Cleburne, TX, USA
N7941L
Beech 23
The private pilot departed on a local flight and had flown for “several” minutes when the oil pressure began decreasing. The pilot elected to return to the departure airport. As he continued on his return, the engine started “shaking” and subsequently stopped. He performed a forced landing on a curving gravel road and subsequently departed the road and impacted bushes and a tree, resulting in substantial damage to the airplane’s right wing. A postimpact examination of the engine revealed the No. 1 cylinder head was separated and the piston exposed. Further examination revealed fatigue cracking through the cylinder wall. The failure initiation was a through-wall crack located on the exhaust side of the cylinder. A review of the airplane logbook revealed debris was found under the oil pressure relief valve seat 4 years prior to the accident, which “caused the oil pressure gage to read zero.” Further review showed that the oil pressure relief valve had been removed, cleaned, and reinstalled 8 months before the accident and that, 3 weeks prior to the accident, the oil pressure relief valve seat was honed, and a new plunger was installed. It is likely the separation of the No. 1 cylinder head was a result of metal fatigue, which in turn, resulted in the complete failure of the engine.
On June 4, 2020, about 0955 central daylight time, a Beech A23A airplane, N7941L, sustained substantial damage when it was involved in an accident near Cleburne, Texas. The private pilot was uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported he departed on the local flight about 0925. He’d flown southwest along a local highway for “several” minutes when he noticed that the oil pressure was “trending toward the bottom of the green arc.” The oil temperature, however, was normal. The pilot elected to return to his departure airport. As he turned the airplane, the engine started “shaking.” He reduced engine power to prevent the engine shaking violently. The pilot re-applied engine power to maintain altitude and the engine stopped. He did not recall if the propeller was windmilling; however, the engine was “silent.” The pilot performed a forced landing on a curving gravel road. As the road curved, the airplane departed the road, impacting bushes and a tree resulting in substantial damage to the airplane’s right wing. Postaccident examination of the engine showed the No. 1 cylinder’s cylinder head had separated and the piston was exposed. Further examination of the engine showed some wear on the tappets and camshaft. Small flakes of aluminum were visible in the piston rod bearing. The bearing exhibited scoring consistent with hard particles passing through it. The oil pump exhibited heavy scoring witness marks consistent with hard particle passage at some point. Other than the separated cylinder, no issues were found that would have precluded operation of the engine. An examination of the No. 1 cylinder revealed fatigue cracking through the cylinder wall around roughly 60º on the exhaust side of the cylinder and secondary fatigue cracking on the intake side of the cylinder. The failure initiation was a through-wall crack located on the exhaust side. A review of the airplane logbook showed that on September 22, 2016, the total time since major engine overhaul had been recalculated from 2,594.0 hours to 190.3 hours; however, a record of a major overhaul was not found. An airplane logbook endorsement dated August 15, 2016, indicated that debris was found under the oil pressure relief valve seat, which “caused the oil pressure gage to read zero.” Additionally, analysis of the oil found metal. A logbook entry stated the oil pressure gauge showing zero pressure was a known problem with the IO-346 engine. Per “talks” with Continental Motors technical personnel, their recommendation was to pull the pressure relief valve and check for debris. The debris was cleared, and the valve re-installed with new washers. A test run of the engine showed no leaks and normal oil pressure. Analysis of the oil showed wear metals with higher-than-normal particles. The logbook entry indicated that this was not a concern as two new cylinders were installed within 50 hours, and the engine “had not been setting and not run consistently.” However, which two cylinders has been replaced was not specified. Spectro Oil Lab, which conducted the oil analysis, recommended the airplane be operated for “another 10 or so hours” and that a clean oil sample be taken and submitted for testing. No logbook entry was found indicating that this was done. An oil and filter change and inspection were completed at the next annual inspection on February 7, 2017. There was no note of any debris found during the inspection. On November 1, 2019, the oil pressure relief valve was removed, cleaned, and reinstalled. The oil temperature sensor was also removed, cleaned, and reinstalled. On June 2, 2020, a logbook endorsement showed that the oil pressure relief valve seat was honed, and a new plunger was installed.
The total loss of engine power due to metal fatigue failure of the No. 1 cylinder wall.
Source: NTSB Aviation Accident Database
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