FAA and CFM International are at odds over the proposed scope of soon-to-be-mandated inspections on certain CFM56-3s to detect corrosion that has been linked to at least one in-service incident.
The checks, proposed in a March 9 draft airworthiness directive (AD) and based on a CFM service bulletin issued last year, call for Boeing 737 Classic operators to inspect CFM56-3, -3B, and 3C variable stator vane (VSV) bores for corrosion.
The issue caused one unidentified operator’s loss of thrust control in both engines, forcing “an air turn back,” FAA’s directive says.
CFM says the problem, which can cause thrust loss by restricting actuator movement, is linked to low-utilization operations in tropical regions. CFM's bulletin recommends inspecting engines that fly 50% or more of their hours in tropical rainforest climate zones and fly less than 150 hours per month.
FAA’s draft directive says that basing mandatory checks "on geography and, to a lesser extent, utilization…is not practical.” The agency wants the checks to be applicable to all CFM56-3s that have not had their bores repaired per CFM's related service instructions. FAA’s rule would cover about 460 engines installed on U.S.-registered aircraft.
In comments on the draft rule, CFM maintains its position that fleet-wide checks would be overkill. "CFM's position is that carriers most susceptible to corrosion are those operating in wet or tropical regions at a low to moderate utilization,” says the manufacturer, a 50/50 joint venture of GE Aviation and Safran Aircraft Engines. "CFM notes successful experience with regional applicable service bulletins, which recommend action based on environmental influences."
Boeing echoed CFM’s view.
"The FAA proposed corrective action to [affect] all engines unnecessarily burdens operators not flying in the noted climate zones adds no enhancement to the safety of the fleet,” the manufacturer says in comments on the rule.
CFM also noted that certain early CFM56-3 configurations with titanium high-pressure compressor front stator cases “are not susceptible to corrosion buildup” and could be exempted from the checks. The later configurations feature steel cases.
FAA proposes initial inspections within 12 months of a final rule’s effective date, and follow-up checks in either three months or 12 months, depending on findings. Damage requiring repairs would need to be fixed before further flight.
Comments on the proposed rule are due by April 24.