Nearly 20 percent of all general aviation accidents are a result of mechanical failure. On April 27, 2002, the student pilot, his flight instructor, and one passenger were killed and another seriously injured when the engine in the Beech Sundowner they were flying, owned by the student, failed during cruise flight over Oak Shores, California.
Prior to the accident, the student/owner and his CFI departed Oakdale, California, flew to Modesto, and picked up the two passengers. They then flew to Hanford, and on to Oceano County Airport. The plan was to stop for dinner on the return flight to Modesto.
According to the surviving passenger, as they neared Oak Shores, the engine sound changed. She heard a roar, and the CFI asked the student about the oil pressure. The student said it was "good," and they both agreed that the fuel pressure was normal as well. There was a private strip nearby, so the CFI instructed the student to head there. The CFI took control of the airplane.
The last thing the passenger remembers was clearing a ridge top and hitting trees about a quarter mile from the strip. At the time of the accident, the CFI had 3,500 hours of experience, and the student pilot had accumulated about 230 hours.
The airplane and engine did not have original logbooks, so new ones were created by a mechanic in 1993. At that time, the total time on the airframe and engine was 1,445 hours. Total airframe and engine time at the time of the accident was 2,079 hours.
It was found that the valve stem in the number one cylinder was broken and remained in the valve guide. It was also found that the exhaust valve stems for the number one and three cylinders were not within recommended specifications.
Textron Lycoming issued a mandatory service bulletin (388B), which described two methods for determining exhaust valve and guide condition. It suggested inspections at a maximum interval of 400 hours time in service. There was no record in the logbooks of this optional inspection being completed.
Textron Lycoming also recommends in Service Instruction 1009 that this engine be overhauled after 2,000 hours or 12 years. The engine in the accident airplane had not been overhauled since new in 1975.
The NTSB determined the cause of this accident to be the fracture and separation of the number one exhaust valve resulting in a loss of engine power and an off-airport forced landing. A finding in the accident was the owner's failure to maintain the engine in accordance with the manufacturer's recommendations.
Although compliance with service bulletins is not mandatory for normal Part 91 operations (there are exceptions, so check the FARs), they are issued because the manufacturer has detected a problem with a component. The service bulletin contains the recommended solution to that problem.
This engine was 79 hours and 15 years over the recommended overhaul time frame. It's highly probable that if the student pilot/owner in this accident had complied with the overhaul or valve inspection recommendations issued by Textron Lycoming, this accident would not have occurred.
Accident reports can be found in ASI's accident database.
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