By  Rytis Beresnevičius

 

The United States Federal Aviation Administration ( FAA ) has issued a new airworthiness directive (AD) for the Boeing 777, following an analysis that similar failures, which were apparent on Boeing 747 aircraft, could also occur on Triple Sevens.

The FAA stated that the electrical bonding of the engine fuel feed tube penetrating the fuel tanks of 777 aircraft, including the 777-200, 777-200LR, 777-300ER, and 777F, which are all powered by the General Electric (GE) GE90 engine, was the main design feature that prevented an ignition source from appearing inside the fuel tank in the event of lightning striking the engine nacelle.

“The fuel feed lightning protection features include the spar bulkhead fitting that redirects the majority of current during a lightning event.”

According to the regulator, separate bonding jumpers outside and inside the fuel tank provide additional electrical paths for current from lightning strikes, with an additional non-bonding feature and an encapsulation seal over the spar fitting inside the fuel tank, providing further protection from sparking.

 

The issue was first discovered on all 747 aircraft, and to address it, the FAA issued a final rule directive in November 2023.

However, analysis from reports related to the AD for the 747s has allowed Boeing and the FAA to determine that 777s are likely to experience similar encapsulation and bonding failures.

“On Model 747 airplanes, the bonding jumper outside the fuel tank is failing at an excessive rate in addition to the known degradation of the primary electrical bonding path through the spar fitting.”

The FAA said that 777 aircraft were expected to be affected to a similar or lesser degree. Nevertheless, the regulator was concerned that while there have not been any failures, these features are typically only inspected and/or replaced conditionally.

 

The FAA added that the fuel feed line encapsulation seals are found within the fuel tank and would generally not be inspected closely unless there was a specific maintenance concern with the fuel feed fitting.

“Poor electrical bonding would not be expected to be detected during on-condition maintenance as electrical bonding is a function of the installation, and the relevant component would likely be removed without a resistance measurement.”

Once replaced, the component installation would be measured and adjusted as necessary to meet the bonding requirements to meet airworthiness standards.

“The spar fitting’s encapsulation is designed to isolate any sparks or arcing generated at the bulkhead fitting during a lightning strike because of failed electrical bonds from flammable fuel vapors in the tank.”

However, the FAA warned that the fitting, including the in-tank coupler connecting the fuel feed line to the fitting, was the most probable location for sparking when electrical bonding is poor.

As a result, an unreliable encapsulation significantly increases the risk of a fuel tank explosion following a lightning strike to the engine nacelle when electrical bonding is poor.

“Analysis has shown that at least a subset of the encapsulation failures seen on Model 747 airplanes is expected on Model 777 airplanes.”

Each 777 has two locations on each wing where the bonding jumpers connect the fuel feed line to the airframe: forward of the front spar and in the upper portion of the engine strut area. The jumpers provide an alternative lightning current path to the airframe, reducing the current that passes into the fuel tank.

“However, airplanes produced with General Electric Company GE90 series engines were designed without the additional bonding jumper installation in the engine strut area that would otherwise sufficiently reduce the risk of a lightning strike that would result in the unsafe condition. The FAA may issue further rulemaking to address all Model 777 airplanes, including those powered by Pratt & Whitney and Rolls-Royce engines.”

The FAA concluded that a lightning strike to an engine nacelle, coupled with a latent failure of the lightning protection features for the fuel feed system, could result in potential ignition sources inside the 777’s fuel tanks.

The condition, combined with flammable fuel vapors, could result in a fuel tank explosion and the consequent loss of an aircraft, which is why the FAA addressed the unsafe condition.

 

FAA Proposes Directive For Boeing 747-400F To Address Risks Of Fuel Tank Explosions

The requirements to ensure fuel tank safety stemmed from the TWA Flight 800 accident in 1996.

Inspections and potential replacements

Issuing the AD, the FAA has skipped the typical rulemaking process of publishing a Notice of Proposed Rulemaking (NPRM), which would have invited stakeholders to comment on the proposed directive.

Instead, the FAA published the AD on October 2, with an effective date of October 17. Boeing issued its Multi Operator Message (MOM) MOM-MOM-24-0463-01B with compliance instructions on September 4, 2024.

The regulator estimated that 174 aircraft in the US will be affected by the directive, which mandates three actions related to inspections of the two fuel feed tube saddle clamps, engine fuel feed tube to wing structure bonding jumpers and tube clamps forward of the wing front spar, and the repetitive measurement of the electrical bonding resistance between the wing structure and engine fuel feed tube.

Boeing’s MOM also outlined the related applicable corrective actions, which include the reworking of the bonding jumper or saddle clamp installation, replacing saddle clamps whose metal contacts the fuel tube, cleaning and replacing damaged bonding jumpers and related hardware, and contacting the manufacturer for repair instructions.

The latter action was required in the event of any resistance measurements exceeding the specified limits, the FAA noted.

“The FAA considers this AD to be an interim action. The manufacturer is currently developing a more comprehensive inspection that will address both out-of-tank and in-tank lightning protection features. Once this inspection is developed, approved, and available, the FAA might consider additional rulemaking.”

Each inspection and resistance measurement should take up to three hours, costing operators $255 per inspection cycle with an estimated cost of labor being $85 per hour.

Replacing the saddle clamp, reworking the bonding jumper installation, and/or replacing the bonding jumper should take operators or maintenance organizations up to an hour, with the former being a 30-minute job, according to the FAA.