The US Federal Aviation Administration has introduced a new airworthiness directive (AD) for specific Boeing 747-400F aircraft following a report indicating that cap seals were omitted from certain fasteners within the fuel tank during production. This omission compromised their electrical insulation, heightening the potential for a fuel tank explosion.
By J Prakash
- The FAA is currently taking steps to tackle the risk of fuel tank explosions on Boeing 747-400F planes.
- Reports have been received by the regulator regarding incorrectly applied cap seals on particular fasteners of certain 747-400F aircraft.
- The FAA cautioned that inadequately applied cap seals may not effectively prevent arcing within the fuel tanks in the event of a lightning strike or a short circuit.
- Changes following the TWA Flight 800 accident.
Adding further context to the matter, the FAA stated that it had thoroughly investigated the safety concerns surrounding fuel tank explosions in various large commercial aircraft and assessed the sufficiency of existing regulations in mitigating these risks. In May 2001, the regulatory body published its findings and conclusions in a finalized rule document titled 'Transport Airplane Fuel Tank System Design Review, Flammability Reduction, and Maintenance and Inspection Requirements.
In a document published to the Federal Register on 12 April, it states, 'This proposed AD would require applying cap seals to certain fastener collars inside the fuel tanks. The FAA is proposing this AD to address the unsafe condition on these products.'
The cap seals serve as an insulating layer on the fasteners, preventing arcing between them during a lightning strike or other high-energy short-circuit. A directive issued in 2022 mandated the installation of cap seals on 747-400Fs with line numbers 645-1363. However, a subsequent production design change omitted eight fasteners on the left and right wings in the inboard main fuel tanks, as discovered by Boeing.
The FAA has determined that the actions specified in the AD are essential to diminish the potential of ignition sources within fuel tanks, 'which, in combination with flammable fuel vapours, could result in fuel tank explosion or fire.' The directive would have an estimated cost to operators of just over $62,000, according to the regulator.
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