What happened
During the takeoff roll, the instructor reported that power increased smoothly; however, the engine suddenly failed at 600 feet AGL. Upon the loss of power, the flight instructor took control of the aircraft and lined up with a major interstate highway. The aircraft touched down in the northbound lanes of the highway. During the landing sequence, the right wing struck a bush, which caused the airplane to spin off the highway and down an adjacent embankment.
The investigation
An engine teardown revealed that the crankshaft idler gear for the left magneto had detached from its mounting pad at the rear of the crankcase. Investigators found that the crankshaft gear was not secured to the crankshaft, and the locating dowel pin had fractured and separated.
Further examination of the idler gear shaft and associated components showed severe wear on the attachment bolts and locking plate. The bolts exhibited signs of prolonged contact with the rotating idler gear. Additionally, the bore at the idler gear shaft-mounting pad was severely worn and elongated. The crankshaft counterbore and its corresponding gear showed evidence of fretting and corrosion at the mating surfaces. The fracture surface of the dowel pin displayed fatigue.
Regarding maintenance history, a propeller overhaul had been completed 20 hours before the accident. While the Hartzell propeller manual prohibits balance weights exceeding 0.9 ounces (25.51 grams) in any single location on the spinner, investigators found weights totaling 36 grams at one location on the airplane spinner.