Reacting to increasing evidence that aging aircraft may pose greater risks of explosion, the Federal Aviation Administration announced plans Thursday for mandatory replacement of fuel tank components and wiring on older Boeing 747s.
The FAA’s proposal, affecting 251 U.S. passenger jets, targets 747s that are 20 years old or older.
This marks the first time a specific age for a plane has been used by the FAA as a safety benchmark. Previously, the agency adopted a position that wiring and internal parts were generally safe for the lifetime of the aircraft.
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Under the new plan, the agency will turn its spotlight from areas outside the tanks to the fuel-filled cores, which contain probes wired to cockpit gauges.
The initiative also includes center fuel tank inspections and, on some planes, the addition of fire safety equipment on fuel lines.
In addition to required repairs, the agency is proposing a mandatory, one-time inspection of the internal workings of the fuel tanks on the older 747s.
The move by the FAA, beleaguered by criticism of slow or inadequate action involving older airplanes, comes in response to evidence that a spark may have triggered the fatal fuel tank explosion that brought down TWA Flight 800 in July 1996, killing 230 people.
FAA spokesman Les Dorr said the proposal, called an airworthiness directive, is part of a continuing drive to prevent accidents by attempting to reduce potential ignition sources in 747-100, 200 and 300 series.
Officials of Boeing Co., manufacturer of the 747 series aircraft, said they support the FAA initiatives, noting that the Seattle-based airplane manufacturer began steps on May 22 to alert operators about specific fuel tank changes.
Earlier that month, the FAA had ordered the immediate grounding of 179 older-model Boeing 737s after mechanics found exposed wiring, signs of chafing and pin-size holes on the fuel pump wiring apparatus of some planes.
Cracked or deteriorated wire insulation has been found on about 60 percent of the 500 planes inspected since May, the FAA reports.
Since May, Boeing has issued three service bulletins outlining new inspections, replacement or repair of fuel tank parts and wire bundles, which can contain hundreds of wires wrapped in huge strands.
The bulletins alert airline operators to forthcoming FAA actions and help establish a maintenance schedule that will not disrupt passenger service, Boeing spokeswoman Kirsti Dunn said.
The bulletins, which do not require action, are based on discussion with FAA officials and Boeing’s ongoing testing. Currently, the manufacturer is testing 20-year-old fuel tank components to track the effects of stress and age.
One immediate change that would be required under Thursday’s proposal involves replacement of a metal clamp that can damage wire shielding and insulation, exposing bare wire.
The FAA will require a new clamping system, already in place on later-model 747s, that incorporates nylon clamps and protective shrink wrap on wires to prevent chafing and damage, Dorr said.
While the cause of the TWA Flight 800 explosion off Long Island, N.Y., in 1996 has not been established, the FAA
concurs with preliminary findings by the National Transportation Safety Board that a combination of elec-
trical failures may have resulted in a high-voltage signal traveling through a fuel tank wire.
One scenario involves two side-by-side wires that may have developed simultaneous breaks in outer insulation coatings, allowing a spark to arc to a wire leading to the fuel tank, investigators said.
As a result, the safety board, an independent federal agency that investigates transportation accidents, has called for mandatory fuel tank inspections of all aircraft, not just selected models.
FAA officials have said there is insufficient evidence to warrant such a widespread and costly mandate.
The FAA’s proposal to inspect more planes, but not all, ignores findings that a thin layer of protective insulation is vulnerable to vibrations, age and other factors, aviation safety experts said.
Safety board investigations have concluded that cracked insulation allows electrical sparks to escape and reach other cracked wires that run to aircraft fuel tanks.
A similar scenario also is considered the probable cause behind a fuel tank explosion of a Philippine Airlines Boeing 737 that exploded on a Manila runway in 1990.
Safety experts believe segregating fuel tank wires from other bundled wires is a necessary and immediate goal for all passenger planes.
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FAA officials are well-versed in structural problems associated with older aircraft, but they now are learning about the equally important role of wiring.
The agency will give the airline industry 45 days to recommend changes before the plan takes effect.
Airlines would be granted up to two years to implement most of the changes, estimated to cost about $35,000 per plane.