Wednesday, November 14, 2007

GO-435 Crankshaft Fracture

This photo shows what happened to the crank in a Lycoming GO-435-C2 engine taken from a 1951 Navion that crashed in Burlington, Ontario in 2005. The engine had 2690 total hours and 101 SMOH when the crankshaft broke in flight after takeoff. (Sadly, the pilot stall-spun into the ground.)

Interestingly, even though this engine had undergone a prop strike some 70 hours earlier, the Transportation Safety Board of Canada did not blame the break on the sudden stop. They noted that the gear faces on the planetary reduction gearing showed no evidence of a hard stop. (The fact that the crank broke at the aft end instead of near the front would also be inconsistent, generally speaking, with a prop strike, but the Board didn't mention that fact.)

The Board found that the fatigue failure was progressive and began at a point on the forward fillet radius to the number six connecting rod journal, where some corrosion pitting was evident.

The Board did a detailed examination of the fillet radius and found that "the journal surface showed an absence of case hardening at the origin of the fatigue crack," adding that "The equivalent location on the aft fillet radius and the forward radius 180ยบ from the fatigue crack origin showed acceptable case hardened layers. The number five connecting rod journal also showed the presence of normal case hardened surface layers."

The Board noted: "The deficiency in the material heat treatment condition is believed to have been the result of a manufacturing error."

The crank was manufactured in 1955.

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