The FAA, siding with airlines that oppose a mandated fix, has ordered CFM56-3 and -7 operators to inspect accessory gearboxes (AGB) for oil leaks, but stopped short of matching a European directive requiring installing new parts that minimize leak risks.

The directive, issued Dec. 30, orders operators of some 2,700 CFM56s flying on U.S.-registered Boeing 737s to ensure AGB hand-cranking pad (HCP) covers are reinstalled correctly following maintenance. Improperly reinstalled HCP covers have been linked to more than 40 in-service reports of engine oil loss. At least 34 of these resulted in in-flight shutdowns or turnbacks, and four affected both engines on a single aircraft at the same time.

In 2009, CFM introduced a new AGB assembly—including a dynamic seal—that fixes the problem. Last year, the European Aviation Safety Agency—over the objections of several carriers—mandated AGB swaps at the next shop visit.

The FAA has opted for a less costly procedure, mandating HCP cover inspections and assuming that operators will take the prudent approach of upgrading AGBs themselves.

“The FAA agrees that the loss of engine oil from unsecured hand-cranking pad covers can be addressed with the introduction of a dynamic seal,” the agency explains in the directive’s preamble. “The FAA has structured its approach to achieving that goal, however, through a combination of inspections and part replacement, which allows each affected operator to manage its own maintenance schedule.”

The FAA’s directive requires an initial inspection, as well as adding recurring checks “as a required inspection item in the approved continuous airworthiness maintenance program for the aircraft.”

The agency reasons that its approach will neither compromise safety nor “have as great an economic effect on the affected operators as mandating a part replacement.” The FAA also “believes that operators will eventually incorporate the dynamic seal to terminate the required inspections.”

Airlines for America, in comments on the FAA’s draft rule, says that “proper” cover installation by “attentive technicians,” combined with the agency’s proposed inspection program, is a prudent approach.

American Airlines suggests increasing safety margins by implementing improved maintenance procedures that minimize the chance of the same error being introduced on both engines.

The National Transportation Safety Board argued for a mandated fix, reasoning that the upgrade is a better solution than “multiple inspections.”