SEATTLE -- Recent discoveries of wire chafing and, in one instance, leaking fuel in a pipe carrying electrical wires inside wing fuel tanks have prompted the Federal Aviation Administration to order that the parts be inspected and replaced as necessary within a week on some older Boeing 737s.
Boeing 747s and 767s must be inspected in 60 days.
Yesterday's announcements are the latest in a series of inspections and design changes ordered after the 1996 crash of TWA Flight 800, which probably was caused by an explosion inside the center wing tank of that 747 -- a blast that may have been of electrical origin.
Boeing said the inspection would take about 15 hours for a maintenance crew of four -- which could disrupt airline schedules.
Airlines typically schedule planes for eight or more hours of service a day. The 737 is the most-used and most-numerous airliner in the world.
Operators of 737-100 and 737-200 models with more than 50,000 hours of flight must complete the inspections within a week.
The inspections affect 152 planes in the domestic 737 fleet, the FAA said. In a service bulletin Boeing issued two weeks ago, the company said 340 planes worldwide are affected.
The FAA said 264 Boeing 747s and 231 Boeing 767s in the United States are subject to the 60-day deadline. It said the compliance time was longer because the issue was different with those models.
Unaffected are next-generation 737-700 and -800 models, 757s and 777s. Those planes do not have conduits or wiring inside fuel tanks.
Officials acknowledged some urgency in their actions but said there was minimal threat to travelers if the inspections and corrective actions are taken within the required times.
The action stems from an investigation into the TWA accident in July 1996, which killed all 230 aboard, over Long Island, N.Y.
While investigators have not determined what caused the crash, they suspect that electricity from wiring around the Boeing 747's center fuel tank may have sparked an explosion.
Since then, Boeing and the FAA have issued service bulletins and airworthiness directives to conduct inspections, or to eliminate other potential flash points in Boeing aircraft having fuel systems similar to the 747's.
In the planes subject to the inspections, metal conduits in the fuel tanks house wiring that carries electricity to fuel pumps in the wings. The pumps are outside the fuel tanks, but the wiring travels through the conduits.
A few months ago, Boeing got a report of chafed insulation inside the conduits of two 737s with more than 50,000 hours in the air. That prompted the company to begin work on last month's service bulletin.
But last week, an airline reported a fuel leak. Boeing found a conduit with pin-sized holes, said Mike Denton, chief project engineer for older-model 737s.
This week, Boeing examined the conduit -- the wires had been discarded by the airline -- and could not rule out electrical "arcing," possibly caused by worn insulation and wires short-circuiting, as the cause of the holes.
That prompted the FAA and Boeing to make the inspections more urgent, Denton said.
Three weeks ago, the FAA announced another proposed order, affecting the design of vent systems and wiring of the fuel-measuring systems in 737-100, -200, -300, -400 and -500 models.
Among other things, the proposed design changes address a potential wiring problem that could create a spark inside a fuel tank big enough to make the contents of the tank explode.
Last month's proposed order for 737s was similar to one announced in November for 747-100s, -200s and -300s. It also has not been implemented.
Pub Date: 5/08/98