The US Federal Aviation Administration has issued emergency airworthiness directives (ADs) ordering inspections of fuel tank wiring in over 1,000 older Boeing 737s, 747s and 767s.

The action follows the chance discovery of a damaged wire conduit during investigations to find the source of a fuel leak on a 737 owned by a US domestic airline, believed to be Continental Airlines. The aluminium conduit, which contains wires supplying power to the fuel boost pumps, had "small pinholes in it", says Richard Breuhaus, chief engineer for fuel system safety at Boeing.

The conduit routes 115V wires through the main fuel tank in the wing. The wires are normally insulated with Teflon material to protect them from chafing during vibration, but in this case the insulation showed signs of wear. This produced the potential for arcing between the exposed wire and the damaged conduit, and set up conditions for a possible fire or explosion in the fuel tank, says the FAA.

The FAA AD is based on a Boeing service bulletin issued on 24 April, and calls for inspections on an estimated 152 US-registered 737-100/200s with more than 50,000h flying time. These must be completed within seven days, or five flight hours. An AD is also proposed requiring inspections of 737 models with fewer than 50,000h. In all, some 340 older 737s around the world are affected.

The action, priced at around $1,800 per aircraft for the 737, includes removals and inspections of the wire bundles and installation of a new layer of Teflon around the entire bundle to augment the layer around individual wires.

The AD for the 747 and 767 fleets is less rigorous, and simply calls for operators to confirm that electrical wiring in the conduits is protected within a Teflon sleeve. Up to 433 older 747s are affected by the ruling which calls for inspections within 60 days.

Source: Flight International