The FAA has issued a Special Airworthiness Information Bulletin (SAIB) to advise owners, operators and repair stations of aircraft equipped with the Rolls-Royce (formerly Allison) 250-C30, -C40 and -C47 series engines that a recent suspected unapproved parts investigation revealed that some combustion liners might have been modified with an unapproved metal alloy. These engines are installed in, but not limited to, the Bell 206, 230, 407 and 430; MD Helicopter 369F, 369FF and 600; and Sikorsky S-76A.

Information obtained from a suspected unapproved parts investigation revealed that some combustion liner mixing cups were overhauled at Aero Weld of San Marcos, Calif., with an alloy (Hastelloy X) not approved for this application. These liners were overhauled between September 2000 and last November. The liner part numbers are 23066675, E23066675, 23064570 and E23064570. Approximately 235 liners reached the field.

This alloy does not have the corrosion resistance of the approved material (L605). However, there is at least one example of a Hastelloy X-equipped liner that reached its scheduled time change interval with satisfactory field service.

The expected failure mode is a degradation of the cup, which affects the performance of the mixing assembly and results in a gradual loss of combustor efficiency. The operator will notice increasing fuel flow and increasing turbine temperatures if the cup deteriorates. FAA SAIBs are recommendations only; operators are not required to comply. For further information contact the FAA Chicago Aircraft Certification Office, Propulsion Branch (ACE-118C) in Des Plaines, Ill.