Two airworthiness instructions will certainly be applied to Boeing’s “Triple Seven” which target the fuel tanks, which present the equivalent of a defect in insulation against electrical discharges and other lightning strikes. A second instruction targets the GE90 turbojets which have metal inclusions on certain engine discs, potentially creating points of weakening of these parts and constituent elements, which could lead to uncontained damage.
An airworthiness directive concerning the tanks of the “Triple sept”…
The United States Federal Aviation Administration (FAA) has issued an Airworthiness Directive (AD) to correct errors made by Boeing in a Requirements Bulletin (RB) for fuel tanks on certain Boeing 777 and for the other, the GE90 turbojets which power the twinjet. The first directive requires airlines and other operators to inspect aircraft and install Teflon sleeves and retaining caps on parts of the center, left and right main fuel tanks. The deposit would potentially concern 282 devices.
…Aiming to prevent the formation of electric arcs
This instruction follows a Boeing service bulletin issued in August. It aims to “prevent the formation of electric arcs inside the main and central fuel tanks in the event of a fault current (i.e. electric current caused by an accidental insulation fault in the facility) or lightning, which, combined with flammable fuel vapors, could result in a fuel tank explosion and subsequent loss of the aircraft,” according to the proposal.
Two previous instructions and errors at Boeing
The FAA originally issued an airworthiness directive in September 2022. This directive followed reports of cracks noted in the lower surface of the front spar of a Boeing 777-300ER, which was flying subject to replacement of the wing spar. These cracks, combined with those in the front spar, presented a safety risk in the form of a fuel leak and could affect the structural integrity of the aircraft. Boeing, for its part, has since discovered that a requirements bulletin dating from October 2021 contains several errors, in particular on the application of seals on the fasteners penetrating the fuel tank. The FAA therefore estimated that it should replace the September 2022 directive. The FAA cited a response time of 90 hours per aircraft to carry out inspections and repairs. Airlines will have 60 days after the directive comes into force to adopt the new inspection program and up to 60 months to carry out repairs.
Inclusion problems on the engine parts constituting the GE90
The “triple seven” is also the subject of a second airworthiness directive, again from the FAA. This concerns more precisely the GE90 turbojets which power the twinjet. This interim airworthiness directive was issued after General Electric (GE) discovered metal (more specifically iron) inclusions on the GE90. The FAA mentions that GE informed the latter of the “detection of an inclusion (of iron) in a turbine disk made from the same powdered metal material used to manufacture certain parts of the high pressure turbine (HPT )”. These include stage 1 HPT discs, stage 2 HPT discs, forward HPT rotor seals, inter-stage HPT seals and stage 7 through 9 compressor rotors for GE90 engines . Some of these elements will need to be replaced in order for airlines to comply with the airworthiness directive. The FAA warned that if the situation was not corrected, it could lead to an “uncontrolled release of debris, damage to the engine and the aircraft”, or what is called uncontained damage with the risks potential that this implies.