Ramon Lopez/WASHINGTON DC
The US Federal Aviation Administration has issued two airworthiness directives (ADs) aimed at removing potential ignition sources in or near the centre-wing fuel tank of older Boeing 747s.
It says that it is best to eliminate ignition sources because it is not possible to purge all potentially explosive fuel vapours from the tanks as urged by the US National Transportation Safety Board (NTSB) .
The measures result from recent tests conducted by the NTSB in its search for the cause of the 17 July 1996 crash of a Trans World Airlines Boeing 747-100 just after departing from New York's Kennedy International Airport.
The Federal Bureau of Investigation (FBI) recently closed its criminal probe of the accident. Although mechanical failure is cited as the most likely cause of the crash, the Safety Board still does not know why the centre-wing tank exploded, and has not located the ignition source.
FAA administrator Jane Garvey says that there is no evidence that the parts played any role in the accident. One AD, which covers the fuel-quantity indication system (FQIS) wiring, comes in the form of a notice of proposed rulemaking (NPRM). It provides for a 90-day comment period and proposes a 12-month compliance deadline for Boeing 747-100/200/300s. Newer 747s are unaffected.
It would guard against voltage spikes or short-circuits in the FQIS. Investigators discovered that the combination of high-voltage wires, low-voltage FQIS wiring and metallic contaminants could produce sparks. The NPRM would require the installation of components to suppress electrical transients and/or the installation of wire shielding and separation of FQIS wiring from other wiring.
The second AD requires immediate inspection of scavenge-pump wiring in the centre-wing tank of some older Boeing 747s for deterioration of insulating materials in the electrical connectors.
It was found that some connectors use a silicone insulating material which is incompatible with the fuel used to cool and lubricate the pump motor. This could cause a fuel leak into the main landing gear wheel well, resulting in a fire.
The AD requires replacement, within 90 days, of pumps found to contain the silicone material.
The FQIS wiring work would cost $13,200 per aircraft, with the NPRM affecting 650 aircraft worldwide. A total of 970 aircraft will require the scavenge pump inspection.
Source: Flight International
