Accident Description
The airplane was in cruise flight at an altitude of 3,500 feet mean sea level when the engine lost power. The pilot subsequently executed a forced landing into trees.
Engine Examination
Inspection of the Lycoming IO-540 series engine found that the zinc-plated crankshaft gear attachment bolt was fractured. The fracture caused the crankshaft to no longer drive the gear. Analysis of the fracture surface indicated intergranular separation resulting from hydrogen-assisted cracking.
Background on Bolt Failures
Prior to the accident, Lycoming had used cadmium-plated bolts in engine construction but switched to zinc-plated bolts due to availability and a perceived positive impact on production. Between November 1996 and July 1998, Lycoming received 8,500 zinc-plated bolts. The company manufactured or overhauled 1,088 engines with these bolts and shipped 2,667 zinc-plated bolts as spares.
In 1998, two zinc-plated gear bolt failures were documented in helicopter engines, attributed to hydrogen embrittlement. One bolt failed after 30 hours of operation, the other after 19 hours. Lycoming and FAA personnel believed the failures were due to higher loads in helicopter applications. As a precaution, Lycoming removed the bolts from stock in July 1998. Bolt recalls were issued by the helicopter manufacturer and Lycoming that same month. The FAA issued an Airworthiness Directive (AD) in February 1999 to replace the bolts in affected helicopter engines worldwide.
In July 1999, a bolt failed on a Piper PA-32R-301 after 327 hours due to hydrogen embrittlement. Between March 2000 and January 2001, two bolts failed on Royal Jordanian Falcon trainer airplanes at 292 and 179 hours. As a precaution, Lycoming replaced bolts in the five-airplane Jordanian Falcon fleet around February 2001. Lycoming and FAA personnel continued to believe failures did not extend beyond helicopter and aerobatic applications, despite the 1999 PA-32R-301 failure.
In June 2002, another bolt failed on a Piper PA-32R-301 at 448 hours. Lycoming and FAA personnel examined the engine, and a Lycoming laboratory report attributed the failure to hydrogen embrittlement. In September 2002, this accident occurred. As a result, Lycoming issued a Service Bulletin (SB) about two weeks later to replace bolts on certain 540-series engines. The FAA issued an Emergency AD in October 2002 to also replace the bolts. The common cause of hydrogen embrittlement is a missed or under-baked condition during plating.