The FAA is calling for “independent inspections” after certain maintenance actions on a large number of CFM56-3 and -7B turbofans in the U.S. fleet, engines that power the Boeing 737 Classic and Next-Generation fleet.

A proposed airworthiness directive (AD), to be published June 10 is designed to prevent total loss of engine oil from the turbofans in situations where mechanics forget to reinstall the “handcranking pad covers” on accessory gearboxes.

The cover, which is opened or closed with the handcrank, gives mechanics access to the engine hot section for the borescope inspections that must be carried out about twice per year.

“We have received reports of 42 incidents of total loss of engine oil from engines while in flight,” says the FAA in the proposed AD. “Thirty-four incidents were single-engine events and resulted in an inflight shutdown of the engine or an air turnback (ATB). Four incidents involved total loss of engine oil in both engines installed on dual-engine airplanes, which caused an immediate ATB of the airplane.”

The AD will require maintenance workers to perform an independent inspection to verify that the handcranking pad cover has been reinstalled each time it is removed. Operators who install a new “dynamic seal assembly” in the gearbox through a CFM service bulletin will no longer have to perform the independent inspections, though the FAA says that several -7B engine types do not yet have dynamic seal retrofits available.

CFM officials dispute that statement, saying dynamic seals are available for all engines. A spokesman tells Aviation Week the company supports both the retrofit dynamic seal, which greatly slows down the leak rate if the cap is left off, and an independent check, also known as a “one-over-one” signoff. He estimates there are about 10,000 engines worldwide yet to be retroffited.

In the U.S., the FAA directive will affect 2,702 CFM56-3 and -7B engines, according to the proposed AD. Independent inspections will take approximately 1 hr. each when the pads are removed and replaced in maintenance, an action the FAA estimates takes place every 1,300 flight cycles per engine.

“Based on an average use of these model engines of approximately 6,000,000 flight cycles per year, we estimate that an independent inspection would be required approximately 4,615 times per year [for the fleet],” says the FAA.

A nearly identical European Aviation Safety Agency AD issued in August 2012 calls for independent inspections as well as installation of the seals, a position CFM says it favors.

The FAA is taking comments on the proposed action through early August.