East Sparta, OH, USA
N1925U
CESSNA T210N
The pilot reported that during cruise flight the engine began to run rough with a sudden decrease in engine manifold pressure and exhaust gas temperature for cylinder Nos. 2 and 4. The engine continued to run erratically for another 100 seconds before it lost total power. The pilot made a forced landing on hilly terrain, during which the nose gear collapsed. The airplane’s engine mount and lower fuselage structure were substantially damaged. Postaccident examination determined that the No. 2 cylinder exhaust valve fractured near the stem-to-fillet transition. The fracture surface exhibited features consistent with a fatigue failure. Additionally, the clearance from the exhaust valve stem to the valve guide exceeded the manufacturer’s service limit. The No. 2 cylinder exhaust valve failed in fatigue likely due to the wear of the valve guide and rocker arm bearing systems. The wear of the guide allowed the exhaust valve to move so that it could no longer consistently contact the valve seat. The asymmetric load induced a bending moment on the exhaust valve stem that eventually resulted in the initiation of a fatigue crack along the stem edge. The wear of the rocker arm shaft and rocker arm bearing likely contributed to the exhaust valve failure by tilting the rocker arm and changing the angle at which it engaged the tip of the exhaust valve stem.
HISTORY OF FLIGHTOn September 28, 2021, about 1608 eastern daylight time, a Cessna T210N airplane, N1925U, was substantially damaged when it was involved in an accident near East Sparta, Ohio. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to automatic dependent surveillance-broadcast (ADS-B) track data, about 1534, the airplane departed from runway 2 at Wadsworth Municipal Airport (3G3), Wadsworth, Ohio. A plot of the airplane’s ground track during the flight is depicted in figure 1. The airplane flew about 2.8 miles north of the airport before entering a climbing turn to a south course. The airplane leveled at 3,500 ft mean sea level (msl) as it continued to the south. At 1555:12, the airplane entered a left turn to the southeast at 3,500 ft msl. Figure 1. Airplane ground track for accident flight. The pilot reported that, during cruise flight, the engine began running rough with a sudden decrease of exhaust gas temperature (EGT) for cylinder Nos. 2 and 4. He contacted Cleveland approach, declared an emergency with the controller, and turned left toward Akron-Canton Regional Airport (CAK), Akron, Ohio, as depicted in figure 2. Figure 2. Airplane ground track showing left turn. According to engine monitor data, at 1604:54, the engine began to run erratically with a sudden decrease in manifold pressure and EGT for cylinders Nos. 2 and 4, as depicted in figures 3 and 4. The engine continued to run with partial power for about 100 seconds before it lost total power. Figure 3. Turbine inlet temperature, cylinder head temperature, and EGT during the accident flight. Figure 4. Engine speed, manifold pressure, and fuel flow during the accident flight. According to ADS-B data, after the loss of engine power, the airplane entered a descent from 3,500 ft msl and decelerated from 130 to 79 knots calibrated airspeed, as depicted in figure 5. Figure 5. Airplane altitude, speed, and vertical speed during the accident flight. At 1608:05, the last ADS-B return was recorded, which showed that the airplane was about 240 ft west of the accident site and about 163 ft above ground level. The pilot chose to make a forced landing into a hilly pasture. The airplane’s ground track and descent profile was consistent with the forced landing occurring on an east-southeast heading. The pilot reported that the nose gear separated during landing roll and that the airplane came to rest nose down. AIRCRAFT INFORMATIONThe airplane’s Continental TSIO-520-R engine, serial No. 1032649, was manufactured on July 18, 2016, and was installed new on the airplane on August 18, 2016. At that time, the airframe total time (AFTT) was 5,262.0 hours. At the time of the airplane’s last annual inspection, the airframe total time and the engine time since new were 6,272 and 1,010 hours, respectively. At the time of the accident, the engine time since new was 1,134 hours. An oil analysis was performed on an oil sample collected during the last oil change, which was completed on August 16, 2021. The results of the oil analysis indicated an increased level of chromium, iron, and nickel. The chromium