Incident Summary
During a climb to cruise altitude, the pilot heard a loud knocking noise from the engine. This was followed by violent vibrations and a loss of power. The pilot performed a forced landing in a newly excavated construction site. After touchdown, the aircraft went over an embankment.
Investigation Findings
Examination of the engine revealed deformation of the number four cylinder valve cover. The number four exhaust valve rocker arm was found to be broken. A metallurgical laboratory analysis of the rocker arm identified fracture features consistent with high cycle fatigue and overstress. The fatigue origin was traced to the reworked oil drip hole, which did not meet the required 0.030-inch radius.
Further inspection of the rocker arm using a measuring microscope found the wall thickness to be 0.055 inches. However, previous measurement using a round anvil micrometer had indicated the wall thickness met the minimum dimension of 0.075 inches.
Maintenance Records
Review of the aircraft maintenance records showed that Airworthiness Directive (AD) 87-10-06, which incorporates Lycoming Service Bulletin (SB) 477A, was accomplished 78.7 hours prior to the accident. The service bulletin requires that rocker arms with a wall thickness less than 0.075 inches be replaced. If the rocker arm passes the thickness inspection, the outer edge of the rocker arm oil drip hole must be reworked (burred) to an approximate 0.030-inch radius before reassembly. The rework performed on the failed rocker arm did not meet the specified radius.