Boeing-developed maintenance information used by regulators to mandate 777 structural inspections in 2022 includes numerous errors that may create more risks for affected aircraft than it addresses, forcing the FAA to issue new requirements that tackle some of the issues while Boeing finishes revamping the instructions.

The original issue—potential cracks in certain structural parts near center fuel tanks—came to light in October 2021 in a Boeing “requirements bulletin” to operators. The 968-page bulletin divided hundreds of 777s of all variants into nine groups, with multiple aircraft configurations in each group. The bulletin recommended inspections at various intervals depending on the aircraft model and configuration. 

The FAA mandated the checks in September 2022 based on the bulletin’s contents and a report of a cracked front spar lower chord found on a 777-300ER undergoing an underwing longeron replacement. Other regulators, including the European Union Aviation Safety Agency, adopted the FAA airworthiness directive (AD).

In late 2022, Boeing discovered some problems with its 2021 bulletin. Chief among them: its instructions did not properly detail removal and replacement of fastener covers, or caps, located inside fuel tanks. The caps protect against a lightning strike triggering a spark and potential fuel tank explosion.

For some airplane groups, Boeing did not clearly explain the cap removal and replacement process. For others, the bulletin references a Boeing “standard overhaul practices manual” that specifies using caps that are too thin to contain possible sparks caused by lightning strikes. Boeing’s instructions also listed specific airplanes that didn’t need the work and omitted a few that did.

Boeing in late 2022 informed the FAA and affected customers that the 2021 bulletin had errors. But a full revision of the instructions is still not done due to the issue’s complexity. 

The FAA has decided to tackle the highest-risk problem—potentially non-compliant fastener caps in center fuel tanks—with a new directive based on Boeing's preliminary revisions already shared with operators while the company completes the service-instruction changes. 

An AD set for publication Aug. 31 lays out new instructions for removing and replacing fastener caps as part of the structural inspections or adding them in places they are needed but were not required by the September 2022 directive. The directive is an immediately adopted rule, bypassing the draft view and public comment process afforded less-pressing requirements.

For aircraft that have already undergone the work mandated last year, the new AD gives operators either 90 or 180 days from its scheduled Sept. 15 effective date, depending on certain parameters, to verify fastener caps are compliant and make any needed modifications.

“Fastener cap seals interior to the airplane’s fuel tanks are a critical lightning protection feature,” the FAA said. “This is particularly true for the center wing fuel tank, which typically contains flammable fuel vapors more frequently than the main wing fuel tanks ... If these seals are not replaced properly, and the associated fastener has poor electrical bonding to the airplane structure for any reason, the fastener may spark during a lightning strike and cause a fuel tank explosion.” Non-compliant caps pose risk for aircraft whether they have flammability reduction or ignition mitigation systems installed, the agency noted, though the risk is greater for aircraft without them.

The directive also removes 777-200s without center fuel tanks from the applicability list and clarifies which 777 freighters (777Fs) need the work. Design changes implemented starting with line number 1743—a 777F delivered to China Cargo Airlines in July—addressed the issues, the FAA said.

Boeing said it “fully supports” the AD, “which is consistent with guidance we have shared with operators previously.”