What happened
While operating in cruise flight at 3,500 feet mean sea level, the Lycoming IO-540 series engine lost power. The pilot was forced to perform an emergency landing into a group of trees. There were no injuries reported following the impact.
The investigation
An examination of the engine revealed that the zinc-plated crankshaft gear attachment bolt had fractured, which prevented the crankshaft from driving the gear. Analysis of the fracture surface showed intergranular separation caused by hydrogen-assisted cracking.
Investigation into the component history showed that while engines were previously built with cadmium-plated bolts, zinc-plated bolts had been introduced to production due to their availability and positive impact on manufacturing efficiency. Lycoming received 8,500 of these zinc-plated bolts between November 1996 and July 1998. During this period, Lycoming manufactured or overhauled 1,088 engines using these bolts and distributed 2,667 as spare parts.
Prior to this accident, two similar failures had been documented in helicopter engines in 1998, occurring at 30 and 19 hours of operation. Although Lycoming and FAA personnel initially believed the issue was limited to the higher loads found in helicopters, subsequent failures occurred in other aircraft, including a Piper PA-32R-301 in July 1999 and two Royal Jordanian Falcon trainer airplanes between March 2000 and January 2001. Another failure occurred on a Piper PA-32R-301 in June 2002 at 448 hours of operation.
Findings
- The engine power loss was caused by the failure of the crankshaft gear attachment bolt.
- The failure mechanism was identified as hydrogen embrittlement.
- The common cause for such hydrogen embrittlement is a missed or under-baked condition during the manufacturing process.