to occur at a time when it can be least tolerated, since it is during such critical operations as takeoff and go-around that maximum power is demanded of the engine and maximum stresses are imposed on its parts."
The failure of the number 9 cylinder connecting rod, due to a primer leak initiation of a partial hydraulic lock. A factor in the accident was the unsuitable terrain, to which the forced landing had to be made.
Source: NTSB Aviation Accident Database
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