FAA, rejecting pleas to narrow the mandate’s scope, has given operators of certain CFM56-3 engines about a year to implement OEM-recommended inspections designed to detect corrosion linked to in-service issues.
The directive, issued July 14 gives operators until August 18, 2018 to start an inspection process that will help detect corrosion in CFM56-3, -3B, and 3C variable stator vane (VSV) bores.
"Corrosion has been found on the inside of the high pressure compressor (HPC) case around the VSV bores resulting in binding, sticking, and seizure of the VSVs during engine operation,” CFM Int'l explains in the service bulletin, issued in April 2016 and updated in November, that FAA used to craft its directive. "The VSV system actuation check will aid operators in the early detection of excessive corrosion build-up on the inside of the compressor case and around the VSV bores.”
The check, which requires rotating VSV actuation rings to ensure they move freely, is to be done at least once every 12 months. Inspections that turn up some binding or sticking trigger follow-up checks each quarter or, in the most extreme cases, repairs within five flight cycles.
CFM says the problem, which has been linked to one in-service thrust-loss incident, stems from operating in tropical regions. The OEM’s recommendation covered engines that fly 50% or more of their hours in tropical rainforest climate zones, including parts of Latin America, Africa, and Southeast Asia, and operate less than 150 hours per month.
CFM, Boeing, and Jet2.com asked FAA to limit the mandate to this sub-group of engines, rather than the entire fleet. FAA rejected the request.
"Operators may experience high moisture environments outside of the specified tropical zone that is described in” the CFM bulletin, FAA explains. "Operators that are outside of the specified tropical zone have experienced restricted VSV movement events."
FAA says its directive affects about 400 engines on U.S.-registered aircraft. The mandate is likely to be adopted globally, though other regulators could choose to limit the applicability.
The directive applies to CFM56-3, -3B, and -3C turbofan engines with steel HPC stator cases installed. Titanium HPC cases do not have the issue and are not subject to inspections.