and iron levels were somewhat similar to the levels that had been reported on a previous oil analysis, dated October 1, 2020, but were above the long-term averages for each. The nickel level, at 51 parts per million, was about 34% higher than the 38 parts per million reported on the October 1, 2020, oil analysis. The material composition of the exhaust valve stem and valve guide included a significant amount of nickel. The comments section of the oil analysis report noted the increased metal wear, which the laboratory thought was not an immediate concern as long the levels did not trend upward. The engine accumulated 20 hours since the last oil change. A review of the available maintenance documentation revealed no evidence of an unresolved engine issue. AIRPORT INFORMATIONThe airplane’s Continental TSIO-520-R engine, serial No. 1032649, was manufactured on July 18, 2016, and was installed new on the airplane on August 18, 2016. At that time, the airframe total time (AFTT) was 5,262.0 hours. At the time of the airplane’s last annual inspection, the airframe total time and the engine time since new were 6,272 and 1,010 hours, respectively. At the time of the accident, the engine time since new was 1,134 hours. An oil analysis was performed on an oil sample collected during the last oil change, which was completed on August 16, 2021. The results of the oil analysis indicated an increased level of chromium, iron, and nickel. The chromium and iron levels were somewhat similar to the levels that had been reported on a previous oil analysis, dated October 1, 2020, but were above the long-term averages for each. The nickel level, at 51 parts per million, was about 34% higher than the 38 parts per million reported on the October 1, 2020, oil analysis. The material composition of the exhaust valve stem and valve guide included a significant amount of nickel. The comments section of the oil analysis report noted the increased metal wear, which the laboratory thought was not an immediate concern as long the levels did not trend upward. The engine accumulated 20 hours since the last oil change. A review of the available maintenance documentation revealed no evidence of an unresolved engine issue. WRECKAGE AND IMPACT INFORMATIONPostaccident examination revealed that the airplane’s engine mount and lower fuselage structure were substantially damaged during the forced landing. A partial teardown of the engine determined that the No. 2 cylinder exhaust valve was fractured. The No. 2 cylinder assembly and ancillary components, shown in figure 6, were examined at the National Transportation Safety Board Materials Laboratory, Washington, DC. The exhaust valve head was situated on edge and embedded in the cylinder head adjacent to the intake valve seat, as shown in figure 7. The intake valve head was fractured in multiple locations and was pushed through the valve seat. The exhaust valve seat was missing, and much of the cylinder head material around the exhaust outlet exhibited erosion and impact damage. Figure 6. Cylinder No. 2 component layout. Figure 7. Cylinder No. 2 intake and exhaust valve heads. The exhaust valve fractured near the stem-to-fillet transition. The fracture surface exhibited a flat morphology with ratchet marks at one location along the edge, as shown in figure 8. The features were consistent with the crack initiating at that location. The fracture origin was obscured by damage, which prevented a detailed examination. Away from the origin, the fracture surface exhibited curved crack arrest marks and features that were consistent with a fatigue fracture. The valve stem surface did not exhibit any pitting, erosion, or other notable features. Figure 8. Cylinder No. 2 exhaust valve stem fracture surface. The clearance from the exhaust valve stem to the valve guide clearance, which was measured in the upper and middle thirds of the valve guide, were 0.0067 and 0.0111 inch, respectively. The manufacturer’s service limit for the exhaust valve stem clearance is 0.0085 inch. The clearance from the exhaust rocker arm shaft to the rocker arm bearing was similarly measured and found to be 0.0032 and 0.0041 inch at the inboard and outboard ends, respectively. The manufacturer’s service limit for the exhaust valve rocker arm shaft clearance was 0.0040 inch.
A total loss of engine power due to a fatigue failure of the No. 2 cylinder exhaust valve.
Source: NTSB Aviation Accident Database
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