FAA issues urgent AD to address potential Boeing 777 fuel tank explosions

The FAA is urgently issuing an airworthiness directive to address an unsafe condition with the center wing fuel tank of the Boeing 777
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The United States Federal Aviation Administration (FAA) urgently issued an airworthiness directive (AD) to address errors made by Boeing in a Requirements Bulletin (RB), which meant that the fuel tanks on some Boeing 777 aircraft were not properly protected against lightning strikes.

Initially, the FAA issued an AD in September 2022 to address “cracking of the left- and right-side ring chords, repair angles, front spar lower chords, and front spar webs (depending on configuration) common to the underwing longeron located at station (STA) 1035”.

The directive was prompted by reports of cracks found on a Boeing 777-300ER’s front spar lower chord that was undergoing underwing longeron replacement, which, combined with cracking in the front spar web, posed a safety hazard in the form of a fuel leak or in more severe cases, could affect the structural integrity of the aircraft.

However, according to the recent AD from the FAA, Boeing has since discovered that its RB from October 2021 “contains errors relating to the application of cap seals to fasteners penetrating the center wing fuel tank which introduce a second, urgent unsafe condition”.

The actions required by the RB from October 2021 and the AD from September 2022, mandate the removal of many cap seals to “accomplish the various modifications and inspections”, the AD continued.

“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,” the FAA said in the directive.

As such, the FAA determined that it needed to supersede the September 2022 directive to address the new unsafe condition.

Affecting Boeing 777 fastener cap seals

The regulator noted that the directive, which now requires an inspection and replacement/modification of the center wing fuel tank fastener cap seals, was now being published “without additional service information” due to the public safety risk.

“The manufacturer submitted an initial report of errors in the requirements bulletin affecting cap sealing instructions in late 2022,” the FAA said, adding that due to the length and complexity of the new RB, Boeing handed over the documentation of these errors in July 2023.

While the planemaker said it would revise the bulletin, the FAA noted that the “work will take longer to accomplish than the risk to public safety allows,” which is why the agency superseded the AD from September 2023.

The FAA highlighted three procedural errors relating to the fastener cap seals.

Firstly, operators did not need to apply cap seals to fasteners associated with the underwing longeron of some Boeing 777s. Without the requirement, “a cap seal may fail to be applied following modification of the underwing longeron, compromising the required fault tolerance of the fuel tank lightning protection design”.

Airlines flying these Triple Sevens will have to apply a cap seal of the correct sealant type to the minimum thickness.

Secondly, some Boeing 777s have no thickness requirement and, furthermore, the RB “mistakenly refers to the Boeing Standard Overhaul Practices Manual (SOPM) section 20-50-19 for procedures to apply the cap seal”. The SOPM’s requirements have a cap seal thickness of half the minimum amount.

Even if the aircraft’s aircraft maintenance manual (AMM) procedure included the correct thickness, the FAA noted that “operators may have used an accepted method other than that specified in the SOPM and AMM” since the RB did not specify whether to refer to the SOPM or AMM. As a result, the cap seal could fail to contain a spark from a lightning strike, resulting in a fuel tank explosion. The FAA now requires operators to ensure the minimum required thickness and sealant type, as well as replacing any seal that was already replaced with improper thickness.

Thirdly, the RB required “the removal of certain fastener cap seals; however, the requirements bulletin either does not require that the cap seal be replaced, or does not provide a thickness requirement for the replaced seal”. This means that the fuel tank’s lighting protection is compromised, and the FAA mandates airlines to replace the removed seal with the correct thickness and type.

Boeing making design changes

Since the issue came to light, Boeing has already implemented a design change, meaning that some newly built Boeing 777s will not be affected by the latest directive.

According to the FAA’s estimates, 291 Boeing 777s in the US will be affected by the directive. After the agency published the now-superseded directive in September 2022, it discovered, along with Boeing, that some 777-200s are not susceptible to the unsafe condition because they do not have a fuel tank between the side-of-body ribs.

With the latest AD, Boeing 777s with the unsafe condition will now be identified as those where the Maximum Taxi Weight (MTW), which includes taxi and run-up fuel for the engines and the Auxiliary Power Unit (APU), is more than 547,000 pounds (248,115 kilograms).

Furthermore, since the September 2022 directive, Boeing has already implemented the required design changes for newly built aircraft of the type where the Line Number (LN) is higher than 1,743. According to ch-aviation.com data, there are seven such aircraft, all Boeing 777F aircraft delivered between June and August 2023, including FedEx’s 50th factory-fresh 777, registered as N874FD.

In its latest AD the FAA has retained three actions from the September 2022 directive, namely an inspection, modification, and post-modification inspection, and has added two more. Now, airlines will have to review their Boeing 777 maintenance records and properly seal the caps.

However, the FAA noted that it “has no way of determining the number of aircraft that might need the cap sealing”.

According to the agency’s estimates, the sealing will take airlines up to 109 workhours. With parts ($90) and labor ($9,265), the action will cost $9,355 per aircraft. Some of the costs related to the actions included in the AD could still be covered under warranty.

The AD will be published on August 31, 2023, and will come into effect 15 days after publication.

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