SEA98LA188

The pilot reported that, while practicing 'S' turns at 2,500 feet above ground, the engine shuddered and then stopped. After an unsuccessful restart, the student maneuvered to a nearby field, and executed a forced landing during which the nose gear folded as the aircraft passed across an irrigation ditch. The aircraft then nosed over. During examination of the engine, the number two cylinder rocker arm for the intake valve was observed to be broken at its hinge point. Inspection of the fracture surface revealed fatigue striations originating in the vicinity of the oil lubrication port on the rocker arm. No mechanical damage or microstructural abnormalities were found associated with the area of apparent fatigue cracking.

On September 29, 1998, approximately 1555 Pacific daylight time, a Cessna 150L, N5867C, registered to David Larkin, operated by Northway Aviation, and being flown by a student pilot, sustained substantial damage during a forced landing near Freeland, Washington. The student pilot was uninjured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was an instructional solo, was to have been operated under 14CFR91, and originated from the Snohomish County (Paine Field) airport, Everett, Washington, at 1524.

The pilot reported that while practicing "S" turns at 2,500 feet above ground near Whidbey Island the engine shuddered and then stopped. After an unsuccessful restart, the student maneuvered to a nearby field, and executed a forced landing during which the nose gear folded as the aircraft passed across an irrigation ditch. The aircraft then nosed over.

The aircraft was subsequently recovered and an engine run was attempted under the oversight of an FAA inspector. Although the engine was started successfully, its performance was less than satisfactory. The number two cylinder rocker box cover was removed and the rocker arm for the intake valve was observed to be broken at its hinge point.

Examination of the number two cylinder and its associated valve train components was conducted at the facilities of Teledyne Continental (TCM) on November 13, 1998, under the supervision of the investigator-in-charge. Inspection of the fracture surface revealed fatigue striations originating in the vicinity of the oil lubrication port on the rocker arm. The inspection also revealed that the cylinder #2 intake guide insert was bronze, and the valve was ground at a 45 degree angle. A TCM service bulletin, SB M76-8 (refer to ATTACHMENT SB-I), which was issued April 1976, addressed changing the chamfer to 30 degrees and incorporation of a stainless steel guide insert to replace the original bronze insert.

Aircraft records showed that the Continental O-200-A engine underwent a major overhaul on May 21, 1981, at a total of 1896.0 hours (nearly five years after the issuance of SB M76-8). The last 100 hour inspection was conducted on August 14, 1998, at a total engine time (since overhaul) of 1914.55 hours. The aircraft was flown an additional 75.66 hours between that date and the time of the accident.

The engine log showed that cylinders #1 and #2 were removed June 3, 1989, with approximately 735 hours since major overhaul, due to low compression. The valve seats and valves were reground and the cylinders honed, and the cylinders were reinstalled with new rings and gaskets.

A compression check of cylinder #2 on June 26, 1998, (at 1768.6 hours time since major overhaul) and on August 14, 1998 (at 1914.8 hours time since major overhaul) were both at 78/80 (differential compression method).

The failed rocker arm was evaluated by the NTSB Materials Laboratory, whose Factual Report is attached. Fracture surfaces were examined, and evidence consistent with fatigue cracking near the parting line on the valve side of the component was identified. No mechanical damage or microstructural abnormalities were found associated with the area of apparent fatigue cracking.