Cylinder  Assemblies Failed Due To Fatigue Cracking

In a letter to acting FAA Administrator Michael Huerta, the NTSB has recommended that the agency require repetitive inspection of Engine Components, Inc. cylinder assemblies produced between May 2003 and October 2009 (serial numbers 7709 through 52884) installed on Teledyne Continental Motors model 520 and 550 engines and removal of these cylinder assemblies once they reach the engine manufacturer’s recommended normal time (hours) in service between overhauls.

NTSB Image

In its letter, the NTSB says that since 2000, "the ...Board has examined numerous Engine Components, Inc. (ECi) reciprocating engine cylinder assemblies that failed due to fatigue cracking that initiated in the root of the cylinder head thread, eventually resulting in loss of compression and/or separation of the cylinder head into two pieces. These failures involved new assemblies installed on Lycoming and Teledyne Continental Motors (TCM)2 engines, and many resulted in fatal accidents.

"Although the FAA has issued several airworthiness directives (AD) applicable to certain new ECi cylinder assemblies used on Lycoming and TCM engines to address this issue, similar fatigue failures in other new ECi cylinder assemblies installed on TCM engines have been identified but are not covered by an existing AD. Because fatigue cracking and separation of cylinder assemblies in piston-engine aircraft is a serious safety issue, this letter recommends corrective action for certain new ECi cylinders used on TCM engines."

NTSB and FAA representatives formed a task force in 2005 to study fatigue failures of cylinder heads, which at that point included domestic and foreign failures. In 2006, the task force visited several facilities, including ECi, and observed the entire manufacturing process, from casting of the cylinder heads to cylinder assembly. The NTSB continued examining failed cylinders and updating the FAA’s aircraft certification office responsible for oversight of ECi. Following a meeting of NTSB, FAA, and ECi representatives in October 2006 to discuss the cylinder head fatigue failure issue, ECi designed and manufactured a fatigue test fixture that simulates the cylinder head temperature and pressure cycles experienced by a cylinder on an operating engine. After running numerous tests, ECi changed the manufacturing process for its cylinder assemblies in October 2009 to provide an increased interference fit at the shrink band and between the cylinder head and the cylinder barrel threads but remaining within the limits of the approved design.

During a meeting in January 2011 between ECi, the FAA, and the NTSB, ECi produced data on the cylinder assembly failures and recommended that the FAA issue an AD to mandate inspection and replacement of any discrepant cylinders; however, the FAA has not taken the recommended action for those cylinders not currently covered by an AD. Since the design improvement in October 2009, there have been no reported cylinder head fatigue failures on ECi cylinder assemblies installed on TCM model 520 and 550 engines. Although ECi’s changes to the manufacturing process may increase the fatigue strength of new production cylinder heads, a large population of potentially discrepant cylinder assemblies remains in service without any required corrective action.

The FAA has not yet commented on the safety recommendations.

FMI: www.ntsb.gov, www.faa.gov